diff --git a/.gitignore b/.gitignore
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--- a/.gitignore
+++ b/.gitignore
@@ -1,14 +1,31 @@
-/tex/*
+/nostarch/[012]*.tex
+/nostarch/hints.tex
+/nostarch/book.*
+/nostarch.pdf
+/pdf/[012]*.tex
+/pdf/hints.tex
+/pdf/book.*
+/pdf/book_mobile.*
+/pdf/*.log
+/book.pdf
+/book_mobile.pdf
/html/[012]*.html
-/html/js/chapter_info.js
+/html/ejs.js
/code/chapter/*
-/code/file_server.js
+/code/chapter_info.js
+/code/file_server.mjs
/code/skillsharing.zip
-/code/solutions/20_4_a_public_space_on_the_web.zip
+/code/solutions/20_3_a_public_space_on_the_web.zip
/code/skillsharing/*
/node_modules
.tern-port
/toc.txt
-/img/generated/*
/img/cover.xcf
-/book.pdf
+/img/generated/*
+/epub/[012]*.xhtml
+/epub/hints.xhtml
+/epub/img/*
+/epub/content.opf
+/epub/toc.xhtml
+/book.epub
+/book.mobi
diff --git a/00_intro.md b/00_intro.md
new file mode 100644
index 000000000..1d14eef57
--- /dev/null
+++ b/00_intro.md
@@ -0,0 +1,277 @@
+{{meta {load_files: ["code/intro.js"]}}}
+
+# Introduction
+
+{{quote {author: "Ellen Ullman", title: "Close to the Machine: Technophilia and Its Discontents", chapter: true}
+
+We think we are creating the system for our own purposes. We believe we are making it in our own image... But the computer is not really like us. It is a projection of a very slim part of ourselves: that portion devoted to logic, order, rule, and clarity.
+
+quote}}
+
+{{figure {url: "img/chapter_picture_00.jpg", alt: "Illustration of a screwdriver next to a circuit board of about the same size", chapter: "framed"}}}
+
+This is a book about instructing ((computer))s. Computers are about as common as screwdrivers today, but they are quite a bit more complex, and making them do what you want them to do isn't always easy.
+
+If the task you have for your computer is a common, well-understood one, such as showing you your email or acting like a calculator, you can open the appropriate ((application)) and get to work. But for unique or open-ended tasks, there often is no appropriate application.
+
+That is where ((programming)) may come in. _Programming_ is the act of constructing a _program_—a set of precise instructions telling a computer what to do. Because computers are dumb, pedantic beasts, programming is fundamentally tedious and frustrating.
+
+{{index [programming, "joy of"], speed}}
+
+Fortunately, if you can get over that fact—and maybe even enjoy the rigor of thinking in terms that dumb machines can deal with—programming can be rewarding. It allows you to do things in seconds that would take _forever_ by hand. It is a way to make your computer tool do things that it couldn't do before. On top of that, it makes for a wonderful game of puzzle solving and abstract thinking.
+
+Most programming is done with ((programming language))s. A _programming language_ is an artificially constructed language used to instruct computers. It is interesting that the most effective way we've found to communicate with a computer borrows so heavily from the way we communicate with each other. Like human languages, computer languages allow words and phrases to be combined in new ways, making it possible to express ever new concepts.
+
+{{index [JavaScript, "availability of"], "casual computing"}}
+
+At one point, language-based interfaces, such as the BASIC and DOS prompts of the 1980s and 1990s, were the main method of interacting with computers. For routine computer use, these have largely been replaced with visual interfaces, which are easier to learn but offer less freedom. But if you know where to look, the languages are still there. One of them, _JavaScript_, is built into every modern web ((browser))—and is thus available on almost every device.
+
+{{indexsee "web browser", browser}}
+
+This book will try to make you familiar enough with this language to do useful and amusing things with it.
+
+## On programming
+
+{{index [programming, "difficulty of"]}}
+
+Besides explaining JavaScript, I will introduce the basic principles of programming. Programming, it turns out, is hard. The fundamental rules are simple and clear, but programs built on top of these rules tend to become complex enough to introduce their own rules and complexity. You're building your own maze, in a way, and you can easily get lost in it.
+
+{{index learning}}
+
+There will be times when reading this book feels terribly frustrating. If you are new to programming, there will be a lot of new material to digest. Much of this material will then be _combined_ in ways that require you to make additional connections.
+
+It is up to you to make the necessary effort. When you are struggling to follow the book, do not jump to any conclusions about your own capabilities. You are fine—you just need to keep at it. Take a break, reread some material, and make sure you read and understand the example programs and ((exercises)). Learning is hard work, but everything you learn is yours and will make further learning easier.
+
+{{quote {author: "Ursula K. Le Guin", title: "The Left Hand of Darkness"}
+
+{{index "Le Guin, Ursula K."}}
+
+When action grows unprofitable, gather information; when information grows unprofitable, sleep.
+
+quote}}
+
+{{index [program, "nature of"], data}}
+
+A program is many things. It is a piece of text typed by a programmer, it is the directing force that makes the computer do what it does, it is data in the computer's memory, and, at the same time, it controls the actions performed on this memory. Analogies that try to compare programs to familiar objects tend to fall short. A superficially fitting one is to compare a program to a machine—lots of separate parts tend to be involved, and to make the whole thing tick, we have to consider the ways in which these parts interconnect and contribute to the operation of the whole.
+
+A ((computer)) is a physical machine that acts as a host for these immaterial machines. Computers themselves can do only stupidly straightforward things. The reason they are so useful is that they do these things at an incredibly high ((speed)). A program can ingeniously combine an enormous number of these simple actions to do very complicated things.
+
+{{index [programming, "joy of"]}}
+
+A program is a building of thought. It is costless to build, it is weightless, and it grows easily under our typing hands. But as a program grows, so does its ((complexity)). The skill of programming is the skill of building programs that don't confuse the programmer. The best programs are those that manage to do something interesting while still being easy to understand.
+
+{{index "programming style", "best practices"}}
+
+Some programmers believe that this complexity is best managed by using only a small set of well-understood techniques in their programs. They have composed strict rules ("best practices") prescribing the form programs should have and carefully stay within their safe little zone.
+
+{{index experiment}}
+
+This is not only boring—it is ineffective. New problems often require new solutions. The field of programming is young and still developing rapidly, and it is varied enough to have room for wildly different approaches. There are many terrible mistakes to make in program design, and you should go ahead and make them at least once so that you understand them. A sense of what a good program looks like is developed with practice, not learned from a list of rules.
+
+## Why language matters
+
+{{index "programming language", "machine code", "binary data"}}
+
+In the beginning, at the birth of computing, there were no programming languages. Programs looked something like this:
+
+```{lang: null}
+00110001 00000000 00000000
+00110001 00000001 00000001
+00110011 00000001 00000010
+01010001 00001011 00000010
+00100010 00000010 00001000
+01000011 00000001 00000000
+01000001 00000001 00000001
+00010000 00000010 00000000
+01100010 00000000 00000000
+```
+
+{{index [programming, "history of"], "punch card", complexity}}
+
+This is a program to add the numbers from 1 to 10 together and print the result: `1 + 2 + ... + 10 = 55`. It could run on a simple hypothetical machine. To program early computers, it was necessary to set large arrays of switches in the right position or punch holes in strips of cardboard and feed them to the computer. You can imagine how tedious and error prone this procedure was. Even writing simple programs required much cleverness and discipline. Complex ones were nearly inconceivable.
+
+{{index bit, "wizard (mighty)"}}
+
+Of course, manually entering these arcane patterns of bits (the ones and zeros) did give the programmer a profound sense of being a mighty wizard. And that has to be worth something in terms of job satisfaction.
+
+{{index memory, instruction}}
+
+Each line of the previous program contains a single instruction. It could be written in English like this:
+
+ 1. Store the number 0 in memory location 0.
+ 2. Store the number 1 in memory location 1.
+ 3. Store the value of memory location 1 in memory location 2.
+ 4. Subtract the number 11 from the value in memory location 2.
+ 5. If the value in memory location 2 is the number 0, continue with instruction 9.
+ 6. Add the value of memory location 1 to memory location 0.
+ 7. Add the number 1 to the value of memory location 1.
+ 8. Continue with instruction 3.
+ 9. Output the value of memory location 0.
+
+{{index readability, naming, binding}}
+
+Although that is already more readable than the soup of bits, it is still rather obscure. Using names instead of numbers for the instructions and memory locations helps.
+
+```{lang: "null"}
+ Set “total” to 0.
+ Set “count” to 1.
+[loop]
+ Set “compare” to “count”.
+ Subtract 11 from “compare”.
+ If “compare” is 0, continue at [end].
+ Add “count” to “total”.
+ Add 1 to “count”.
+ Continue at [loop].
+[end]
+ Output “total”.
+```
+
+{{index loop, jump, "summing example"}}
+
+Can you see how the program works at this point? The first two lines give two memory locations their starting values: `total` will be used to build up the result of the computation, and `count` will keep track of the number that we are currently looking at. The lines using `compare` are probably the most confusing ones. The program wants to see whether `count` is equal to 11 to decide whether it can stop running. Because our hypothetical machine is rather primitive, it can test only whether a number is zero and make a decision based on that. It therefore uses the memory location labeled `compare` to compute the value of `count - 11` and makes a decision based on that value. The next two lines add the value of `count` to the result and increment `count` by 1 every time the program decides that `count` is not 11 yet.
+
+Here is the same program in JavaScript:
+
+```
+let total = 0, count = 1;
+while (count <= 10) {
+ total += count;
+ count += 1;
+}
+console.log(total);
+// → 55
+```
+
+{{index "while loop", loop, [braces, block]}}
+
+This version gives us a few more improvements. Most importantly, there is no need to specify the way we want the program to jump back and forth anymore—the `while` construct takes care of that. It continues executing the block (wrapped in braces) below it as long as the condition it was given holds. That condition is `count <= 10`, which means “the count is less than or equal to 10”. We no longer have to create a temporary value and compare that to zero, which was just an uninteresting detail. Part of the power of programming languages is that they can take care of uninteresting details for us.
+
+{{index "console.log"}}
+
+At the end of the program, after the `while` construct has finished, the `console.log` operation is used to write out the result.
+
+{{index "sum function", "range function", abstraction, function}}
+
+Finally, here is what the program could look like if we happened to have the convenient operations `range` and `sum` available, which respectively create a ((collection)) of numbers within a range and compute the sum of a collection of numbers:
+
+```{startCode: true}
+console.log(sum(range(1, 10)));
+// → 55
+```
+
+{{index readability}}
+
+The moral of this story is that the same program can be expressed in both long and short, unreadable and readable ways. The first version of the program was extremely obscure, whereas this last one is almost English: `log` the `sum` of the `range` of numbers from 1 to 10. (We will see in [later chapters](data) how to define operations like `sum` and `range`.)
+
+{{index ["programming language", "power of"], composability}}
+
+A good programming language helps the programmer by allowing them to talk about the actions that the computer has to perform on a higher level. It helps omit details, provides convenient building blocks (such as `while` and `console.log`), allows you to define your own building blocks (such as `sum` and `range`), and makes those blocks easy to compose.
+
+## What is JavaScript?
+
+{{index history, Netscape, browser, "web application", JavaScript, [JavaScript, "history of"], "World Wide Web"}}
+
+{{indexsee WWW, "World Wide Web"}}
+
+{{indexsee Web, "World Wide Web"}}
+
+JavaScript was introduced in 1995 as a way to add programs to web pages in the Netscape Navigator browser. The language has since been adopted by all other major graphical web browsers. It has made modern web applications possible—that is, applications with which you can interact directly without doing a page reload for every action. JavaScript is also used in more traditional websites to provide various forms of interactivity and cleverness.
+
+{{index Java, naming}}
+
+It is important to note that JavaScript has almost nothing to do with the programming language named Java. The similar name was inspired by marketing considerations rather than good judgment. When JavaScript was being introduced, the Java language was being heavily marketed and was gaining popularity. Someone thought it was a good idea to try to ride along on this success. Now we are stuck with the name.
+
+{{index ECMAScript, compatibility}}
+
+After its adoption outside of Netscape, a ((standard)) document was written to describe the way the JavaScript language should work so that the various pieces of software that claimed to support JavaScript could make sure they actually provided the same language. This is called the ECMAScript standard, after the Ecma International organization that conducted the standardization. In practice, the terms ECMAScript and JavaScript can be used interchangeably—they are two names for the same language.
+
+{{index [JavaScript, "weaknesses of"], debugging}}
+
+There are those who will say _terrible_ things about JavaScript. Many of these things are true. When I was required to write something in JavaScript for the first time, I quickly came to despise it. It would accept almost anything I typed but interpret it in a way that was completely different from what I meant. This had a lot to do with the fact that I did not have a clue what I was doing, of course, but there is a real issue here: JavaScript is ridiculously liberal in what it allows. The idea behind this design was that it would make programming in JavaScript easier for beginners. In actuality, it mostly makes finding problems in your programs harder because the system will not point them out to you.
+
+{{index [JavaScript, "flexibility of"], flexibility}}
+
+This flexibility also has its advantages, though. It leaves room for techniques that are impossible in more rigid languages and makes for a pleasant, informal style of programming. After ((learning)) the language properly and working with it for a while, I have come to actually _like_ JavaScript.
+
+{{index future, [JavaScript, "versions of"], ECMAScript, "ECMAScript 6"}}
+
+There have been several versions of JavaScript. ECMAScript version 3 was the widely supported version during JavaScript's ascent to dominance, roughly between 2000 and 2010. During this time, work was underway on an ambitious version 4, which planned a number of radical improvements and extensions to the language. Changing a living, widely used language in such a radical way turned out to be politically difficult, and work on version 4 was abandoned in 2008. A much less ambitious version 5, which made only some uncontroversial improvements, came out in 2009. In 2015, version 6 came out, a major update that included some of the ideas planned for version 4. Since then we've had new, small updates every year.
+
+The fact that JavaScript is evolving means that browsers have to constantly keep up. If you're using an older browser, it may not support every feature. The language designers are careful to not make any changes that could break existing programs, so new browsers can still run old programs. In this book, I'm using the 2024 version of JavaScript.
+
+{{index [JavaScript, "uses of"]}}
+
+Web browsers are not the only platforms on which JavaScript is used. Some databases, such as MongoDB and CouchDB, use JavaScript as their scripting and query language. Several platforms for desktop and server programming, most notably the ((Node.js)) project (the subject of [Chapter ?](node)), provide an environment for programming JavaScript outside of the browser.
+
+## Code, and what to do with it
+
+{{index "reading code", "writing code"}}
+
+_Code_ is the text that makes up programs. Most chapters in this book contain quite a lot of code. I believe reading code and writing ((code)) are indispensable parts of ((learning)) to program. Try to not just glance over the examples—read them attentively and understand them. This may be slow and confusing at first, but I promise that you'll quickly get the hang of it. The same goes for the ((exercises)). Don't assume you understand them until you've actually written a working solution.
+
+{{index interpretation}}
+
+I recommend you try your solutions to exercises in an actual JavaScript interpreter. That way, you'll get immediate feedback on whether what you are doing is working, and, I hope, you'll be tempted to ((experiment)) and go beyond the exercises.
+
+{{if interactive
+
+When reading this book in your browser, you can edit (and run) all example programs by clicking them.
+
+if}}
+
+{{if book
+
+{{index download, sandbox, "running code"}}
+
+The easiest way to run the example code in the book—and to experiment with it—is to look it up in the online version of the book at [_https://eloquentjavascript.net_](https://eloquentjavascript.net/). There, you can click any code example to edit and run it and to see the output it produces. To work on the exercises, go to [_https://eloquentjavascript.net/code_](https://eloquentjavascript.net/code), which provides starting code for each coding exercise and allows you to look at the solutions.
+
+if}}
+
+{{index "developer tools", "JavaScript console"}}
+
+Running the programs defined in this book outside of the book's website requires some care. Many examples stand on their own and should work in any JavaScript environment. But code in later chapters is often written for a specific environment (the browser or Node.js) and can run only there. In addition, many chapters define bigger programs, and the pieces of code that appear in them depend on each other or on external files. The [sandbox](https://eloquentjavascript.net/code) on the website provides links to ZIP files containing all the scripts and data files necessary to run the code for a given chapter.
+
+## Overview of this book
+
+This book contains roughly three parts. The first 12 chapters discuss the JavaScript language. The next seven chapters are about web ((browsers)) and the way JavaScript is used to program them. Finally, two chapters are devoted to ((Node.js)), another environment to program JavaScript in. There are five _project chapters_ in the book that describe larger example programs to give you a taste of actual programming.
+
+The language part of the book starts with four chapters that introduce the basic structure of the JavaScript language. They discuss [control structures](program_structure) (such as the `while` word you saw in this introduction), [functions](functions) (writing your own building blocks), and [data structures](data). After these, you will be able to write basic programs. Next, Chapters [?](higher_order) and [?](object) introduce techniques to use functions and objects to write more _abstract_ code and keep complexity under control.
+
+After a [first project chapter](robot) that builds a crude delivery robot, the language part of the book continues with chapters on [error handling and bug fixing](error), [regular expressions](regexp) (an important tool for working with text), [modularity](modules) (another defense against complexity), and [asynchronous programming](async) (dealing with events that take time). The [second project chapter](language), where we implement a programming language, concludes the first part of the book.
+
+The second part of the book, Chapters [?](browser) to [?](paint), describes the tools that browser JavaScript has access to. You'll learn to display things on the screen (Chapters [?](dom) and [?](canvas)), respond to user input ([Chapter ?](event)), and communicate over the network ([Chapter ?](http)). There are again two project chapters in this part: building a [platform game](game) and a [pixel paint program](paint).
+
+[Chapter ?](node) describes Node.js, and [Chapter ?](skillsharing) builds a small website using that tool.
+
+{{if commercial
+
+Finally, [Chapter ?](fast) describes some of the considerations that come up when optimizing JavaScript programs for speed.
+
+if}}
+
+## Typographic conventions
+
+{{index "factorial function"}}
+
+In this book, text written in a `monospaced` font will represent elements of programs. Sometimes these are self-sufficient fragments, and sometimes they just refer to part of a nearby program. Programs (of which you have already seen a few) are written as follows:
+
+```
+function factorial(n) {
+ if (n == 0) {
+ return 1;
+ } else {
+ return factorial(n - 1) * n;
+ }
+}
+```
+
+{{index "console.log"}}
+
+Sometimes, to show the output that a program produces, the expected output is written after it, with two slashes and an arrow in front.
+
+```
+console.log(factorial(8));
+// → 40320
+```
+
+Good luck!
diff --git a/00_intro.txt b/00_intro.txt
deleted file mode 100644
index 3954796a5..000000000
--- a/00_intro.txt
+++ /dev/null
@@ -1,420 +0,0 @@
-:next_link: 01_values
-:load_files: ["js/intro.js"]
-
-= Introduction =
-
-This is a book about getting ((computer))s to do what you want them to
-do. Computers are about as common as screwdrivers today, but contain a
-lot more hidden complexity, and thus are harder to operate and
-understand. To many, they remain alien, slightly threatening things.
-
-image::img/generated/computer.png[alt="Communicating with a computer"]
-
-(((graphical user interface)))We've found two effective ways of
-bridging the communication gap between us, squishy biological
-organisms with a talent for social and spatial reasoning, and the
-computer, unfeeling manipulator of meaningless data. The first is to
-appeal to our sense of the physical world, and build interfaces that
-mimic that world, and allow us to manipulate shapes on a screen with
-our fingers. This works very well for casual machine interaction.
-
-(((programming language)))But we have not yet found a good way to use
-the point-and-click approach to communicate things to the computer
-that the designer of the interface did not anticipate. For open-ended
-interfaces, such as instructing the computer to perform arbitrary
-tasks, we've had more luck with an approach that makes use of our
-talent for language: teaching the machine a language.
-
-(((human language)))(((expressivity)))Human languages allow words and
-subsentences to be combined in many, many ways, allowing us to say
-many, many different things. Computer languages, though typically less
-grammatically flexible, follow a similar principle.
-
-(((JavaScript,availability of)))(((casual computing)))Casual computing
-has become much more widespread in the past twenty years, and
-language-based interfaces, which once were the default way in which
-people interacted with computers, have largely been replaced with
-graphical interfaces. But they are still there, if you know where to
-look. One such language, _JavaScript_, is built into just about every
-web ((browser)), and thus available on just about every consumer
-device.
-
-indexsee:[web browser,browser]
-
-This book intends to make you familiar enough with this language to be
-able to make a computer do what you want.
-
-== On programming ==
-
-[quote, Confucius]
-____
-(((Confucius)))I do not enlighten those who are not eager to learn,
-nor arouse those who are not anxious to give an explanation
-themselves. If I have presented one corner of the square and they
-cannot come back to me with the other three, I should not go over the
-points again.
-____
-
-(((programming,difficulty of)))Besides explaining JavaScript, I also
-want to introduce the basic principles of programming. Programming, it
-turns out, is hard. The fundamental rules are typically simple and
-clear. But programs, built on top of these basic rules, tend to become
-complex enough to introduce their own rules and complexity. You're
-building your own maze, in a way, and might just get lost in it.
-
-(((learning)))There will be times at which reading this book feels terribly
-frustrating. If you are new to programming, there will be a lot of new
-material to digest. Much of this material will then be _combined_ in
-ways that require you to make additional connections.
-
-It is up to you to make the effort necessary. When you are struggling
-to follow the book, do not jump to any conclusions about your own
-capabilities. You are fine—you just need to keep at it. Take a break,
-re-read some material, and _always_ make sure you read and understand
-the example programs and ((exercises)). Learning is hard work, but
-everything you learn is yours, and will make subsequent learning
-easier.
-
-[quote, Joseph Weizenbaum, Computer Power and Human Reason]
-____
-(((Weizenbaum+++,+++ Joseph)))The computer programmer is a creator of
-universes for which he [sic] alone is responsible. Universes of virtually
-unlimited complexity can be created in the form of computer programs.
-____
-
-(((program,nature of)))(((data)))A program is many things. It is a
-piece of text typed by a programmer, it is the directing force that
-makes the computer do what it does, it is data in the computer's
-memory, yet it controls the actions performed on this same memory.
-Analogies that try to compare programs to objects we are familiar with
-tend to fall short. A superficially fitting one is that of a
-machine—lots of separate parts tend to be involved, and to make the
-whole thing tick we have to consider the ways in which these parts
-interconnect and contribute to the operation of the whole.
-
-(((computer)))A computer is a machine built to act as a host for these
-immaterial machines. Computers themselves can only do stupidly
-straightforward things. The reason they are so useful is that they do
-these things at an incredibly high speed. A program can ingeniously
-combine enormous numbers of these simple actions in order to do very
-complicated things.
-
-(((programming,joy of)))To some of us, writing computer programs is a
-fascinating game. A program is a building of thought. It is costless
-to build, it is weightless, and it grows easily under our typing
-hands.
-
-If we are not careful, its size and ((complexity)) will grow out of
-control, confusing even the person who created it. This is the main
-problem of programming: keeping programs under control. When a program
-works, it is beautiful. The art of programming is the skill of
-controlling complexity. The great program is subdued, made simple in
-its complexity.
-
-(((programming style)))(((best practices)))Many programmers believe
-that this complexity is best managed by using only a small set of
-well-understood techniques in their programs. They have composed
-strict rules (_“best practices”_) prescribing the form programs should
-have, and the more zealous among them will consider those that go
-outside of this little safe zone to be _bad_ programmers.
-
-(((experiment)))(((learning)))What hostility to the richness of
-programming—to try to reduce it to something straightforward and
-predictable, to place a taboo on all the weird and beautiful programs!
-The landscape of programming techniques is enormous, fascinating in
-its diversity, and still largely unexplored. It is certainly dangerous
-going, luring the inexperienced programmer into all kinds of
-confusion, but that only means you should proceed with caution and
-keep your wits about you. As you learn, there will always be new
-challenges and new territory to explore. Programmers who refuse to
-keep exploring will stagnate, forget their joy, and get bored with
-their craft.
-
-== Why language matters ==
-
-(((programming language)))(((machine code)))(((binary data)))In the
-beginning, at the birth of computing, there were no programming
-languages. Programs looked something like this:
-
-----
-00110001 00000000 00000000
-00110001 00000001 00000001
-00110011 00000001 00000010
-01010001 00001011 00000010
-00100010 00000010 00001000
-01000011 00000001 00000000
-01000001 00000001 00000001
-00010000 00000010 00000000
-01100010 00000000 00000000
-----
-
-(((programming,history of)))(((punch card)))(((complexity)))That is a
-program to add the numbers from 1 to 10 together and print out the
-result `(1 + 2 + ... + 10 = 55)`. It could run on a very simple,
-hypothetical machine. To program early computers, it was necessary to
-set large arrays of switches in the right position, or punch holes in
-strips of cardboard and feed them to the computer. You can imagine how
-this was a tedious, error-prone procedure. Even the writing of simple
-programs required much cleverness and discipline. Complex ones were
-nearly inconceivable.
-
-(((bit)))(((wizard (mighty))))Of course, manually entering these
-arcane patterns of bits (the ones and zeros) did give the programmer
-a profound sense of being a mighty wizard. And that has to be worth
-something in terms of job satisfaction.
-
-(((memory)))(((instruction)))Each line of the program above contains a
-single instruction. It could be written in English like this:
-
-[source,text/plain]
-----
-1. Store the number 0 in memory location 0.
-2. Store the number 1 in memory location 1.
-3. Store the value of memory location 1 in memory location 2.
-4. Subtract the number 11 from the value in memory location 2.
-5. If the value in memory location 2 is the number 0,
- continue with instruction 9.
-6. Add the value of memory location 1 to memory location 0.
-7. Add the number 1 to the value of memory location 1.
-8. Continue with instruction 3.
-9. Output the value of memory location 0.
-----
-
-(((readability)))(((naming)))(((variable)))Although that is already
-more readable than the soup of bits, it is still rather unpleasant. It
-might help to use names instead of numbers for the instructions and
-memory locations:
-
-[source,text/plain]
-----
- Set “total” to 0
- Set “count” to 1
-[loop]
- Set “compare” to “count”
- Subtract 11 from “compare”
- If “compare” is zero, continue at [end]
- Add “count” to “total”
- Add 1 to “count”
- Continue at [loop]
-[end]
- Output “total”
-----
-
-(((loop)))(((jump)))(((summing example)))At this point it is not too
-hard to see how the program works. Can you? The first two lines give
-two memory locations their starting values: `total` will be used to
-build up the result of the computation, and `count` keeps track of the
-number that we are currently looking at. The lines using `compare` are
-probably the weirdest ones. What the program wants to do is see
-whether `count` is equal to 11 in order to decide whether it can stop
-yet. Because our hypothetical machine is rather primitive, it can only
-test whether a number is zero and make a decision (jump) based on
-that. So, it uses the memory location labeled `compare` to compute the
-value of `count - 11` and makes a decision based on that value. The
-next two lines add the value of `count` to the result and increment
-`count` by 1 every time the program has decided that it is not 11 yet.
-
-Here is the same program in JavaScript:
-
-[source,javascript]
-----
-var total = 0, count = 1;
-while (count <= 10) {
- total += count;
- count += 1;
-}
-console.log(total);
-// → 55
-----
-
-(((while loop)))(((loop)))This gives us a few more improvements.
-Most importantly, there is no need to specify the way we want the
-program to jump back and forth any more. The `while` language
-construct takes care of that. It continues executing the block
-(wrapped in braces) below it as long as the condition it was given
-holds: `count <= 10`, which means “`count` is less than or equal to
-10”. We no longer have to create a temporary value and compare that
-to zero. This was an uninteresting detail, and the power of
-programming languages is that they take care of uninteresting details
-for us.
-
-(((console.log)))At the end of the program, after the `while` has
-finished, the `console.log` operation is applied to the result in
-order to write it as output.
-
-(((sum function)))(((range
-function)))(((abstraction)))(((function)))Finally, here is what the
-program could look like if we happened to have the convenient
-operations `range` and `sum` available, which respectively create a
-((collection)) of numbers within a range and compute the sum of a
-collection of numbers:
-
-[source,javascript]
-----
-console.log(sum(range(1, 10)));
-// → 55
-----
-
-(((readability)))The moral of this story is that the same program can
-be expressed in long and short, unreadable and readable ways. The
-first version of the program was extremely obscure, whereas this last
-one is almost English: `log` the `sum` of the `range` of numbers from
-1 to 10. (We will see in link:04_data.html#data[later chapters] how to
-build things like `sum` and `range`.)
-
-(((programming language,power of)))(((composability)))A good
-programming language helps the programmer by allowing them to talk
-about the actions that the computer has to perform on a higher level.
-It helps omit uninteresting details, provides convenient building
-blocks (such as `while` and `console.log`), allows you to define your
-own building blocks (such as `sum` and `range`), and makes it easy to
-compose these blocks.
-
-== What is JavaScript? ==
-
-indexsee:[WWW,World Wide Web] indexsee:[Web,World Wide Web]
-
-(((history)))(((Netscape)))(((browser)))(((web
-application)))(((JavaScript)))(((JavaScript,history of)))(((World Wide
-Web))) JavaScript was introduced in 1995, as a way to add programs to
-Web pages in the Netscape Navigator browser. The language has since
-been adapted by all other major graphical web browsers. It has made
-the current generation of web applications possible—browser-based
-email clients, maps, and social networks—and is also used in more
-traditional sites to provide various forms of interactivity and
-cleverness.
-
-(((Java)))(((naming)))It is important to note that JavaScript has
-almost nothing to do with the programming language named Java. The
-similar name was inspired by marketing considerations, rather than
-good judgment. When JavaScript was being introduced, the Java language
-was being heavily marketed and was gaining in popularity. Someone
-thought it a good idea to try to ride along on this success. Now we
-are stuck with the name.
-
-(((ECMAScript)))(((compatibility)))After its adoption outside of
-Netscape, a ((standard)) document was written to describe the way the
-JavaScript language should work, to make sure the various pieces of
-software that claimed to support JavaScript were actually talking
-about the same language. This is called the ECMAScript standard, after
-the ECMA organization, which did the standardization. In practice, the
-terms ECMAScript and JavaScript can be used interchangeably—they are
-two names for the same language.
-
-(((JavaScript,weaknesses of)))(((debugging)))There are those who will
-say _terrible_ things about the JavaScript language. Many of these
-things are true. When I was required to write something in JavaScript
-for the first time, I quickly came to despise it—it would accept
-almost anything I typed but interpret it in a way that was completely
-different from what I meant. This had a lot to do with the fact that I
-did not have a clue what I was doing, of course, but there is a real
-issue here: JavaScript is ridiculously liberal in what it allows. The
-idea behind this design was that it would make programming in
-JavaScript easier for beginners. In actuality, it mostly makes finding
-problems in your programs harder, because the system will not point
-them out to you.
-
-(((JavaScript,flexibility of)))(((flexibility)))This flexibility also
-has its advantages, though. It leaves space for a lot of techniques
-that are impossible in more rigid languages, and, as we will see, for
-example, in the link:10_modules.html#modules[chapter on modules], it
-can be used to overcome some of JavaScript's shortcomings. After
-((learning)) the language properly and working with it for a while, I have
-learned to actually _like_ JavaScript.
-
-(((future)))(((JavaScript,versions of)))(((ECMAScript)))(((ECMAScript
-6)))There have been several _versions_ of JavaScript. ECMAScript
-version 3 was the dominant, widely supported version in the time of
-JavaScript's ascent to dominance, roughly between 2000 and 2010.
-During this time, work was underway on an ambitious version 4, which
-planned a number of radical improvements and extensions to the
-language. Changing a living, widely used language in such a radical
-way turned out to be politically difficult, and work on the 4th
-edition was abandoned in 2008, leading to the much less ambitious 5th
-edition coming out in 2009. We're now at the point where all major
-browsers support this 5th edition, which is the language version that
-this book will be focusing on. A 6th edition is in the process of
-being finalized, and some browsers are starting to support new
-features from this edition.
-
-(((JavaScript,uses of)))Web browsers are not the only platforms on
-which JavaScript is used. Some databases, such as MongoDB and CouchDB,
-use JavaScript as their scripting and query language. Several
-platforms for desktop and server programming, most notably the
-_((Node.js))_ project, the subject of link:20_node.html#node[Chapter
-20], are providing a powerful environment for programming JavaScript
-outside of the browser.
-
-== Code, and what to do with it ==
-
-(((reading code)))(((writing code)))Code is the text that makes up
-programs. Most chapters in this book contain quite a lot of it. In my
-experience, reading and writing ((code)) is an indispensable part of
-((learning)) to program. Try to not just glance over the examples, read
-them attentively and understand them. This will be slow and confusing
-at first, but I promise that you will quickly get the hang of it. The
-same goes for the ((exercises)). Don't assume you understand them
-until you've actually written a working solution.
-
-(((interpretation)))I recommend to try out your solutions to exercises
-in an actual JavaScript interpreter, to get immediate feedback on
-whether what you are doing is working or not, and, hopefully, to be
-tempted to ((experiment)) and go beyond the exercises.
-
-ifdef::html_target[]
-
-When reading this book in your browser, you can edit (and run) all
-example programs by clicking on them.
-
-endif::html_target[]
-
-ifdef::tex_target[]
-
-(((download)))(((sandbox)))(((running code)))Most of the example code
-in this book can be found on the book's ((website)), at
-http://eloquentjavascript.net/code[_eloquentjavascript.net/code_],
-which also provides an easy way to run the programs and experiment
-with writing your own code.
-
-endif::tex_target[]
-
-(((developer tools)))(((JavaScript console)))Running JavaScript
-programs outside of this book's sandbox is also possible. You can opt
-to install Node.js, and use it run text files that contain programs.
-Or you can use your browser's developer console (typically found
-somewhere under a “tools” or “developer” menu) and play around in
-there. In link:12_browser.html#script_tag[Chapter 12], the way in
-which JavaScript programs are embedded in web pages (HTML files) is
-explained. There are also websites like http://jsbin.com[_jsbin.com_]
-which provide a friendly interface for running JavaScript code in a
-browser.
-
-== Typographic conventions ==
-
-(((factorial function)))In this book, text written in a `monospaced`
-font should be understood to represent elements of programs—sometimes
-they are self-sufficient fragments, and sometimes they just refer to
-part of a nearby program. Programs (of which you have already seen a
-few), are written as follows:
-
-[source,javascript]
-----
-function fac(n) {
- if (n == 0)
- return 1;
- else
- return fac(n - 1) * n;
-}
-----
-
-(((console.log)))Sometimes, in order to show the output that a program
-produces, the expected output is written below it, with two slashes
-and an arrow in front:
-
-[source,javascript]
-----
-console.log(fac(8));
-// → 40320
-----
-
-Good luck!
diff --git a/01_values.md b/01_values.md
new file mode 100644
index 000000000..bbf9de95a
--- /dev/null
+++ b/01_values.md
@@ -0,0 +1,452 @@
+{{meta {docid: values}}}
+
+# Values, Types, and Operators
+
+{{quote {author: "Master Yuan-Ma", title: "The Book of Programming", chapter: true}
+
+Below the surface of the machine, the program moves. Without effort, it expands and contracts. In great harmony, electrons scatter and regroup. The forms on the monitor are but ripples on the water. The essence stays invisibly below.
+
+quote}}
+
+{{index "Yuan-Ma", "Book of Programming"}}
+
+{{figure {url: "img/chapter_picture_1.jpg", alt: "Illustration of a sea of dark and bright dots (bits) with islands in it", chapter: framed}}}
+
+{{index "binary data", data, bit, memory}}
+
+In the computer's world, there is only data. You can read data, modify data, create new data—but that which isn't data cannot be mentioned. All this data is stored as long sequences of bits and is thus fundamentally alike.
+
+{{index CD, signal}}
+
+_Bits_ are any kind of two-valued things, usually described as zeros and ones. Inside the computer, they take forms such as a high or low electrical charge, a strong or weak signal, or a shiny or dull spot on the surface of a CD. Any piece of discrete information can be reduced to a sequence of zeros and ones and thus represented in bits.
+
+{{index "binary number", "decimal number"}}
+
+For example, we can express the number 13 in bits. This works the same way as a decimal number, but instead of 10 different ((digit))s, we have only 2, and the weight of each increases by a factor of 2 from right to left. Here are the bits that make up the number 13, with the weights of the digits shown below them:
+
+```{lang: null}
+ 0 0 0 0 1 1 0 1
+ 128 64 32 16 8 4 2 1
+```
+
+That's the binary number 00001101. Its nonzero digits stand for 8, 4, and 1, and add up to 13.
+
+## Values
+
+{{index [memory, organization], "volatile data storage", "hard drive"}}
+
+Imagine a sea of bits—an ocean of them. A typical modern computer has more than 100 billion bits in its volatile data storage (working memory). Nonvolatile storage (the hard disk or equivalent) tends to have yet a few orders of magnitude more.
+
+To be able to work with such quantities of bits without getting lost, we separate them into chunks that represent pieces of information. In a JavaScript environment, those chunks are called _((value))s_. Though all values are made of bits, they play different roles. Every value has a ((type)) that determines its role. Some values are numbers, some values are pieces of text, some values are functions, and so on.
+
+{{index "garbage collection"}}
+
+To create a value, you must merely invoke its name. This is convenient. You don't have to gather building material for your values or pay for them. You just call for one, and _whoosh_, you have it. Of course, values are not really created from thin air. Each one has to be stored somewhere, and if you want to use a gigantic number of them at the same time, you might run out of computer memory. Fortunately, this is a problem only if you need them all simultaneously. As soon as you no longer use a value, it will dissipate, leaving behind its bits to be recycled as building material for the next generation of values.
+
+The remainder of this chapter introduces the atomic elements of JavaScript programs, that is, the simple value types and the operators that can act on such values.
+
+## Numbers
+
+{{index [syntax, number], number, [number, notation]}}
+
+Values of the _number_ type are, unsurprisingly, numeric values. In a JavaScript program, they are written as follows:
+
+```
+13
+```
+
+{{index "binary number"}}
+
+Using that in a program will cause the bit pattern for the number 13 to come into existence inside the computer's memory.
+
+{{index [number, representation], bit}}
+
+JavaScript uses a fixed number of bits, 64 of them, to store a single number value. There are only so many patterns you can make with 64 bits, which limits the number of different numbers that can be represented. With _N_ decimal ((digit))s, you can represent 10^N^ numbers. Similarly, given 64 binary digits, you can represent 2^64^ different numbers, which is about 18 quintillion (an 18 with 18 zeros after it). That's a lot.
+
+Computer memory used to be much smaller, and people tended to use groups of 8 or 16 bits to represent their numbers. It was easy to accidentally _((overflow))_ such small numbers—to end up with a number that did not fit into the given number of bits. Today, even computers that fit in your pocket have plenty of memory, so you are free to use 64-bit chunks, and you need to worry about overflow only when dealing with truly astronomical numbers.
+
+{{index sign, "floating-point number", "sign bit"}}
+
+Not all whole numbers less than 18 quintillion fit in a JavaScript number, though. Those bits also store negative numbers, so one bit indicates the sign of the number. A bigger issue is representing nonwhole numbers. To do this, some of the bits are used to store the position of the decimal point. The actual maximum whole number that can be stored is more in the range of 9 quadrillion (15 zeros)—which is still pleasantly huge.
+
+{{index [number, notation], "fractional number"}}
+
+Fractional numbers are written using a dot:
+
+```
+9.81
+```
+
+{{index exponent, "scientific notation", [number, notation]}}
+
+For very big or very small numbers, you may also use scientific notation by adding an _e_ (for _exponent_), followed by the exponent of the number.
+
+```
+2.998e8
+```
+
+That's 2.998 × 10^8^ = 299,800,000.
+
+{{index pi, [number, "precision of"], "floating-point number"}}
+
+Calculations with whole numbers (also called _((integer))s_) that are smaller than the aforementioned 9 quadrillion are guaranteed to always be precise. Unfortunately, calculations with fractional numbers are generally not. Just as π (pi) cannot be precisely expressed by a finite number of decimal digits, many numbers lose some precision when only 64 bits are available to store them. This is a shame, but it causes practical problems only in specific situations. The important thing is to be aware of it and treat fractional digital numbers as approximations, not as precise values.
+
+### Arithmetic
+
+{{index [syntax, operator], operator, "binary operator", arithmetic, addition, multiplication}}
+
+The main thing to do with numbers is arithmetic. Arithmetic operations such as addition or multiplication take two number values and produce a new number from them. Here is what they look like in JavaScript:
+
+```{meta: "expr"}
+100 + 4 * 11
+```
+
+{{index [operator, application], asterisk, "plus character", "* operator", "+ operator"}}
+
+The `+` and `*` symbols are called _operators_. The first stands for addition and the second stands for multiplication. Putting an operator between two values will apply it to those values and produce a new value.
+
+{{index grouping, parentheses, precedence}}
+
+Does this example mean "Add 4 and 100, and multiply the result by 11", or is the multiplication done before the adding? As you might have guessed, the multiplication happens first. As in mathematics, you can change this by wrapping the addition in parentheses.
+
+```{meta: "expr"}
+(100 + 4) * 11
+```
+
+{{index "hyphen character", "slash character", division, subtraction, minus, "- operator", "/ operator"}}
+
+For subtraction, there is the `-` operator. Division can be done with the `/` operator.
+
+When operators appear together without parentheses, the order in which they are applied is determined by the _((precedence))_ of the operators. The example shows that multiplication comes before addition. The `/` operator has the same precedence as `*`. Likewise, `+` and `-` have the same precedence. When multiple operators with the same precedence appear next to each other, as in `1 - 2 + 1`, they are applied left to right: `(1 - 2) + 1`.
+
+Don't worry too much about these precedence rules. When in doubt, just add parentheses.
+
+{{index "modulo operator", division, "remainder operator", "% operator"}}
+
+There is one more arithmetic operator, which you might not immediately recognize. The `%` symbol is used to represent the _remainder_ operation. `X % Y` is the remainder of dividing `X` by `Y`. For example, `314 % 100` produces `14`, and `144 % 12` gives `0`. The remainder operator's precedence is the same as that of multiplication and division. You'll also often see this operator referred to as _modulo_.
+
+### Special numbers
+
+{{index [number, "special values"], infinity}}
+
+There are three special values in JavaScript that are considered numbers but don't behave like normal numbers. The first two are `Infinity` and `-Infinity`, which represent the positive and negative infinities. `Infinity - 1` is still `Infinity`, and so on. Don't put too much trust in infinity-based computation, though. It isn't mathematically sound, and it will quickly lead to the next special number: `NaN`.
+
+{{index NaN, "not a number", "division by zero"}}
+
+`NaN` stands for "not a number", even though it _is_ a value of the number type. You'll get this result when you, for example, try to calculate `0 / 0` (zero divided by zero), `Infinity - Infinity`, or any number of other numeric operations that don't yield a meaningful result.
+
+## Strings
+
+{{indexsee "grave accent", backtick}}
+
+{{index [syntax, string], text, character, [string, notation], "single-quote character", "double-quote character", "quotation mark", backtick}}
+
+The next basic data type is the _((string))_. Strings are used to represent text. They are written by enclosing their content in quotes.
+
+```
+`Down on the sea`
+"Lie on the ocean"
+'Float on the ocean'
+```
+
+You can use single quotes, double quotes, or backticks to mark strings, as long as the quotes at the start and the end of the string match.
+
+{{index "line break", "newline character"}}
+
+You can put almost anything between quotes to have JavaScript make a string value out of it. But a few characters are more difficult. You can imagine how putting quotes between quotes might be hard, since they will look like the end of the string. _Newlines_ (the characters you get when you press [enter]{keyname}) can be included only when the string is quoted with backticks (`` ` ``).
+
+{{index [escaping, "in strings"], ["backslash character", "in strings"]}}
+
+To make it possible to include such characters in a string, the following notation is used: a backslash (`\`) inside quoted text indicates that the character after it has a special meaning. This is called _escaping_ the character. A quote that is preceded by a backslash will not end the string but be part of it. When an `n` character occurs after a backslash, it is interpreted as a newline. Similarly, a `t` after a backslash means a ((tab character)). Take the following string:
+
+```
+"This is the first line\nAnd this is the second"
+```
+
+This is the actual text in that string:
+
+```{lang: null}
+This is the first line
+And this is the second
+```
+
+There are, of course, situations where you want a backslash in a string to be just a backslash, not a special code. If two backslashes follow each other, they will collapse together, and only one will be left in the resulting string value. This is how the string "_A newline character is written like `"`\n`"`._" can be expressed:
+
+```
+"A newline character is written like \"\\n\"."
+```
+
+{{id unicode}}
+
+{{index [string, representation], Unicode, character}}
+
+Strings, too, have to be modeled as a series of bits to be able to exist inside the computer. The way JavaScript does this is based on the _((Unicode))_ standard. This standard assigns a number to virtually every character you would ever need, including characters from Greek, Arabic, Japanese, Armenian, and so on. If we have a number for every character, a string can be described by a sequence of numbers. And that's what JavaScript does.
+
+{{index "UTF-16", emoji}}
+
+There's a complication though: JavaScript's representation uses 16 bits per string element, which can describe up to 2^16^ different characters. However, Unicode defines more characters than that—about twice as many, at this point. So some characters, such as many emoji, take up two "character positions" in JavaScript strings. We'll come back to this in [Chapter ?](higher_order#code_units).
+
+{{index "+ operator", concatenation}}
+
+Strings cannot be divided, multiplied, or subtracted. The `+` operator _can_ be used on them, not to add, but to _concatenate_—to glue two strings together. The following line will produce the string `"concatenate"`:
+
+```{meta: "expr"}
+"con" + "cat" + "e" + "nate"
+```
+
+String values have a number of associated functions (_methods_) that can be used to perform other operations on them. I'll say more about these in [Chapter ?](data#methods).
+
+{{index interpolation, backtick}}
+
+Strings written with single or double quotes behave very much the same—the only difference lies in which type of quote you need to escape inside of them. Backtick-quoted strings, usually called _((template literals))_, can do a few more tricks. Apart from being able to span lines, they can also embed other values.
+
+```{meta: "expr"}
+`half of 100 is ${100 / 2}`
+```
+
+When you write something inside `${}` in a template literal, its result will be computed, converted to a string, and included at that position. This example produces the string `"half of 100 is 50"`.
+
+## Unary operators
+
+{{index operator, "typeof operator", type}}
+
+Not all operators are symbols. Some are written as words. One example is the `typeof` operator, which produces a string value naming the type of the value you give it.
+
+```
+console.log(typeof 4.5)
+// → number
+console.log(typeof "x")
+// → string
+```
+
+{{index "console.log", output, "JavaScript console"}}
+
+{{id "console.log"}}
+
+We will use `console.log` in example code to indicate that we want to see the result of evaluating something. (More about that in the [next chapter](program_structure).)
+
+{{index negation, "- operator", "binary operator", "unary operator"}}
+
+The other operators shown so far in this chapter all operated on two values, but `typeof` takes only one. Operators that use two values are called _binary_ operators, while those that take one are called _unary_ operators. The minus operator (`-`) can be used both as a binary operator and as a unary operator.
+
+```
+console.log(- (10 - 2))
+// → -8
+```
+
+## Boolean values
+
+{{index Boolean, operator, true, false, bit}}
+
+It is often useful to have a value that distinguishes between only two possibilities, like "yes" and "no" or "on" and "off". For this purpose, JavaScript has a _Boolean_ type, which has just two values, true and false, written as those words.
+
+### Comparison
+
+{{index comparison}}
+
+Here is one way to produce Boolean values:
+
+```
+console.log(3 > 2)
+// → true
+console.log(3 < 2)
+// → false
+```
+
+{{index [comparison, "of numbers"], "> operator", "< operator", "greater than", "less than"}}
+
+The `>` and `<` signs are the traditional symbols for "is greater than" and "is less than", respectively. They are binary operators. Applying them results in a Boolean value that indicates whether they hold true in this case.
+
+Strings can be compared in the same way.
+
+```
+console.log("Aardvark" < "Zoroaster")
+// → true
+```
+
+{{index [comparison, "of strings"]}}
+
+The way strings are ordered is roughly alphabetic but not really what you'd expect to see in a dictionary: uppercase letters are always "less" than lowercase ones, so `"Z" < "a"`, and nonalphabetic characters (!, -, and so on) are also included in the ordering. When comparing strings, JavaScript goes over the characters from left to right, comparing the ((Unicode)) codes one by one.
+
+{{index equality, ">= operator", "<= operator", "== operator", "!= operator"}}
+
+Other similar operators are `>=` (greater than or equal to), `<=` (less than or equal to), `==` (equal to), and `!=` (not equal to).
+
+```
+console.log("Garnet" != "Ruby")
+// → true
+console.log("Pearl" == "Amethyst")
+// → false
+```
+
+{{index [comparison, "of NaN"], NaN}}
+
+There is only one value in JavaScript that is not equal to itself, and that is `NaN` ("not a number").
+
+```
+console.log(NaN == NaN)
+// → false
+```
+
+`NaN` is supposed to denote the result of a nonsensical computation, and as such, it isn't equal to the result of any _other_ nonsensical computations.
+
+### Logical operators
+
+{{index reasoning, "logical operators"}}
+
+There are also some operations that can be applied to Boolean values themselves. JavaScript supports three logical operators: _and_, _or_, and _not_. These can be used to "reason" about Booleans.
+
+{{index "&& operator", "logical and"}}
+
+The `&&` operator represents logical _and_. It is a binary operator, and its result is true only if both the values given to it are true.
+
+```
+console.log(true && false)
+// → false
+console.log(true && true)
+// → true
+```
+
+{{index "|| operator", "logical or"}}
+
+The `||` operator denotes logical _or_. It produces true if either of the values given to it is true.
+
+```
+console.log(false || true)
+// → true
+console.log(false || false)
+// → false
+```
+
+{{index negation, "! operator"}}
+
+_Not_ is written as an exclamation mark (`!`). It is a unary operator that flips the value given to it—`!true` produces `false` and `!false` gives `true`.
+
+{{index precedence}}
+
+When mixing these Boolean operators with arithmetic and other operators, it is not always obvious when parentheses are needed. In practice, you can usually get by with knowing that of the operators we have seen so far, `||` has the lowest precedence, then comes `&&`, then the comparison operators (`>`, `==`, and so on), and then the rest. This order has been chosen such that, in typical expressions like the following one, as few parentheses as possible are necessary:
+
+```{meta: "expr"}
+1 + 1 == 2 && 10 * 10 > 50
+```
+
+{{index "conditional execution", "ternary operator", "?: operator", "conditional operator", "colon character", "question mark"}}
+
+The last logical operator we will look at is not unary, not binary, but _ternary_, operating on three values. It is written with a question mark and a colon, like this:
+
+```
+console.log(true ? 1 : 2);
+// → 1
+console.log(false ? 1 : 2);
+// → 2
+```
+
+This one is called the _conditional_ operator (or sometimes just _the ternary operator_ since it is the only such operator in the language). The operator uses the value to the left of the question mark to decide which of the two other values to "pick". If you write `a ? b : c`, the result will be `b` when `a` is true and `c` otherwise.
+
+## Empty values
+
+{{index undefined, null}}
+
+There are two special values, written `null` and `undefined`, that are used to denote the absence of a _meaningful_ value. They are themselves values, but they carry no information.
+
+Many operations in the language that don't produce a meaningful value yield `undefined` simply because they have to yield _some_ value.
+
+The difference in meaning between `undefined` and `null` is an accident of JavaScript's design, and it doesn't matter most of the time. In cases where you actually have to concern yourself with these values, I recommend treating them as mostly interchangeable.
+
+## Automatic type conversion
+
+{{index NaN, "type coercion"}}
+
+In the [introduction](intro), I mentioned that JavaScript goes out of its way to accept almost any program you give it, even programs that do odd things. This is nicely demonstrated by the following expressions:
+
+```
+console.log(8 * null)
+// → 0
+console.log("5" - 1)
+// → 4
+console.log("5" + 1)
+// → 51
+console.log("five" * 2)
+// → NaN
+console.log(false == 0)
+// → true
+```
+
+{{index "+ operator", arithmetic, "* operator", "- operator"}}
+
+When an operator is applied to the "wrong" type of value, JavaScript will quietly convert that value to the type it needs, using a set of rules that often aren't what you want or expect. This is called _((type coercion))_. The `null` in the first expression becomes `0` and the `"5"` in the second expression becomes `5` (from string to number). Yet in the third expression, `+` tries string concatenation before numeric addition, so the `1` is converted to `"1"` (from number to string).
+
+{{index "type coercion", [number, "conversion to"]}}
+
+When something that doesn't map to a number in an obvious way (such as `"five"` or `undefined`) is converted to a number, you get the value `NaN`. Further arithmetic operations on `NaN` keep producing `NaN`, so if you find yourself getting one of those in an unexpected place, look for accidental type conversions.
+
+{{index null, undefined, [comparison, "of undefined values"], "== operator"}}
+
+When comparing values of the same type using the `==` operator, the outcome is easy to predict: you should get true when both values are the same, except in the case of `NaN`. But when the types differ, JavaScript uses a complicated and confusing set of rules to determine what to do. In most cases, it just tries to convert one of the values to the other value's type. However, when `null` or `undefined` occurs on either side of the operator, it produces true only if both sides are one of `null` or `undefined`.
+
+```
+console.log(null == undefined);
+// → true
+console.log(null == 0);
+// → false
+```
+
+That behavior is often useful. When you want to test whether a value has a real value instead of `null` or `undefined`, you can compare it to `null` with the `==` or `!=` operator.
+
+{{index "type coercion", [Boolean, "conversion to"], "=== operator", "!== operator", comparison}}
+
+What if you want to test whether something refers to the precise value `false`? Expressions like `0 == false` and `"" == false` are also true because of automatic type conversion. When you do _not_ want any type conversions to happen, there are two additional operators: `===` and `!==`. The first tests whether a value is _precisely_ equal to the other, and the second tests whether it is not precisely equal. Thus `"" === false` is false, as expected.
+
+I recommend using the three-character comparison operators defensively to prevent unexpected type conversions from tripping you up. But when you're certain the types on both sides will be the same, there is no problem with using the shorter operators.
+
+### Short-circuiting of logical operators
+
+{{index "type coercion", [Boolean, "conversion to"], operator}}
+
+The logical operators `&&` and `||` handle values of different types in a peculiar way. They will convert the value on their left side to Boolean type in order to decide what to do, but depending on the operator and the result of that conversion, they will return either the _original_ left-hand value or the right-hand value.
+
+{{index "|| operator"}}
+
+The `||` operator, for example, will return the value to its left when that value can be converted to true and will return the value on its right otherwise. This has the expected effect when the values are Boolean and does something analogous for values of other types.
+
+```
+console.log(null || "user")
+// → user
+console.log("Agnes" || "user")
+// → Agnes
+```
+
+{{index "default value"}}
+
+We can use this functionality as a way to fall back on a default value. If you have a value that might be empty, you can put `||` after it with a replacement value. If the initial value can be converted to false, you'll get the replacement instead. The rules for converting strings and numbers to Boolean values state that `0`, `NaN`, and the empty string (`""`) count as false, while all the other values count as true. That means `0 || -1` produces `-1`, and `"" || "!?"` yields `"!?"`.
+
+{{index "?? operator", null, undefined}}
+
+The `??` operator resembles `||` but returns the value on the right only if the one on the left is `null` or `undefined`, not if it is some other value that can be converted to `false`. Often, this is preferable to the behavior of `||`.
+
+```
+console.log(0 || 100);
+// → 100
+console.log(0 ?? 100);
+// → 0
+console.log(null ?? 100);
+// → 100
+```
+
+{{index "&& operator"}}
+
+The `&&` operator works similarly but the other way around. When the value to its left is something that converts to false, it returns that value, and otherwise it returns the value on its right.
+
+Another important property of these two operators is that the part to their right is evaluated only when necessary. In the case of `true || X`, no matter what `X` is—even if it's a piece of program that does something _terrible_—the result will be true, and `X` is never evaluated. The same goes for `false && X`, which is false and will ignore `X`. This is called _((short-circuit evaluation))_.
+
+{{index "ternary operator", "?: operator", "conditional operator"}}
+
+The conditional operator works in a similar way. Of the second and third values, only the one that is selected is evaluated.
+
+## Summary
+
+We looked at four types of JavaScript values in this chapter: numbers, strings, Booleans, and undefined values. Such values are created by typing in their name (`true`, `null`) or value (`13`, `"abc"`).
+
+You can combine and transform values with operators. We saw binary operators for arithmetic (`+`, `-`, `*`, `/`, and `%`), string concatenation (`+`), comparison (`==`, `!=`, `===`, `!==`, `<`, `>`, `<=`, `>=`), and logic (`&&`, `||`, `??`), as well as several unary operators (`-` to negate a number, `!` to negate logically, and `typeof` to find a value's type) and a ternary operator (`?:`) to pick one of two values based on a third value.
+
+This gives you enough information to use JavaScript as a pocket calculator but not much more. The [next chapter](program_structure) will start tying these expressions together into basic programs.
diff --git a/01_values.txt b/01_values.txt
deleted file mode 100644
index 6d950b20f..000000000
--- a/01_values.txt
+++ /dev/null
@@ -1,621 +0,0 @@
-:chap_num: 1
-:prev_link: 00_intro
-:next_link: 02_program_structure
-:docid: values
-
-= Values, Types, and Operators =
-
-[chapterquote="true"]
-[quote, Master Yuan-Ma, The Book of Programming]
-____
-Below the surface of the
-machine, the program moves. Without effort, it expands and contracts.
-In great harmony, electrons scatter and regroup. The forms on the
-monitor are but ripples on the water. The essence stays invisibly
-below.
-____
-
-(((Yuan-Ma)))(((Book of Programming)))(((binary
-data)))(((data)))(((bit)))(((memory)))Inside the computer's world,
-there is only data. You can read data, modify data, create new
-data—but anything that isn't data simply does not exist. All this data
-is stored as long sequences of bits and is thus fundamentally alike.
-
-(((CD)))(((signal)))Bits are any kind of two-valued things, usually
-described as zeroes and ones. Inside the computer, they take forms
-such as a high or low electrical charge, a strong or weak signal, or a
-shiny or dull spot on the surface of a CD. Any piece of discrete
-information can be reduced to a sequence of zeros and ones and thus
-represented in bits.
-
-(((binary number)))(((radix)))(((decimal number)))For example, think
-about how you might show the number 13 in bits. It works the same way
-you write decimal numbers, but instead of 10 different ((digit))s, you
-have only 2, and the weight of each increases by a factor of 2 from
-right to left. Here are the bits that make up the number 13, with the
-weights of the digits shown after them:
-
-----
- 0 0 0 0 1 1 0 1
- 128 64 32 16 8 4 2 1
-----
-
-So that's the binary number 00001101, or 8 + 4 + 1, which equals 13.
-
-== Values ==
-
-(((memory)))(((volatile data storage)))(((hard drive)))Imagine a sea of
-bits. An ocean of them. A typical modern computer has more than 30
-billion bits in its volatile data storage. Nonvolatile storage (the
-hard disk or equivalent) tends to have yet a few orders of magnitude
-more.
-
-image::img/bit-sea.png[alt="The Ocean of Bits"]
-
-To be able to work with such quantities of bits without getting lost,
-you can separate them into chunks that represent pieces of
-information. In a JavaScript environment, those chunks are called
-_((value))s_. Though all values are made of bits, they play different
-roles. Every value has a ((type)) that determines its role. There are
-six basic types of values in JavaScript: numbers, strings, Booleans,
-objects, functions, and undefined values.
-
-(((garbage collection)))To create a value, you must merely invoke its
-name. This is convenient. You don't have to gather building material
-for your values or pay for them. You just call for one, and _woosh_,
-you have it. They are not created from thin air, of course. Every
-value has to be stored somewhere, and if you want to use a gigantic
-amount of them at the same time, you might run out of bits.
-Fortunately, this is a problem only if you need them all
-simultaneously. As soon as you no longer use a value, it will
-dissipate, leaving behind its bits to be recycled as building material
-for the next generation of values.
-
-This chapter introduces the atomic elements of JavaScript programs,
-that is, the simple value types and the operators that can act on such
-values.
-
-== Numbers ==
-
-(((syntax)))(((number)))(((number,notation)))Values of the
-_number_ type are, unsurprisingly, numeric values. In a JavaScript
-program, they are written as follows:
-
-[source,javascript]
-----
-13
-----
-
-(((binary number)))Use that in a program, and it will cause the bit
-pattern for the number 13 to come into existence inside the computer's
-memory.
-
-(((number,representation)))(((bit)))JavaScript uses a fixed
-number of bits, namely 64 of them, to store a single number value.
-There are only so many patterns you can make with 64 bits, which means
-that the amount of different numbers that can be represented is
-limited. For N decimal ((digit))s, the amount of numbers that can be
-represented is 10^N^. Similarly, given 64 binary digits, you can
-represent 2^64^ different numbers, which is about 18 quintillion (an
-18 with 18 zeroes after it). This is a lot.
-
-Computer memory used to be a lot smaller, and people tended to use
-groups of 8 or 16 bits to represent their numbers. It was easy to
-accidentally _((overflow))_ such small numbers—to end up with a number
-that did not fit into the given amount of bits. Today, even personal
-computers have plenty of memory, so you are free to use 64-bit chunks,
-which means you need to worry about overflow only when dealing with
-truly astronomical numbers.
-
-(((sign)))(((floating-point number)))(((fractional number)))(((sign bit)))Not
-all whole numbers below 18 quintillion fit in a JavaScript number,
-though. Those bits also store negative numbers, so one bit indicates
-the sign of the number. A bigger issue is that nonwhole numbers must
-also be represented. To do this, some of the bits are used to store
-the position of the decimal point. The actual maximum whole number
-that can be stored is more in the range of 9 quadrillion (15 zeroes),
-which is still pleasantly huge.
-
-(((number,notation)))Fractional numbers are written by using a
-dot.
-
-[source,javascript]
-----
-9.81
-----
-
-(((exponent)))(((scientific notation)))(((number,notation)))For
-very big or very small numbers, you can also use scientific notation
-by adding an “e” (for “exponent”), followed by the exponent of the
-number:
-
-[source,javascript]
-----
-2.998e8
-----
-
-That is 2.998 × 10^8^ = 299800000.
-
-(((pi)))(((number,precision of)))(((floating-point
-number)))Calculations with whole numbers (also called _((integer))s_)
-smaller than the aforementioned 9 quadrillion are guaranteed to always
-be precise. Unfortunately, calculations with fractional numbers are
-generally not. Just as π (pi) cannot be precisely expressed by a
-finite number of decimal digits, many numbers lose some precision when
-only 64 bits are available to store them. This is a shame, but it
-causes practical problems only in specific situations. The important
-thing is to be aware of it and treat fractional digital numbers as
-approximations, not as precise values.
-
-=== Arithmetic ===
-
-(((syntax)))(((operator)))(((binary
-operator)))(((arithmetic)))(((addition)))(((multiplication))) The main
-thing to do with numbers is arithmetic. Arithmetic operations such as
-addition or multiplication take two number values and produce a new
-number from them. Here is what they look like in JavaScript:
-
-[source,javascript]
-----
-100 + 4 * 11
-----
-
-(((operator,application)))(((asterisk)))(((plus
-character)))(((pass:[*] operator)))(((+ operator)))The `+` and `*`
-symbols are called _operators_. The first stands for addition, and the
-second stands for multiplication. Putting an operator between two
-values will apply it to those values and produce a new value.
-
-(((grouping)))(((parentheses)))(((precedence)))Does the example mean
-“add 4 and 100, and multiply the result by 11”, or is the
-multiplication done before the adding? As you might have guessed, the
-multiplication happens first. But, as in mathematics, you can change
-this by wrapping the addition in parentheses.
-
-[source,javascript]
-----
-(100 + 4) * 11
-----
-
-(((dash character)))(((slash
-character)))(((division)))(((subtraction)))(((minus)))(((-
-operator)))(((/ operator)))For subtraction, there is the `-` operator,
-and division can be done with the `/` operator.
-
-When operators appear together without parentheses, the order in which
-they are applied is determined by the _((precedence))_ of the
-operators. The example shows that multiplication comes before
-addition. The `/` operator has the same precedence as `*`. Likewise
-for `+` and `-`. When multiple operators with the same precedence
-appear next to each other (as in `1 - 2 + 1`), they are applied left
-to right (`(1 - 2) + 1`).
-
-These rules of precedence are not something you should worry about.
-When in doubt, just add parentheses.
-
-(((modulo operator)))(((division)))(((remainder operator)))(((%
-operator)))There is one more arithmetic operator, which you might not
-immediately recognize. The `%` symbol is used to represent the
-_remainder_ operation. `X % Y` is the remainder of dividing `X` by
-`Y`. For example, `314 % 100` produces `14`, and `144 % 12` gives `0`.
-Remainder's precedence is the same as that of multiplication and
-division. You'll often see this operator referred to as _modulo_,
-though technically _remainder_ is more accurate.
-
-=== Special numbers ===
-
-(((number,special values)))There are three special values in
-JavaScript that are considered numbers but don't behave like normal
-numbers.
-
-(((infinity)))The first two are `Infinity` and `-Infinity`, which
-represent the positive and negative infinities. `Infinity - 1` is
-still `Infinity`, and so on. Don't put too much trust in
-infinity-based computation. It isn't mathematically solid, and it will
-quickly lead to our next special number: `NaN`.
-
-(((NaN)))(((not a number)))(((division by zero)))`NaN` stands for “not
-a number”, even though it is a value of the number type. You'll get
-this result when you, for example, try to calculate `0 / 0` (zero
-divided by zero), `Infinity - Infinity`, or any number of other
-numeric operations that don't yield a precise, meaningful result.
-
-== Strings ==
-
-(((syntax)))(((text)))(((character)))(((string,notation)))(((single-quote
-character)))(((double-quote character)))(((quotation mark)))The next
-basic data type is the _((string))_. Strings are used to represent
-text. They are written by enclosing their content in quotes.
-
-[source,javascript]
-----
-"Patch my boat with chewing gum"
-'Monkeys wave goodbye'
-----
-
-Both single and double quotes can be used to mark strings as long as
-the quotes at the start and the end of the string match.
-
-(((line break)))(((newline character)))Almost anything can be put
-between quotes, and JavaScript will make a string value out of it. But
-a few characters are more difficult. You can imagine how putting
-quotes between quotes might be hard. _Newlines_ (the characters you
-get when you press Enter) also can't be put between quotes. The string
-has to stay on a single line.
-
-(((escaping,in strings)))(((backslash character)))To be able to have
-such characters in a string, the following notation is used: whenever
-a backslash (“\”) is found inside quoted text, it indicates that the
-character after it has a special meaning. This is called _escaping_
-the character. A quote that is preceded by a backslash will not end
-the string but be part of it. When an “n” character occurs after a
-backslash, it is interpreted as a newline. Similarly, a “t” after a
-backslash means a ((tab character)). Take the following string:
-
-[source,javascript]
-----
-"This is the first line\nAnd this is the second"
-----
-
-The actual text contained is this:
-
-----
-This is the first line
-And this is the second
-----
-
-There are, of course, situations where you want a backslash in a
-string to be just a backslash, not a special code. If two backslashes
-follow each other, they will collapse together, and only one will be
-left in the resulting string value. This is how the string “`A newline
-character is written like "\n"`” can be written:
-
-[source,javascript]
-----
-"A newline character is written like \"\\n\"."
-----
-
-(((+ operator)))(((concatenation)))Strings cannot be divided,
-multiplied, or subtracted, but the `+` operator _can_ be used on them.
-It does not add, but it __concatenates__—it glues two strings together.
-The following line will produce the string `"concatenate"`:
-
-[source,javascript]
-----
-"con" + "cat" + "e" + "nate"
-----
-
-There are more ways of manipulating strings, which we will discuss
-when we get to methods in link:04_data.html#methods[Chapter 4].
-
-== Unary operators ==
-
-(((operator)))(((typeof operator)))(((type)))Not all operators are
-symbols. Some are written as words. One example is the `typeof`
-operator, which produces a string value naming the type of the value
-you give it.
-
-[source,javascript]
-----
-console.log(typeof 4.5)
-// → number
-console.log(typeof "x")
-// → string
-----
-
-[[console.log]]
-
-(((console.log)))(((output)))(((JavaScript console)))We will use
-`console.log` in example code to indicate that we want to see the
-result of evaluating something. When you run such code, the value
-produced should be shown on the screen, though how it appears will
-depend on the JavaScript environment you use to run it.
-
-(((negation)))(((- operator)))(((binary operator)))(((unary
-operator)))The other operators we saw all operated on two values, but
-`typeof` takes only one. Operators that use two values are called
-_binary_ operators, while those that take one are called _unary_
-operators. The minus operator can be used both as a binary operator
-and as a unary operator.
-
-[source,javascript]
-----
-console.log(- (10 - 2))
-// → -8
-----
-
-== Boolean values ==
-
-(((Boolean)))(((operator)))(((true)))(((false)))(((bit)))Often,
-you will need a value that simply distinguishes between two
-possibilities, like “yes” and “no”, or “on” and “off”. For this,
-JavaScript has a _Boolean_ type, which has just two values: true and
-false (which are written simply as those words).
-
-=== Comparisons ===
-
-(((comparison)))Here is one way to produce Boolean values:
-
-[source,javascript]
-----
-console.log(3 > 2)
-// → true
-console.log(3 < 2)
-// → false
-----
-
-(((comparison,of numbers)))(((> operator)))(((< operator)))(((greater
-than)))(((less than)))The `>` and `<` signs are the traditional
-symbols for “is greater than” and “is less than”, respectively. They
-are binary operators. Applying them results in a Boolean value that
-indicates whether they hold true in this case.
-
-Strings can be compared in the same way.
-
-[source,javascript]
-----
-console.log("Aardvark" < "Zoroaster")
-// → true
-----
-
-(((comparison,of strings)))The way strings are ordered is more or less
-alphabetic: uppercase letters are always “less” than lowercase ones,
-so `"Z" < "a"` is true, and nonalphabetic characters (!, -, and so on)
-are also included in the ordering. The actual comparison is based on
-the _((Unicode))_ standard. This standard assigns a number to
-virtually every character you would ever need, including characters
-from Greek, Arabic, Japanese, Tamil, and so on. Having such numbers is
-useful for storing strings inside a computer because it makes it
-possible to represent them as a sequence of numbers. When comparing
-strings, JavaScript goes over them from left to right, comparing the
-numeric codes of the characters one by one.
-
-(((equality)))(((>= operator)))(((pass:[<=] operator)))(((==
-operator)))(((!= operator)))Other similar operators are `>=` (greater
-than or equal to), `<=` (less than or equal to), `==` (equal to), and
-`!=` (not equal to).
-
-[source,javascript]
-----
-console.log("Itchy" != "Scratchy")
-// → true
-----
-
-(((comparison,of NaN)))(((NaN)))There is only one value in JavaScript
-that is not equal to itself, and that is `NaN` (which stands for "not
-a number").
-
-[source,javascript]
-----
-console.log(NaN == NaN)
-// → false
-----
-
-`NaN` is supposed to denote the result of a nonsensical computation,
-and as such, it isn't equal to the result of any _other_ nonsensical
-computations.
-
-=== Logical operators ===
-
-(((reasoning)))(((logical operators)))There are also some operations
-that can be applied to Boolean values themselves. JavaScript supports
-three logical operators: _and_, _or_, and _not_. These can be used to
-“reason” about Booleans.
-
-(((&& operator)))(((logical and)))The `&&` operator represents logical
-_and_. It is a binary operator, and its result is true only if both
-the values given to it are true.
-
-[source,javascript]
-----
-console.log(true && false)
-// → false
-console.log(true && true)
-// → true
-----
-
-(((|| operator)))(((logical or)))The `||` operator denotes logical
-_or_. It produces true if either of the values given to it is true.
-
-[source,javascript]
-----
-console.log(false || true)
-// → true
-console.log(false || false)
-// → false
-----
-
-(((negation)))(((! operator)))_Not_ is written as an exclamation mark
-(`!`). It is a unary operator that flips the value given to it—`!true`
-produces `false` and `!false` gives `true`.
-
-(((precedence)))When mixing these Boolean operators with arithmetic
-and other operators, it is not always obvious when parentheses are
-needed. In practice, you can usually get by with knowing that of the
-operators we have seen so far, `||` has the lowest precedence, then
-comes `&&`, then the comparison operators (`>`, `==`, and so on), and
-then the rest. This order has been chosen such that, in typical
-expressions like the following one, as few parentheses as possible are
-necessary:
-
-[source,javascript]
-----
-1 + 1 == 2 && 10 * 10 > 50
-----
-
-(((conditional execution)))(((ternary operator)))(((?:
-operator)))(((conditional operator)))(((colon character)))(((question
-mark)))The last logical operator I will discuss is not unary, not
-binary, but _ternary_, operating on three values. It is written with a
-question mark and a colon, like this:
-
-[source,javascript]
-----
-console.log(true ? 1 : 2);
-// → 1
-console.log(false ? 1 : 2);
-// → 2
-----
-
-This one is called the _conditional_ operator (or sometimes just
-_ternary_ operator since it is the only such operator in the
-language). The value on the left of the question mark “picks” which of
-the other two values will come out. When it is true, the middle value
-is chosen, and when it is false, the value on the right comes out.
-
-== Undefined values ==
-
-(((undefined)))(((null)))There are two special values, written `null`
-and `undefined`, that are used to denote the absence of a meaningful
-value. They are themselves values, but they carry no
-information.
-
-Many operations in the language that don't produce a meaningful value
-(you'll see some later) yield `undefined` simply because they have to
-yield _some_ value.
-
-The difference in meaning between `undefined` and `null` is an accident
-of JavaScript's design, and it doesn't matter most of the time. In the cases
-where you actually have to concern yourself with these values, I
-recommend treating them as interchangeable (more on that in a moment).
-
-== Automatic type conversion ==
-
-(((NaN)))(((type coercion)))In the introduction, I mentioned that
-JavaScript goes out of its way to accept almost any program you give
-it, even programs that do odd things. This is nicely demonstrated by
-the following expressions:
-
-[source,javascript]
-----
-console.log(8 * null)
-// → 0
-console.log("5" - 1)
-// → 4
-console.log("5" + 1)
-// → 51
-console.log("five" * 2)
-// → NaN
-console.log(false == 0)
-// → true
-----
-
-(((+ operator)))(((arithmetic)))(((pass:[*] operator)))(((-
-operator)))When an operator is applied to the “wrong” type of value,
-JavaScript will quietly convert that value to the type it wants, using
-a set of rules that often aren't what you want or expect. This is
-called _((type coercion))_. So the `null` in the first expression becomes
-`0`, and the `"5"` in the second expression becomes `5` (from string
-to number). Yet in the third expression, `+` tries string
-concatenation before numeric addition, so the `1` is converted to
-`"1"` (from number to string).
-
-(((type coercion)))(((number,conversion to)))When something that
-doesn't map to a number in an obvious way (such as `"five"` or
-`undefined`) is converted to a number, the value `NaN` is produced.
-Further arithmetic operations on `NaN` keep producing `NaN`, so if you
-find yourself getting one of those in an unexpected place, look for
-accidental type conversions.
-
-(((null)))(((undefined)))(((comparison,of undefined values)))(((==
-operator)))When comparing values of the same type using `==`, the
-outcome is easy to predict: you should get true when both values are
-the same, except in the case of `NaN`. But when the types differ,
-JavaScript uses a complicated and confusing set of rules to determine
-what to do. In most cases, it just tries to convert one of the values
-to the other value's type. However, when `null` or `undefined` occurs
-on either side of the operator, it produces true only if both sides
-are one of `null` or `undefined`.
-
-[source,javascript]
-----
-console.log(null == undefined);
-// → true
-console.log(null == 0);
-// → false
-----
-
-That last piece of behavior is often useful. When you want to test
-whether a value has a real value instead of `null` or `undefined`, you
-can simply compare it to `null` with the `==` (or `!=`) operator.
-
-(((type coercion)))(((Boolean,conversion to)))(((=== operator)))(((!==
-operator)))(((comparison)))But what if you want to test whether
-something refers to the precise value `false`? The rules for
-converting strings and numbers to Boolean values state that `0`,
-`NaN`, and the empty string (`""`) count as `false`, while all the
-other values count as `true`. Because of this, expressions like `0 ==
-false` and `"" == false` are also true. For cases like this, where you
-do _not_ want any automatic type conversions to happen, there are two
-extra operators: `===` and `!==`. The first tests whether a value is
-precisely equal to the other, and the second tests whether it is not
-precisely equal. So `"" === false` is false as expected.
-
-I recommend using the three-character comparison operators defensively to
-prevent unexpected type conversions from tripping you up. But when you're
-certain the types on both sides will be the same, there is no problem with
-using the shorter operators.
-
-=== Short-circuiting of logical operators ===
-
-(((type coercion)))(((Boolean,conversion to)))(((operator)))The
-logical operators `&&` and `||` handle values of different types in a
-peculiar way. They will convert the value on their left side to
-Boolean type in order to decide what to do, but depending on the
-operator and the result of that conversion, they return either the
-_original_ left-hand value or the right-hand value.
-
-(((|| operator)))The `||` operator, for example, will return the value
-to its left when that can be converted to true and will return the
-value on its right otherwise. This conversion works as you'd expect
-for Boolean values and should do something analogous for values of
-other types.
-
-[source,javascript]
-----
-console.log(null || "user")
-// → user
-console.log("Karl" || "user")
-// → Karl
-----
-
-(((default value)))This functionality allows the `||` operator to be
-used as a way to fall back on a default value. If you give it an
-expression that might produce an empty value on the left, the value on
-the right will be used as a replacement in that case.
-
-(((&& operator)))The `&&` operator works similarly, but the other way
-around. When the value to its left is something that converts to
-false, it returns that value, and otherwise it returns the value on
-its right.
-
-(((short-circuit evaluation)))Another important property of these two
-operators is that the expression to their right is evaluated only when
-necessary. In the case of `true || X`, no matter what `X` is—even if
-it's an expression that does something __terrible__—the result will be
-true, and `X` is never evaluated. The same goes for `false && X`,
-which is false and will ignore `X`. This is called _short-circuit
-evaluation_.
-
-(((ternary operator)))(((?: operator)))(((conditional operator)))The
-conditional operator works in a similar way. The first expression is
-always evaluated, but the second or third value, the one that is not
-picked, is not.
-
-== Summary ==
-
-We looked at four types of JavaScript values in this chapter: numbers,
-strings, Booleans, and undefined values.
-
-Such values are created by typing in their name (`true`, `null`) or
-value (`13`, `"abc"`). You can combine and transform values with
-operators. We saw binary operators for arithmetic (`+`, `-`, `*`, `/`,
-and `%`), string concatenation (`+`), comparison (`==`, `!=`, `===`,
-`!==`, `<`, `>`, `<=`, `>=`), and logic (`&&`, `||`), as well as
-several unary operators (`-` to negate a number, `!` to negate
-logically, and `typeof` to find a value's type).
-
-This gives you enough information to use JavaScript as a pocket
-calculator, but not much more. The
-link:02_program_structure.html#program_structure[next chapter] will
-start tying these basic expressions together into basic programs.
diff --git a/02_program_structure.md b/02_program_structure.md
new file mode 100644
index 000000000..7f9db00e2
--- /dev/null
+++ b/02_program_structure.md
@@ -0,0 +1,754 @@
+# Program Structure
+
+{{quote {author: "_why", title: "Why's (Poignant) Guide to Ruby", chapter: true}
+
+And my heart glows bright red under my filmy, translucent skin and they have to administer 10cc of JavaScript to get me to come back. (I respond well to toxins in the blood.) Man, that stuff will kick the peaches right out your gills!
+
+quote}}
+
+{{index why, "Poignant Guide"}}
+
+{{figure {url: "img/chapter_picture_2.jpg", alt: "Illustration showing a number of tentacles holding chess pieces", chapter: framed}}}
+
+In this chapter, we will start to do things that can actually be called _programming_. We will expand our command of the JavaScript language beyond the nouns and sentence fragments we've seen so far to the point where we can express meaningful prose.
+
+## Expressions and statements
+
+{{index grammar, [syntax, expression], [code, "structure of"], grammar, [JavaScript, syntax]}}
+
+In [Chapter ?](values), we made values and applied operators to them to get new values. Creating values like this is the main substance of any JavaScript program. But that substance has to be framed in a larger structure to be useful. That's what we'll cover in this chapter.
+
+{{index "literal expression", [parentheses, expression]}}
+
+A fragment of code that produces a value is called an _((expression))_. Every value that is written literally (such as `22` or `"psychoanalysis"`) is an expression. An expression between parentheses is also an expression, as is a ((binary operator)) applied to two expressions or a ((unary operator)) applied to one.
+
+{{index [nesting, "of expressions"], "human language"}}
+
+This shows part of the beauty of a language-based interface. Expressions can contain other expressions in a way similar to how subsentences in human languages are nested—a subsentence can contain its own subsentences, and so on. This allows us to build expressions that describe arbitrarily complex computations.
+
+{{index statement, semicolon, program}}
+
+If an expression corresponds to a sentence fragment, a JavaScript _statement_ corresponds to a full sentence. A program is a list of statements.
+
+{{index [syntax, statement]}}
+
+The simplest kind of statement is an expression with a semicolon after it. This is a program:
+
+```
+1;
+!false;
+```
+
+It is a useless program, though. An ((expression)) can be content to just produce a value, which can then be used by the enclosing code. However, a ((statement)) stands on its own, so if it doesn't affect the world, it's useless. It may display something on the screen, as with `console.log`, or change the state of the machine in a way that will affect the statements that come after it. These changes are called _((side effect))s_. The statements in the previous example just produce the values `1` and `true` and then immediately throw them away. This leaves no impression on the world at all. When you run this program, nothing observable happens.
+
+{{index "programming style", "automatic semicolon insertion", semicolon}}
+
+In some cases, JavaScript allows you to omit the semicolon at the end of a statement. In other cases, it has to be there, or the next ((line)) will be treated as part of the same statement. The rules for when it can be safely omitted are somewhat complex and error prone. So in this book, every statement that needs a semicolon will always get one. I recommend you do the same, at least until you've learned more about the subtleties of missing semicolons.
+
+## Bindings
+
+{{indexsee variable, binding}}
+{{index [syntax, statement], [binding, definition], "side effect", [memory, organization], [state, in binding]}}
+
+How does a program keep an internal state? How does it remember things? We have seen how to produce new values from old values, but this does not change the old values, and the new value must be used immediately or it will dissipate again. To catch and hold values, JavaScript provides a thing called a _binding_, or _variable_.
+
+```
+let caught = 5 * 5;
+```
+
+{{index "let keyword"}}
+
+That gives us a second kind of ((statement)). The special word (_((keyword))_) `let` indicates that this sentence is going to define a binding. It is followed by the name of the binding and, if we want to immediately give it a value, by an `=` operator and an expression.
+
+The example creates a binding called `caught` and uses it to grab hold of the number that is produced by multiplying 5 by 5.
+
+After a binding has been defined, its name can be used as an ((expression)). The value of such an expression is the value the binding currently holds. Here's an example:
+
+```
+let ten = 10;
+console.log(ten * ten);
+// → 100
+```
+
+{{index "= operator", assignment, [binding, assignment]}}
+
+When a binding points at a value, that does not mean it is tied to that value forever. The `=` operator can be used at any time on existing bindings to disconnect them from their current value and have them point to a new one:
+
+```
+let mood = "light";
+console.log(mood);
+// → light
+mood = "dark";
+console.log(mood);
+// → dark
+```
+
+{{index [binding, "model of"], "tentacle (analogy)"}}
+
+You should imagine bindings as tentacles rather than boxes. They do not _contain_ values; they _grasp_ them—two bindings can refer to the same value. A program can access only the values to which it still has a reference. When you need to remember something, you either grow a tentacle to hold on to it or reattach one of your existing tentacles to it.
+
+Let's look at another example. To remember the number of dollars that Luigi still owes you, you create a binding. When he pays back $35, you give this binding a new value.
+
+```
+let luigisDebt = 140;
+luigisDebt = luigisDebt - 35;
+console.log(luigisDebt);
+// → 105
+```
+
+{{index undefined}}
+
+When you define a binding without giving it a value, the tentacle has nothing to grasp, so it ends in thin air. If you ask for the value of an empty binding, you'll get the value `undefined`.
+
+{{index "let keyword"}}
+
+A single `let` statement may define multiple bindings. The definitions must be separated by commas:
+
+```
+let one = 1, two = 2;
+console.log(one + two);
+// → 3
+```
+
+The words `var` and `const` can also be used to create bindings, in a similar fashion to `let`.
+
+```
+var name = "Ayda";
+const greeting = "Hello ";
+console.log(greeting + name);
+// → Hello Ayda
+```
+
+{{index "var keyword"}}
+
+The first of these, `var` (short for "variable"), is the way bindings were declared in pre-2015 JavaScript, when `let` didn't exist yet. I'll get back to the precise way it differs from `let` in the [next chapter](functions). For now, remember that it mostly does the same thing, but we'll rarely use it in this book because it behaves oddly in some situations.
+
+{{index "const keyword", naming}}
+
+The word `const` stands for _((constant))_. It defines a constant binding, which points at the same value for as long as it lives. This is useful for bindings that just give a name to a value so that you can easily refer to it later.
+
+## Binding names
+
+{{index "underscore character", "dollar sign", [binding, naming]}}
+
+Binding names can be any sequence of one or more letters. Digits can be part of binding names—`catch22` is a valid name, for example—but the name must not start with a digit. A binding name may include dollar signs (`$`) or underscores (`_`) but no other punctuation or special characters.
+
+{{index [syntax, identifier], "implements (reserved word)", "interface (reserved word)", "package (reserved word)", "private (reserved word)", "protected (reserved word)", "public (reserved word)", "static (reserved word)", "void operator", "yield (reserved word)", "enum (reserved word)", "reserved word", [binding, naming]}}
+
+Words with a special meaning, such as `let`, are _((keyword))s_, and may not be used as binding names. There are also a number of words that are "reserved for use" in ((future)) versions of JavaScript, which also can't be used as binding names. The full list of keywords and reserved words is rather long:
+
+```{lang: "null"}
+break case catch class const continue debugger default
+delete do else enum export extends false finally for
+function if implements import interface in instanceof let
+new package private protected public return static super
+switch this throw true try typeof var void while with yield
+```
+
+{{index [syntax, error]}}
+
+Don't worry about memorizing this list. When creating a binding produces an unexpected syntax error, check whether you're trying to define a reserved word.
+
+## The environment
+
+{{index "standard environment", [browser, environment]}}
+
+The collection of bindings and their values that exist at a given time is called the _((environment))_. When a program starts up, this environment is not empty. It always contains bindings that are part of the language ((standard)), and most of the time, it also has bindings that provide ways to interact with the surrounding system. For example, in a browser, there are functions to interact with the currently loaded website and to read ((mouse)) and ((keyboard)) input.
+
+## Functions
+
+{{indexsee "application (of functions)", [function, application]}}
+{{indexsee "invoking (of functions)", [function, application]}}
+{{indexsee "calling (of functions)", [function, application]}}
+{{index output, function, [function, application], [browser, environment]}}
+
+A lot of the values provided in the default environment have the type _((function))_. A function is a piece of program wrapped in a value. Such values can be _applied_ in order to run the wrapped program. For example, in a browser environment, the binding `prompt` holds a function that shows a little ((dialog)) asking for user input. It is used like this:
+
+```
+prompt("Enter passcode");
+```
+
+{{figure {url: "img/prompt.png", alt: "A prompt dialog that says 'enter passcode'", width: "8cm"}}}
+
+{{index parameter, [function, application], [parentheses, arguments]}}
+
+Executing a function is called _invoking_, _calling_, or _applying_ it. You can call a function by putting parentheses after an expression that produces a function value. Usually you'll directly use the name of the binding that holds the function. The values between the parentheses are given to the program inside the function. In the example, the `prompt` function uses the string that we give it as the text to show in the dialog box. Values given to functions are called _((argument))s_. Different functions might need a different number or different types of arguments.
+
+The `prompt` function isn't used much in modern web programming, mostly because you have no control over the way the resulting dialog looks, but it can be helpful in toy programs and experiments.
+
+## The console.log function
+
+{{index "JavaScript console", "developer tools", "Node.js", "console.log", output, [browser, environment]}}
+
+In the examples, I used `console.log` to output values. Most JavaScript systems (including all modern web browsers and Node.js) provide a `console.log` function that writes out its arguments to _some_ text output device. In browsers, the output lands in the ((JavaScript console)). This part of the browser interface is hidden by default, but most browsers open it when you press F12 or, on a Mac, [command]{keyname}-[option]{keyname}-I. If that does not work, search through the menus for an item named Developer Tools or similar.
+
+{{if interactive
+
+When running the examples (or your own code) on the pages of this book, `console.log` output will be shown after the example, instead of in the browser's JavaScript console.
+
+```
+let x = 30;
+console.log("the value of x is", x);
+// → the value of x is 30
+```
+
+if}}
+
+{{index [object, property], [property, access]}}
+
+Though binding names cannot contain ((period character))s, `console.log` does have one. This is because `console.log` isn't a simple binding, but an expression that retrieves the `log` property from the value held by the `console` binding. We'll find out exactly what this means in [Chapter ?](data#properties).
+
+{{id return_values}}
+## Return values
+
+{{index [comparison, "of numbers"], "return value", "Math.max function", maximum}}
+
+Showing a dialog box or writing text to the screen is a _((side effect))_. Many functions are useful because of the side effects they produce. Functions may also produce values, in which case they don't need to have a side effect to be useful. For example, the function `Math.max` takes any amount of number arguments and gives back the greatest.
+
+```
+console.log(Math.max(2, 4));
+// → 4
+```
+
+{{index [function, application], minimum, "Math.min function"}}
+
+When a function produces a value, it is said to _return_ that value. Anything that produces a value is an ((expression)) in JavaScript, which means that function calls can be used within larger expressions. In the following code, a call to `Math.min`, which is the opposite of `Math.max`, is used as part of a plus expression:
+
+```
+console.log(Math.min(2, 4) + 100);
+// → 102
+```
+
+[Chapter ?](functions) will explain how to write your own functions.
+
+## Control flow
+
+{{index "execution order", program, "control flow"}}
+
+When your program contains more than one ((statement)), the statements are executed as though they were a story, from top to bottom. For example, the following program has two statements. The first asks the user for a number, and the second, which is executed after the first, shows the ((square)) of that number:
+
+```
+let theNumber = Number(prompt("Pick a number"));
+console.log("Your number is the square root of " +
+ theNumber * theNumber);
+```
+
+{{index [number, "conversion to"], "type coercion", "Number function", "String function", "Boolean function", [Boolean, "conversion to"]}}
+
+The function `Number` converts a value to a number. We need that conversion because the result of `prompt` is a string value, and we want a number. There are similar functions called `String` and `Boolean` that convert values to those types.
+
+Here is the rather trivial schematic representation of straight-line control flow:
+
+{{figure {url: "img/controlflow-straight.svg", alt: "Diagram showing a straight arrow", width: "4cm"}}}
+
+## Conditional execution
+
+{{index Boolean, ["control flow", conditional]}}
+
+Not all programs are straight roads. We may, for example, want to create a branching road where the program takes the proper branch based on the situation at hand. This is called _((conditional execution))_.
+
+{{figure {url: "img/controlflow-if.svg", alt: "Diagram of an arrow that splits in two, and then rejoins again",width: "4cm"}}}
+
+{{index [syntax, statement], "Number function", "if keyword"}}
+
+Conditional execution is created with the `if` keyword in JavaScript. In the simple case, we want some code to be executed if, and only if, a certain condition holds. We might, for example, want to show the square of the input only if the input is actually a number:
+
+```{test: wrap}
+let theNumber = Number(prompt("Pick a number"));
+if (!Number.isNaN(theNumber)) {
+ console.log("Your number is the square root of " +
+ theNumber * theNumber);
+}
+```
+
+With this modification, if you enter "parrot", no output is shown.
+
+{{index [parentheses, statement]}}
+
+The `if` keyword executes or skips a statement depending on the value of a Boolean expression. The deciding expression is written after the keyword, between parentheses, followed by the statement to execute.
+
+{{index "Number.isNaN function"}}
+
+The `Number.isNaN` function is a standard JavaScript function that returns `true` only if the argument it is given is `NaN`. The `Number` function happens to return `NaN` when you give it a string that doesn't represent a valid number. Thus, the condition translates to "unless `theNumber` is not-a-number, do this".
+
+{{index grouping, "{} (block)", [braces, "block"]}}
+
+The statement after the `if` is wrapped in braces (`{` and `}`) in this example. The braces can be used to group any number of statements into a single statement, called a _((block))_. You could also have omitted them in this case, since they hold only a single statement, but to avoid having to think about whether they are needed, most JavaScript programmers use them in every wrapped statement like this. We'll mostly follow that convention in this book, except for the occasional one-liner.
+
+```
+if (1 + 1 == 2) console.log("It's true");
+// → It's true
+```
+
+{{index "else keyword"}}
+
+You often won't just have code that executes when a condition holds true, but also code that handles the other case. This alternate path is represented by the second arrow in the diagram. You can use the `else` keyword, together with `if`, to create two separate, alternative execution paths:
+
+```{test: wrap}
+let theNumber = Number(prompt("Pick a number"));
+if (!Number.isNaN(theNumber)) {
+ console.log("Your number is the square root of " +
+ theNumber * theNumber);
+} else {
+ console.log("Hey. Why didn't you give me a number?");
+}
+```
+
+{{index ["if keyword", chaining]}}
+
+If you have more than two paths to choose from, you can "chain" multiple `if`/`else` pairs together. Here's an example:
+
+```
+let num = Number(prompt("Pick a number"));
+
+if (num < 10) {
+ console.log("Small");
+} else if (num < 100) {
+ console.log("Medium");
+} else {
+ console.log("Large");
+}
+```
+
+The program will first check whether `num` is less than 10. If it is, it chooses that branch, shows `"Small"`, and is done. If it isn't, it takes the `else` branch, which itself contains a second `if`. If the second condition (`< 100`) holds, that means the number is at least 10 but below 100, and `"Medium"` is shown. If it doesn't, the second and last `else` branch is chosen.
+
+The schema for this program looks something like this:
+
+{{figure {url: "img/controlflow-nested-if.svg", alt: "Diagram showing arrow that splits in two, with on the branches splitting again, before all branches rejoin again", width: "4cm"}}}
+
+{{id loops}}
+## while and do loops
+
+Consider a program that outputs all ((even number))s from 0 to 12. One way to write this is as follows:
+
+```
+console.log(0);
+console.log(2);
+console.log(4);
+console.log(6);
+console.log(8);
+console.log(10);
+console.log(12);
+```
+
+{{index ["control flow", loop]}}
+
+That works, but the idea of writing a program is to make something _less_ work, not more. If we needed all even numbers less than 1,000, this approach would be unworkable. What we need is a way to run a piece of code multiple times. This form of control flow is called a _((loop))_.
+
+{{figure {url: "img/controlflow-loop.svg", alt: "Diagram showing an arrow to a point which has a cyclic arrow going back to itself and another arrow going further", width: "4cm"}}}
+
+{{index [syntax, statement], "counter variable"}}
+
+Looping control flow allows us to go back to some point in the program where we were before and repeat it with our current program state. If we combine this with a binding that counts, we can do something like this:
+
+```
+let number = 0;
+while (number <= 12) {
+ console.log(number);
+ number = number + 2;
+}
+// → 0
+// → 2
+// … etcetera
+```
+
+{{index "while loop", Boolean, [parentheses, statement]}}
+
+A ((statement)) starting with the keyword `while` creates a loop. The word `while` is followed by an ((expression)) in parentheses and then a statement, much like `if`. The loop keeps entering that statement as long as the expression produces a value that gives `true` when converted to Boolean.
+
+{{index [state, in binding], [binding, as state]}}
+
+The `number` binding demonstrates the way a ((binding)) can track the progress of a program. Every time the loop repeats, `number` gets a value that is 2 more than its previous value. At the beginning of every repetition, it is compared with the number 12 to decide whether the program's work is finished.
+
+{{index exponentiation}}
+
+As an example that actually does something useful, we can now write a program that calculates and shows the value of 2^10^ (2 to the 10th power). We use two bindings: one to keep track of our result and one to count how often we have multiplied this result by 2. The loop tests whether the second binding has reached 10 yet and, if not, updates both bindings.
+
+```
+let result = 1;
+let counter = 0;
+while (counter < 10) {
+ result = result * 2;
+ counter = counter + 1;
+}
+console.log(result);
+// → 1024
+```
+
+The counter could also have started at `1` and checked for `<= 10`, but for reasons that will become apparent in [Chapter ?](data#array_indexing), it is a good idea to get used to counting from 0.
+
+{{index "** operator"}}
+
+Note that JavaScript also has an operator for exponentiation (`2 ** 10`), which you would use to compute this in real code—but that would have ruined the example.
+
+{{index "loop body", "do loop", ["control flow", loop]}}
+
+A `do` loop is a control structure similar to a `while` loop. It differs only on one point: a `do` loop always executes its body at least once, and it starts testing whether it should stop only after that first execution. To reflect this, the test appears after the body of the loop:
+
+```
+let yourName;
+do {
+ yourName = prompt("Who are you?");
+} while (!yourName);
+console.log("Hello " + yourName);
+```
+
+{{index [Boolean, "conversion to"], "! operator"}}
+
+This program will force you to enter a name. It will ask again and again until it gets something that is not an empty string. Applying the `!` operator will convert a value to Boolean type before negating it, and all strings except `""` convert to `true`. This means the loop continues going round until you provide a non-empty name.
+
+## Indenting Code
+
+{{index [code, "structure of"], [whitespace, indentation], "programming style"}}
+
+In the examples, I've been adding spaces in front of statements that are part of some larger statement. These spaces are not required—the computer will accept the program just fine without them. In fact, even the ((line)) breaks in programs are optional. You could write a program as a single long line if you felt like it.
+
+The role of this ((indentation)) inside ((block))s is to make the structure of the code stand out to human readers. In code where new blocks are opened inside other blocks, it can become hard to see where one block ends and another begins. With proper indentation, the visual shape of a program corresponds to the shape of the blocks inside it. I like to use two spaces for every open block, but tastes differ—some people use four spaces, and some people use ((tab character))s. The important thing is that each new block adds the same amount of space.
+
+```
+if (false != true) {
+ console.log("That makes sense.");
+ if (1 < 2) {
+ console.log("No surprise there.");
+ }
+}
+```
+
+Most code ((editor)) programs[ (including the one in this book)]{if interactive} will help by automatically indenting new lines the proper amount.
+
+## for loops
+
+{{index [syntax, statement], "while loop", "counter variable"}}
+
+Many loops follow the pattern shown in the `while` examples. First a "counter" binding is created to track the progress of the loop. Then comes a `while` loop, usually with a test expression that checks whether the counter has reached its end value. At the end of the loop body, the counter is updated to track progress.
+
+{{index "for loop", loop}}
+
+Because this pattern is so common, JavaScript and similar languages provide a slightly shorter and more comprehensive form, the `for` loop:
+
+```
+for (let number = 0; number <= 12; number = number + 2) {
+ console.log(number);
+}
+// → 0
+// → 2
+// … etcetera
+```
+
+{{index ["control flow", loop], state}}
+
+This program is exactly equivalent to the [earlier](program_structure#loops) even-number-printing example. The only change is that all the ((statement))s that are related to the "state" of the loop are grouped together after `for`.
+
+{{index [binding, as state], [parentheses, statement]}}
+
+The parentheses after a `for` keyword must contain two ((semicolon))s. The part before the first semicolon _initializes_ the loop, usually by defining a binding. The second part is the ((expression)) that _checks_ whether the loop must continue. The final part _updates_ the state of the loop after every iteration. In most cases, this is shorter and clearer than a `while` construct.
+
+{{index exponentiation}}
+
+This is the code that computes 2^10^ using `for` instead of `while`:
+
+```{test: wrap}
+let result = 1;
+for (let counter = 0; counter < 10; counter = counter + 1) {
+ result = result * 2;
+}
+console.log(result);
+// → 1024
+```
+
+## Breaking Out of a Loop
+
+{{index [loop, "termination of"], "break keyword"}}
+
+Having the looping condition produce `false` is not the only way a loop can finish. The `break` statement has the effect of immediately jumping out of the enclosing loop. Its use is demonstrated in the following program, which finds the first number that is both greater than or equal to 20 and divisible by 7:
+
+```
+for (let current = 20; ; current = current + 1) {
+ if (current % 7 == 0) {
+ console.log(current);
+ break;
+ }
+}
+// → 21
+```
+
+{{index "remainder operator", "% operator"}}
+
+Using the remainder (`%`) operator is an easy way to test whether a number is divisible by another number. If it is, the remainder of their division is zero.
+
+{{index "for loop"}}
+
+The `for` construct in the example does not have a part that checks for the end of the loop. This means that the loop will never stop unless the `break` statement inside is executed.
+
+If you were to remove that `break` statement or you accidentally write an end condition that always produces `true`, your program would get stuck in an _((infinite loop))_. A program stuck in an infinite loop will never finish running, which is usually a bad thing.
+
+{{if interactive
+
+If you create an infinite loop in one of the examples on these pages, you'll usually be asked whether you want to stop the script after a few seconds. If that fails, you will have to close the tab that you're working in to recover.
+
+if}}
+
+{{index "continue keyword"}}
+
+The `continue` keyword is similar to `break` in that it influences the progress of a loop. When `continue` is encountered in a loop body, control jumps out of the body and continues with the loop's next iteration.
+
+## Updating bindings succinctly
+
+{{index assignment, "+= operator", "-= operator", "/= operator", "*= operator", [state, in binding], "side effect"}}
+
+Especially when looping, a program often needs to "update" a binding to hold a value based on that binding's previous value.
+
+```{test: no}
+counter = counter + 1;
+```
+
+JavaScript provides a shortcut for this:
+
+```{test: no}
+counter += 1;
+```
+
+Similar shortcuts work for many other operators, such as `result *= 2` to double `result` or `counter -= 1` to count downward.
+
+This allows us to further shorten our counting example:
+
+```
+for (let number = 0; number <= 12; number += 2) {
+ console.log(number);
+}
+```
+
+{{index "++ operator", "-- operator"}}
+
+For `counter += 1` and `counter -= 1`, there are even shorter equivalents: `counter++` and `counter--`.
+
+## Dispatching on a value with switch
+
+{{index [syntax, statement], "conditional execution", dispatch, ["if keyword", chaining]}}
+
+It is not uncommon for code to look like this:
+
+```{test: no}
+if (x == "value1") action1();
+else if (x == "value2") action2();
+else if (x == "value3") action3();
+else defaultAction();
+```
+
+{{index "colon character", "switch keyword"}}
+
+There is a construct called `switch` that is intended to express such a "dispatch" in a more direct way. Unfortunately, the syntax JavaScript uses for this (which it inherited from the C/Java line of programming languages) is somewhat awkward—a chain of `if` statements may look better. Here is an example:
+
+```
+switch (prompt("What is the weather like?")) {
+ case "rainy":
+ console.log("Remember to bring an umbrella.");
+ break;
+ case "sunny":
+ console.log("Dress lightly.");
+ case "cloudy":
+ console.log("Go outside.");
+ break;
+ default:
+ console.log("Unknown weather type!");
+ break;
+}
+```
+
+{{index fallthrough, "break keyword", "case keyword", "default keyword"}}
+
+You may put any number of `case` labels inside the block opened by `switch`. The program will start executing at the label that corresponds to the value that `switch` was given, or at `default` if no matching value is found. It will continue executing, even across other labels, until it reaches a `break` statement. In some cases, such as the `"sunny"` case in the example, this can be used to share some code between cases (it recommends going outside for both sunny and cloudy weather). Be careful, though—it is easy to forget such a `break`, which will cause the program to execute code you do not want executed.
+
+## Capitalization
+
+{{index capitalization, [binding, naming], [whitespace, syntax]}}
+
+Binding names may not contain spaces, yet it is often helpful to use multiple words to clearly describe what the binding represents. These are pretty much your choices for writing a binding name with several words in it:
+
+```{lang: null}
+fuzzylittleturtle
+fuzzy_little_turtle
+FuzzyLittleTurtle
+fuzzyLittleTurtle
+```
+
+{{index "camel case", "programming style", "underscore character"}}
+
+The first style can be hard to read. I rather like the look of the underscores, though that style is a little painful to type. The ((standard)) JavaScript functions, and most JavaScript programmers, follow the final style—they capitalize every word except the first. It is not hard to get used to little things like that, and code with mixed naming styles can be jarring to read, so we follow this ((convention)).
+
+{{index "Number function", constructor}}
+
+In a few cases, such as the `Number` function, the first letter of a binding is also capitalized. This was done to mark this function as a constructor. It will become clear what a constructor is in [Chapter ?](object#constructors). For now, the important thing is to not be bothered by this apparent lack of ((consistency)).
+
+## Comments
+
+{{index readability}}
+
+Often, raw code does not convey all the information you want a program to convey to human readers, or it conveys it in such a cryptic way that people might not understand it. At other times, you might just want to include some related thoughts as part of your program. This is what _((comment))s_ are for.
+
+{{index "slash character", "line comment"}}
+
+A comment is a piece of text that is part of a program but is completely ignored by the computer. JavaScript has two ways of writing comments. To write a single-line comment, you can use two slash characters (`//`) and then the comment text after it:
+
+```{test: no}
+let accountBalance = calculateBalance(account);
+// It's a green hollow where a river sings
+accountBalance.adjust();
+// Madly catching white tatters in the grass.
+let report = new Report();
+// Where the sun on the proud mountain rings:
+addToReport(accountBalance, report);
+// It's a little valley, foaming like light in a glass.
+```
+
+{{index "block comment"}}
+
+A `//` comment goes only to the end of the line. A section of text between `/*` and `*/` will be ignored in its entirety, regardless of whether it contains line breaks. This is useful for adding blocks of information about a file or a chunk of program:
+
+```
+/*
+ I first found this number scrawled on the back of an old
+ notebook. Since then, it has often dropped by, showing up in
+ phone numbers and the serial numbers of products that I've
+ bought. It obviously likes me, so I've decided to keep it.
+*/
+const myNumber = 11213;
+```
+
+## Summary
+
+You now know that a program is built out of statements, which themselves sometimes contain more statements. Statements tend to contain expressions, which themselves can be built out of smaller expressions.
+
+Putting statements after one another gives you a program that is executed from top to bottom. You can introduce disturbances in the flow of control by using conditional (`if`, `else`, and `switch`) and looping (`while`, `do`, and `for`) statements.
+
+Bindings can be used to file pieces of data under a name, and they are useful for tracking state in your program. The environment is the set of bindings that are defined. JavaScript systems always put a number of useful standard bindings into your environment.
+
+Functions are special values that encapsulate a piece of program. You can invoke them by writing `functionName(argument1, argument2)`. Such a function call is an expression and may produce a value.
+
+## Exercises
+
+{{index exercises}}
+
+If you are unsure how to test your solutions to the exercises, refer to the [introduction](intro).
+
+Each exercise starts with a problem description. Read this description and try to solve the exercise. If you run into problems, consider reading the hints [after the exercise]{if interactive}[at the [end of the book](hints)]{if book}. You can find full solutions to the exercises online at [_https://eloquentjavascript.net/code_](https://eloquentjavascript.net/code#2). If you want to learn something from the exercises, I recommend looking at the solutions only after you've solved the exercise, or at least after you've attacked it long and hard enough to have a slight headache.
+
+### Looping a triangle
+
+{{index "triangle (exercise)"}}
+
+Write a ((loop)) that makes seven calls to `console.log` to output the following triangle:
+
+```{lang: null}
+#
+##
+###
+####
+#####
+######
+#######
+```
+
+{{index [string, length]}}
+
+It may be useful to know that you can find the length of a string by writing `.length` after it.
+
+```
+let abc = "abc";
+console.log(abc.length);
+// → 3
+```
+
+{{if interactive
+
+Most exercises contain a piece of code that you can modify to solve the exercise. Remember that you can click code blocks to edit them.
+
+```
+// Your code here.
+```
+if}}
+
+{{hint
+
+{{index "triangle (exercise)"}}
+
+You can start with a program that prints out the numbers 1 to 7, which you can derive by making a few modifications to the [even number printing example](program_structure#loops) given earlier in the chapter, where the `for` loop was introduced.
+
+Now consider the equivalence between numbers and strings of hash characters. You can go from 1 to 2 by adding 1 (`+= 1`). You can go from `"#"` to `"##"` by adding a character (`+= "#"`). Thus, your solution can closely follow the number-printing program.
+
+hint}}
+
+### FizzBuzz
+
+{{index "FizzBuzz (exercise)", loop, "conditional execution"}}
+
+Write a program that uses `console.log` to print all the numbers from 1 to 100, with two exceptions. For numbers divisible by 3, print `"Fizz"` instead of the number, and for numbers divisible by 5 (and not 3), print `"Buzz"` instead.
+
+When you have that working, modify your program to print `"FizzBuzz"` for numbers that are divisible by both 3 and 5 (and still print `"Fizz"` or `"Buzz"` for numbers divisible by only one of those).
+
+(This is actually an ((interview question)) that has been claimed to weed out a significant percentage of programmer candidates. So if you solved it, your labor market value just went up.)
+
+{{if interactive
+```
+// Your code here.
+```
+if}}
+
+{{hint
+
+{{index "FizzBuzz (exercise)", "remainder operator", "% operator"}}
+
+Going over the numbers is clearly a looping job, and selecting what to print is a matter of conditional execution. Remember the trick of using the remainder (`%`) operator for checking whether a number is divisible by another number (has a remainder of zero).
+
+In the first version, there are three possible outcomes for every number, so you'll have to create an `if`/`else if`/`else` chain.
+
+{{index "|| operator", ["if keyword", chaining]}}
+
+The second version of the program has a straightforward solution and a clever one. The simple solution is to add another conditional "branch" to precisely test the given condition. For the clever solution, build up a string containing the word or words to output and print either this word or the number if there is no word, potentially by making good use of the `||` operator.
+
+hint}}
+
+### Chessboard
+
+{{index "chessboard (exercise)", loop, [nesting, "of loops"], "newline character"}}
+
+Write a program that creates a string that represents an 8×8 grid, using newline characters to separate lines. At each position of the grid there is either a space or a "#" character. The characters should form a chessboard.
+
+Passing this string to `console.log` should show something like this:
+
+```{lang: null}
+ # # # #
+# # # #
+ # # # #
+# # # #
+ # # # #
+# # # #
+ # # # #
+# # # #
+```
+
+When you have a program that generates this pattern, define a binding `size = 8` and change the program so that it works for any `size`, outputting a grid of the given width and height.
+
+{{if interactive
+```
+// Your code here.
+```
+if}}
+
+{{hint
+
+{{index "chess board (exercise)"}}
+
+You can build the string by starting with an empty one (`""`) and repeatedly adding characters. A newline character is written `"\n"`.
+
+{{index [nesting, "of loops"], [braces, "block"]}}
+
+To work with two ((dimensions)), you will need a ((loop)) inside of a loop. Put braces around the bodies of both loops to make it easy to see where they start and end. Try to properly indent these bodies. The order of the loops must follow the order in which we build up the string (line by line, left to right, top to bottom). So the outer loop handles the lines, and the inner loop handles the characters on a line.
+
+{{index "counter variable", "remainder operator", "% operator"}}
+
+You'll need two bindings to track your progress. To know whether to put a space or a hash sign at a given position, you could test whether the sum of the two counters is even (`% 2`).
+
+Terminating a line by adding a newline character must happen after the line has been built up, so do this after the inner loop but inside the outer loop.
+
+hint}}
diff --git a/02_program_structure.txt b/02_program_structure.txt
deleted file mode 100644
index 8bb586803..000000000
--- a/02_program_structure.txt
+++ /dev/null
@@ -1,1016 +0,0 @@
-:chap_num: 2
-:prev_link: 01_values
-:next_link: 03_functions
-
-= Program Structure =
-
-[chapterquote="true"]
-[quote, _why, Why's (Poignant) Guide to Ruby]
-____
-And my heart glows bright red under my
-filmy, translucent skin and they have to administer 10cc of JavaScript
-to get me to come back. (I respond well to toxins in the blood.) Man,
-that stuff will kick the peaches right out your gills!
-____
-
-(((why)))(((Poignant Guide)))In this chapter, we will start to do
-things that can actually be called _programming_. We will expand our
-command of the JavaScript language beyond the nouns and sentence
-fragments we've seen so far, to the point where we can actually
-express some meaningful prose.
-
-== Expressions and statements ==
-
-(((grammar)))(((syntax)))(((code,structure
-of)))(((grammar)))(((JavaScript,syntax)))In
-link:01_values.html#values[Chapter 1], we made some values and then
-applied operators to them to get new values. Creating values like this
-is an essential part of every JavaScript program, but it is still only
-a part.
-
-(((literal expression)))A fragment of code that produces a value is
-called an _((expression))_. Every value that is written literally
-(such as `22` or `"psychoanalysis"`) is an expression. An expression
-between ((parentheses)) is also an expression, as is a ((binary
-operator)) applied to two expressions or a unary operator applied to
-one.
-
-(((nesting,of expressions)))(((human language)))This shows part of the
-beauty of a language-based interface. Expressions can nest in a way
-very similar to the way subsentences in human languages are nested—a
-subsentence can contain its own subsentences, and so on. This allows
-us to combine expressions to express arbitrarily complex computations.
-
-(((statement)))(((semicolon)))(((program)))If an expression
-corresponds to a sentence fragment, a JavaScript _statement_
-corresponds to a full sentence in a human language. A program is
-simply a list of statements.
-
-(((syntax)))The simplest kind of statement is an expression with a
-semicolon after it. This is a program:
-
-[source,javascript]
-----
-1;
-!false;
-----
-
-It is a useless program, though. An ((expression)) can be content to
-just produce a value, which can then be used by the enclosing
-expression. A ((statement)) stands on its own and amounts to something
-only if it affects the world. It could display something on the
-screen—that counts as changing the world—or it could change the
-internal state of the machine in a way that will affect the statements
-that come after it. These changes are called _((side effect))s_. The
-statements in the previous example just produce the values `1` and
-`true` and then immediately throw them away. This leaves no impression
-on the world at all. When executing the program, nothing observable
-happens.
-
-(((programming style)))(((automatic semicolon
-insertion)))(((semicolon)))In some cases, JavaScript allows you to
-omit the semicolon at the end of a statement. In other cases, it has
-to be there, the next ((line)) will be treated as part of the same
-statement. The rules for when it can be safely omitted are somewhat
-complex and error-prone. In this book, every statement that needs a
-semicolon will always be terminated by one. I recommend you do the
-same in your own programs, at least until you've learned more about
-subtleties involved in leaving out semicolons.
-
-== Variables ==
-
-(((syntax)))(((variable,definition)))(((side effect)))(((memory)))How
-does a program keep an internal ((state))? How does it remember
-things? We have seen how to produce new values from old values, but
-this does not change the old values, and the new value has to be
-immediately used or it will dissipate again. To catch and hold values,
-JavaScript provides a thing called a _variable_.
-
-[source,javascript]
-----
-var caught = 5 * 5;
-----
-
-(((var keyword)))And that gives us our second kind of ((statement)).
-The special word (_((keyword))_) `var` indicates that this sentence is
-going to define a variable. It is followed by the name of the variable
-and, if we want to immediately give it a value, by an `=` operator and
-an expression.
-
-The previous statement creates a variable called `caught` and uses it
-to grab hold of the number that is produced by multiplying 5 by 5.
-
-After a variable has been defined, its name can be used as an
-((expression)). The value of such an expression is the value the
-variable currently holds. Here's an example:
-
-[source,javascript]
-----
-var ten = 10;
-console.log(ten * ten);
-// → 100
-----
-
-(((underscore character)))(((dollar
-sign)))(((variable,naming)))Variable names can be any word that isn't
-reserved as a keyword (such as `var`). They may not include spaces.
-Digits can also be part of variable names—`catch22` is a valid name,
-for example—but the name must not start with a digit. A variable name
-cannot include punctuation, except for the characters “$” and “_”.
-
-(((= operator)))(((assignment)))(((variable,assignment)))When a
-variable points at a value, that does not mean it is tied to that
-value forever. The `=` operator can be used at any time on existing
-variables to disconnect them from their current value and have them
-point to a new one.
-
-[source,javascript]
-----
-var mood = "light";
-console.log(mood);
-// → light
-mood = "dark";
-console.log(mood);
-// → dark
-----
-
-(((variable,model of)))(((tentacle (analogy))))You should
-imagine variables as tentacles, rather than boxes. They do not
-_contain_ values; they _grasp_ them—two variables can refer to the
-same value. A program can access only the values that it still has a
-hold on. When you need to remember something, you grow a tentacle to
-hold on to it or you reattach one of your existing tentacles to it.
-
-image::img/octopus.jpg[alt="Variables as tentacles"]
-
-Let's look at an example. To remember the number of dollars that Luigi
-still owes you, you create a variable. And then when he pays back $35,
-you give this variable a new value.
-
-[source,javascript]
-----
-var luigisDebt = 140;
-luigisDebt = luigisDebt - 35;
-console.log(luigisDebt);
-// → 105
-----
-
-(((undefined)))When you define a variable without giving it a value,
-the tentacle has nothing to grasp, so it ends in thin air. If you ask
-for the value of an empty variable, you'll get the value `undefined`.
-
-(((var keyword)))A single `var` statement may define multiple
-variables. The definitions must be separated by commas.
-
-[source,javascript]
-----
-var one = 1, two = 2;
-console.log(one + two);
-// → 3
-----
-
-== Keywords and reserved words ==
-
-(((syntax)))(((implements (reserved word))))(((interface (reserved
-word))))(((let keyword)))(((package (reserved word))))(((private
-(reserved word))))(((protected (reserved word))))(((public (reserved
-word))))(((static (reserved word))))(((void operator)))(((yield
-(reserved word))))(((reserved word)))(((variable,naming)))Words with
-a special meaning, such as `var`, are _((keyword))s_, and they may not
-be used as variable names. There are also a number of words that are
-“reserved for use” in ((future)) versions of JavaScript. These are also
-officially not allowed to be used as variable names, though some
-JavaScript environments do allow them. The full list of keywords and
-reserved words is rather long.
-
-[source,text/plain]
-----
-break case catch continue debugger default delete
-do else false finally for function if implements
-in instanceof interface let new null package private
-protected public return static switch throw true
-try typeof var void while with yield this
-----
-
-Don't worry about memorizing these, but remember that this might be
-the problem when a variable definition does not work as expected.
-
-== The environment ==
-
-(((standard environment)))The collection of variables and their values
-that exist at a given time is called the _((environment))_. When a
-program starts up, this environment is not empty. It always contains
-variables that are part of the language ((standard)), and most of the
-time, it has variables that provide ways to interact with the
-surrounding system. For example, in a ((browser)), there are variables
-and functions to inspect and influence the currently loaded website
-and to read ((mouse)) and ((keyboard)) input.
-
-== Functions ==
-
-indexsee:[application (of functions),function application]
-indexsee:[invoking (of functions),function application]
-indexsee:[calling (of functions),function application]
-
-(((output)))(((function)))(((function,application)))(((alert
-function)))(((message box)))A lot of the values provided in the
-default environment have the type _((function))_. A function is a
-piece of program wrapped in a value. Such values can be _applied_ in
-order to run the wrapped program. For example, in a ((browser))
-environment, the variable `alert` holds a function that shows a little
-((dialog box)) with a message. It is used like this:
-
-[source,javascript]
-----
-alert("Good morning!");
-----
-
-image::img/alert.png[alt="An alert dialog",width="8cm"]
-
-(((parameter)))(((function,application)))Executing a function is
-called _invoking_, _calling_, or _applying_ it. You can call a
-function by putting ((parentheses)) after an expression that produces a
-function value. Usually you'll directly use the name of a variable
-that holds a function. The values between the parentheses are given to
-the program inside the function. In the example, the `alert` function
-uses the string that we give it as the text to show in the dialog box.
-Values given to functions are called _((argument))s_. The `alert`
-function needs only one of them, but other functions might need a
-different number or different types of arguments.
-
-== The console.log function ==
-
-(((JavaScript console)))(((developer
-tools)))(((Node.js)))(((console.log)))(((output)))The `alert` function
-can be useful as an output device when experimenting, but clicking
-away all those little windows will get on your nerves. In past
-examples, we've used `console.log` to output values. Most JavaScript
-systems (including all modern web ((browser))s and Node.js) provide a
-`console.log` function that writes out its arguments to _some_ text
-output device. In browsers, the output lands in the ((JavaScript
-console)). This part of the browser interface is hidden by default,
-but most browsers open it when you press F12 or, on Mac, when you
-press Command-Option-I. If that does not work, search through the
-menus for an item named “web console” or “developer tools”.
-
-ifdef::html_target[]
-
-When running the examples, or your own code, on the pages of this
-book, `console.log` output will be shown after the example, instead of
-in the browser's JavaScript console.
-
-endif::html_target[]
-
-[source,javascript]
-----
-var x = 30;
-console.log("the value of x is", x);
-// → the value of x is 30
-----
-
-(((object)))Though ((variable)) names cannot contain ((period
-character))s, `console.log` clearly has one. This is because
-`console.log` isn't a simple variable. It is actually an expression
-that retrieves the `log` ((property)) from the value held by the
-`console` variable. We will find out exactly what this means in
-link:04_data.html#properties[Chapter 4].
-
-[[return_values]]
-== Return values ==
-
-(((comparison,of numbers)))(((return value)))(((Math.max
-function)))(((maximum)))Showing a dialog box or writing text to
-the screen is a _((side effect))_. A lot of functions are useful
-because of the side effects they produce. Functions may also produce
-values, and in that case, they don't need to have a side effect to be
-useful. For example, the function `Math.max` takes any number of
-number values and gives back the greatest.
-
-[source,javascript]
-----
-console.log(Math.max(2, 4));
-// → 4
-----
-
-(((function,application)))(((minimum)))(((Math.min
-function)))When a function produces a value, it is said to _return_
-that value. Anything that produces a value is an ((expression)) in
-JavaScript, which means function calls can be used within larger
-expressions. Here a call to `Math.min`, which is the opposite of
-`Math.max`, is used as an input to the plus operator:
-
-[source,javascript]
-----
-console.log(Math.min(2, 4) + 100);
-// → 102
-----
-
-The link:03_functions.html#functions[next chapter] explains how to
-write your own functions.
-
-== prompt and confirm ==
-
-(((dialog box)))(((input)))(((browser)))(((confirm function)))Browser
-environments contain other functions besides `alert` for popping up
-windows. You can ask the user an “OK”/“Cancel” question using
-`confirm`. This returns a Boolean: `true` if the user clicks OK and
-`false` if the user clicks Cancel.
-
-[source,javascript]
-----
-confirm("Shall we, then?");
-----
-
-image::img/confirm.png[alt="A confirm dialog",width="8cm"]
-
-(((input)))(((prompt function)))(((text input)))The `prompt` function
-can be used to ask an “open” question. The first argument is the
-question, the second one is the text that the user starts with. A line
-of text can be typed into the dialog window, and the function will
-return this text as a string.
-
-[source,javascript]
-----
-prompt("Tell me everything you know.", "...");
-----
-
-image::img/prompt.png[alt="An prompt dialog",width="8cm"]
-
-These two functions aren't used much in modern web programming, mostly
-because you have no control over the way the resulting windows look,
-but they are useful for toy programs and experiments.
-
-== Control flow ==
-
-(((execution order)))(((program)))(((control flow)))When your program
-contains more than one ((statement)), the statements are executed,
-predictably, from top to bottom. As a basic example, this program has
-two statements. The first one asks the user for a number, and the
-second, which is executed afterward, shows the ((square)) of that
-number.
-
-[source,javascript]
-----
-var theNumber = Number(prompt("Pick a number", ""));
-alert("Your number is the square root of " +
- theNumber * theNumber);
-----
-
-(((number,conversion to)))(((type coercion)))(((Number
-function)))(((String function)))(((Boolean
-function)))(((Boolean,conversion to)))The function `Number` converts a
-value to a number. We need that conversion because the result of
-`prompt` is a string value, and we want a number. There are similar
-functions called `String` and `Boolean` that convert values to those
-types.
-
-Here is the rather trivial schematic representation of straight
-control flow:
-
-image::img/controlflow-straight.svg[alt="Trivial control flow",width="4cm"]
-
-== Conditional execution ==
-
-(((Boolean)))(((control flow)))Executing statements in straight-line
-order isn't the only option we have. An alternative is _((conditional
-execution))_, where we choose between two different routes based on a
-Boolean value.
-
-image::img/controlflow-if.svg[alt="Conditional control flow",width="4cm"]
-
-(((syntax)))(((Number function)))(((if keyword)))Conditional execution
-is written with the `if` keyword in JavaScript. In the simple case, we
-just want some code to be executed if, and only if, a certain
-condition holds. For example, in the previous program, we might want
-to show the square of the input only if the input is actually a
-number.
-
-[source,javascript]
-----
-var theNumber = Number(prompt("Pick a number", ""));
-if (!isNaN(theNumber))
- alert("Your number is the square root of " +
- theNumber * theNumber);
-----
-
-With this modification, if you enter “cheese”, no output will be shown.
-
-The keyword `if` executes or skips a statement depending on the value
-of a Boolean expression. The deciding expression is written after the
-keywords, between ((parentheses)), followed by the statement to execute.
-
-(((isNaN function)))The `isNaN` function is a standard JavaScript
-function that returns `true` only if the argument it is given is
-`NaN`. The `Number` function happens to return `NaN` when you give it
-a string that doesn't represent a valid number. Thus, the condition
-translates to “unless `theNumber` is not-a-number, do this”.
-
-(((else keyword)))You often won't just have code that executes when a
-condition holds true, but also code that handles the other case. This
-alternate path is represented by the second arrow in the previous
-diagram. The `else` keyword can be used, together with `if`, to create
-two separate, alternative execution paths.
-
-[source,javascript]
-----
-var theNumber = Number(prompt("Pick a number", ""));
-if (!isNaN(theNumber))
- alert("Your number is the square root of " +
- theNumber * theNumber);
-else
- alert("Hey. Why didn't you give me a number?");
-----
-
-(((if keyword,chaining)))If we have more than two paths to choose
-from, multiple `if`/`else` pairs can be “chained” together. Here's an
-example:
-
-[source,javascript]
-----
-var num = Number(prompt("Pick a number", "0"));
-
-if (num < 10)
- alert("Small");
-else if (num < 100)
- alert("Medium");
-else
- alert("Large");
-----
-
-The program will first check whether `num` is less than 10. If it is,
-it chooses that branch, shows `"Small"`, and is done. If it isn't, it
-takes the `else` branch, which itself contains a second `if`. If the
-second condition (`< 100`) holds, that means the number is between 10
-and 100, and `"Medium"` is shown. If it doesn't, the second, and last,
-`else` branch is chosen.
-
-The flow chart for this program looks something like this:
-
-image::img/controlflow-nested-if.svg[alt="Nested if control flow",width="4cm"]
-
-[[loops]]
-== while and do loops ==
-
-(((even number)))Consider a program that prints all even numbers from
-0 to 12. One way to write this is as follows:
-
-[source,javascript]
-----
-console.log(0);
-console.log(2);
-console.log(4);
-console.log(6);
-console.log(8);
-console.log(10);
-console.log(12);
-----
-
-(((control flow)))That works, but the idea of writing a program is to
-make something _less_ work, not more. If we needed all even numbers
-less than 1,000, the previous would be unworkable. What we need is a
-way to repeat some code—a _((loop))_.
-
-image::img/controlflow-loop.svg[alt="Loop control flow",width="4cm"]
-
-(((syntax)))(((counter variable)))Looping control flow allows us to go
-back to some point in the program where we were before and repeat it
-with our current program state. If we combine this with a variable
-that counts, we can do something like this:
-
-[source,javascript]
-----
-var number = 0;
-while (number <= 12) {
- console.log(number);
- number = number + 2;
-}
-// → 0
-// → 2
-// … etcetera
-----
-
-(((while loop)))(((Boolean)))A ((statement)) starting with the
-keyword `while` creates a loop. The word `while` is followed by an
-((expression)) in ((parentheses)) and then a statement, much like `if`.
-The loop executes that statement as long as the expression produces a
-value that is `true` when converted to Boolean type.
-
-(((grouping)))((({} (block))))(((block)))In this loop, we want to both
-print the current number and add two to our variable. Whenever we need
-to execute multiple ((statement))s inside a loop, we wrap them in
-((curly braces)) (`{` and `}`). Braces do for statements what
-((parentheses)) do for expressions: they group them together, making
-them count as a single statement. A sequence of statements wrapped in
-braces is called a _block_.
-
-(((programming style)))Many JavaScript programmers wrap every single
-loop or `if` body in braces. They do this both for the sake of
-consistency and to avoid having to add or remove braces when changing
-the number of statements in the body later. In this book, I will write
-most single-statement bodies without braces, since I value brevity.
-You are free to go with whichever style you prefer.
-
-(((comparison)))(((state)))The variable `number` demonstrates the way
-a ((variable)) can track the progress of a program. Every time the
-loop repeats, `number` is incremented by `2`. Then, at the beginning
-of every repetition, it is compared with the number `12` to decide
-whether the program has done all the work it intended to do.
-
-(((exponentiation)))As an example that actually does something useful,
-we can now write a program that calculates and shows the value of
-2^10^ (2 to the 10th power). We use two variables: one to keep
-track of our result and one to count how often we have multiplied this
-result by 2. The loop tests whether the second variable has reached 10
-yet and then updates both variables.
-
-[source,javascript]
-----
-var result = 1;
-var counter = 0;
-while (counter < 10) {
- result = result * 2;
- counter = counter + 1;
-}
-console.log(result);
-// → 1024
-----
-
-The counter could also start at `1` and check for `<= 10`, but, for
-reasons that will become apparent in
-link:04_data.html#array_indexing[Chapter 4], it is a good idea to get
-used to counting from 0.
-
-(((loop body)))(((do loop)))(((control flow)))The `do` loop is a
-control structure similar to the `while` loop. It differs only on one
-point: a `do` loop always executes its body at least once, and it
-starts testing whether it should stop only after that first execution.
-To reflect this, the test appears after the body of the loop:
-
-[source,javascript]
-----
-do {
- var name = prompt("Who are you?");
-} while (!name);
-console.log(name);
-----
-
-(((Boolean,conversion to)))(((! operator)))This program will
-force you to enter a name. It will ask again and again until it gets
-something that is not an empty string. Applying the `!` operator will
-convert a value to Boolean type before negating it, and all strings
-except `""` convert to `true`.
-
-== Indenting Code ==
-
-(((block)))(((code structure)))(((whitespace)))(((programming
-style)))You've probably noticed the spaces I put in front of some
-statements. In JavaScript, these are not required—the computer will
-accept the program just fine without them. In fact, even the ((line))
-breaks in programs are optional. You could write a program as a single
-long line if you felt like it. The role of the ((indentation)) inside
-blocks is to make the structure of the code stand out. In complex
-code, where new blocks are opened inside other blocks, it can become
-hard to see where one block ends and another begins. With proper
-indentation, the visual shape of a program corresponds to the shape of
-the blocks inside it. I like to use two spaces for every open block,
-but tastes differ—some people use four spaces, and some people use
-((tab character))s.
-
-== for loops ==
-
-(((syntax)))(((while loop)))(((counter variable)))Many loops follow
-the pattern seen in the previous `while` examples. First, a “counter”
-variable is created to track the progress of the loop. Then comes a
-`while` loop, whose test expression usually checks whether the counter
-has reached some boundary yet. At the end of the loop body, the
-counter is updated to track progress.
-
-(((for loop)))(((loop)))Because this pattern is so common, JavaScript and
-similar languages provide a slightly shorter and more comprehensive
-form, the `for` loop.
-
-[source,javascript]
-----
-for (var number = 0; number <= 12; number = number + 2)
- console.log(number);
-// → 0
-// → 2
-// … etcetera
-----
-
-(((control flow)))(((state)))This program is exactly equivalent to the
-link:02_program_structure.html#loops[earlier] even-number-printing
-example. The only change is that all the ((statement))s that are
-related to the “state” of the loop are now grouped together.
-
-The ((parentheses)) after a `for` keyword must contain two
-((semicolon))s. The part before the first semicolon _initializes_ the
-loop, usually by defining a ((variable)). The second part is the
-((expression)) that _checks_ whether the loop must continue. The final
-part _updates_ the state of the loop after every iteration. In most
-cases, this is shorter and clearer than a `while` construct.
-
-(((exponentiation)))Here is the code that computes 2^10^, using `for`
-instead of `while`:
-
-[source,javascript]
-----
-var result = 1;
-for (var counter = 0; counter < 10; counter = counter + 1)
- result = result * 2;
-console.log(result);
-// → 1024
-----
-
-(((programming style)))(((indentation)))Note that even though no block
-is opened with a `{`, the statement in the loop is still indented two
-spaces to make it clear that it “belongs” to the line before it.
-
-== Breaking Out of a Loop ==
-
-(((loop,termination of)))(((break keyword)))Having the loop's
-condition produce `false` is not the only way a loop can finish. There
-is a special statement called `break` that has the effect of
-immediately jumping out of the enclosing loop.
-
-This program illustrates the `break` statement. It finds the first number
-that is both greater than or equal to 20 and divisible by 7.
-
-[source,javascript]
-----
-for (var current = 20; ; current++) {
- if (current % 7 == 0)
- break;
-}
-console.log(current);
-// → 21
-----
-
-(((remainder operator)))(((% operator)))The trick with the remainder
-(`%`) operator is an easy way to test whether a number is divisible by
-another number. If it is, the remainder of their division is zero.
-
-(((for loop)))The `for` construct in the example does not have a part
-that checks for the end of the loop. This means that the loop will
-never stop unless the `break` statement inside is executed.
-
-If you were to leave out that `break` statement or accidentally write
-a condition that always produces `true`, your program would get stuck
-in an _((infinite loop))_. A program stuck in an infinite loop will
-never finish running, which is usually a bad thing.
-
-ifdef::html_target[]
-
-If you create an infinite loop in one of the examples on these pages,
-you'll usually be asked whether you want to stop the script after a
-few seconds. If that fails, you will have to close the tab that you're
-working in, or on some browsers close your whole browser, in order to
-recover.
-
-endif::html_target[]
-
-(((continue keyword)))The `continue` keyword is similar to `break`, in
-that it influences the progress of a loop. When `continue` is
-encountered in a loop body, control jumps out of the loop body, and
-continues with the loop's next iteration.
-
-== Updating variables succinctly ==
-
-(((assignment)))(((+= operator)))(((-= operator)))(((/=
-operator)))(((*= operator)))(((state)))(((side effect)))Especially
-when looping, a program often needs to “update” a variable to hold a
-value based on that variable's previous value.
-
-// test: no
-
-[source,javascript]
-----
-counter = counter + 1;
-----
-
-JavaScript provides a shortcut for this:
-
-// test: no
-
-[source,javascript]
-----
-counter += 1;
-----
-
-Similar shortcuts work for many other operators, such as `result *= 2` to
-double `result` or `counter -= 1` to count downward.
-
-This allows us to shorten our counting example a little more.
-
-[source,javascript]
-----
-for (var number = 0; number <= 12; number += 2)
- console.log(number);
-----
-
-(((++ operator)))(((-- operator)))For `counter += 1` and `counter -=
-1`, there are even shorter equivalents: `counter++` and `counter--`.
-
-== Dispatching on a value with switch ==
-
-(((syntax)))(((conditional execution)))(((dispatching)))(((if
-keyword,chaining)))It is common for code to look like this:
-
-// test: no
-
-[source,javascript]
-----
-if (variable == "value1") action1();
-else if (variable == "value2") action2();
-else if (variable == "value3") action3();
-else defaultAction();
-----
-
-(((colon character)))(((switch keyword)))There is a construct called
-`switch` that is intended to solve such a “dispatch” in a more direct
-way. Unfortunately, the syntax JavaScript uses for this (which it
-inherited from the C/Java line of programming languages) is somewhat
-awkward—a chain of `if` statements often looks better. Here is an
-example:
-
-[source,javascript]
-----
-switch (prompt("What is the weather like?")) {
- case "rainy":
- console.log("Remember to bring an umbrella.");
- break;
- case "sunny":
- console.log("Dress lightly.");
- case "cloudy":
- console.log("Go outside.");
- break;
- default:
- console.log("Unknown weather type!");
- break;
-}
-----
-
-(((fallthrough)))(((comparison)))(((break keyword)))(((case
-keyword)))(((default keyword)))You may put any number of `case` labels
-inside the block opened by `switch`. The program will jump to the
-label that corresponds to the value that `switch` was given or to
-`default` if no matching value is found. It starts executing
-statements there, even if they're under another label, until it
-reaches a `break` statement. In some cases, such as the `"sunny"` case
-in the example, this can be used to share some code between cases (it
-recommends going outside for both sunny and cloudy weather). But
-beware: it is easy to forget such a `break`, which will cause the
-program to execute code you do not want executed.
-
-== Capitalization ==
-
-(((capitalization)))(((variable,naming)))(((whitespace)))Variable
-names may not contain spaces, yet it is often helpful to use multiple
-words to clearly describe what the variable represents. These are
-pretty much your choices for writing a variable name with several
-words in it:
-
-----
-fuzzylittleturtle
-fuzzy_little_turtle
-FuzzyLittleTurtle
-fuzzyLittleTurtle
-----
-
-(((camel case)))(((programming style)))(((underscore character)))The
-first style can be hard to read. Personally, I like the look of the
-underscores, though that style is a little painful to type. The
-((standard)) JavaScript functions, and most JavaScript programmers,
-follow the bottom style—they capitalize every word except the first.
-It is not hard to get used to little things like that, and code with
-mixed naming styles can be jarring to read, so we will just follow
-this ((convention)).
-
-(((Number function)))(((constructor)))In a few cases, such as the
-`Number` function, the first letter of a variable is also capitalized.
-This was done to mark this function as a constructor. What a
-constructor is will become clear in
-link:06_object.html#constructors[Chapter 6]. For now, the important
-thing is not to be bothered by this apparent lack of ((consistency)).
-
-== Comments ==
-
-(((readability)))Often, raw code does not convey all the information
-you want a program to convey to human readers, or it conveys it in
-such a cryptic way that people might not understand it. At other
-times, you might just feel poetic or want to include some thoughts as
-part of your program. This is what _((comment))s_ are for.
-
-(((slash character)))(((line comment)))A comment is a piece of text
-that is part of a program but is completely ignored by the computer.
-JavaScript has two ways of writing comments. To write a single-line
-comment, you can use two slash characters (`//`) and then the comment
-text after it.
-
-// test: no
-
-[source,javascript]
-----
-var accountBalance = calculateBalance(account);
-// It's a green hollow where a river sings
-accountBalance.adjust();
-// Madly catching white tatters in the grass.
-var report = new Report();
-// Where the sun on the proud mountain rings:
-addToReport(accountBalance, report);
-// It's a little valley, foaming like light in a glass.
-----
-
-(((block comment)))A `//` comment goes only to the end of the line. A
-section of text between `/*` and `*/` will be ignored, regardless of
-whether it contains line breaks. This is often useful for adding
-blocks of information about a file or a chunk of program.
-
-[source,javascript]
-----
-/*
- I first found this number scrawled on the back of one of
- my notebooks a few years ago. Since then, it has often
- dropped by, showing up in phone numbers and the serial
- numbers of products that I've bought. It obviously likes
- me, so I've decided to keep it.
-*/
-var myNumber = 11213;
-----
-
-== Summary ==
-
-You now know that a program is built out of statements, which
-themselves sometimes contain more statements. Statements tend to
-contain expressions, which themselves can be built out of smaller
-expressions.
-
-Putting statements after one another gives you a program that is
-executed from top to bottom. You can introduce disturbances in the
-flow of control by using conditional (`if`, `else`, and `switch`) and
-looping (`while`, `do`, and `for`) statements.
-
-Variables can be used to file pieces of data under a name, and they
-are useful for tracking state in your program. The environment is the
-set of variables that are defined in a program. JavaScript systems
-always put a number of useful standard variables into your
-environment.
-
-Functions are special values that encapsulate a piece of program. You
-can invoke them by writing `functionName(argument1, argument2)`. Such
-a function call is an expression, and may produce a value.
-
-== Exercises ==
-
-(((exercises)))If you are unsure how to try your solutions to
-exercises, refer to the end of the introduction.
-
-Each exercise starts with a problem description. Read that and try to
-solve the exercise. If you run into problems, consider reading the
-hints (!html after the exercise!)(!tex at the link:solutions[end of the book]!).
-Full solutions to the exercises are not included in this
-book, but you can find them online at
-http://eloquentjavascript.net/code[_eloquentjavascript.net/code_].
-If you want to learn something from the exercises, I recommend looking
-at the solutions only after you've solved the exercise, or at least
-after you've attacked it long and hard enough to have a slight
-headache.
-
-=== Looping a triangle ===
-
-(((triangle (exercise))))(((loop)))Write a program that makes seven
-calls to `console.log` to output the following triangle:
-
-----
-#
-##
-###
-####
-#####
-######
-#######
-----
-
-ifdef::html_target[]
-[source,javascript]
-----
-// Your code here.
-// (Click code listings to edit them.)
-----
-endif::html_target[]
-
-!!solution!!
-
-(((triangle (exercise))))You can start with a program that simply
-prints out the numbers 1 to 7, which you can derive by making a few
-modifications to the
-link:02_program_structure.html#loops[even-number-printing example]
-given earlier in the chapter, where the `for` loop was introduced.
-
-Now consider the equivalence between numbers and strings of hash
-characters. You can go from 1 to 2 by adding 1 (`+= 1`). You can go
-from `"#"` to `"##"` by adding a character (`+= "#"`). Thus, your
-solution can closely follow the number-printing program.
-
-!!solution!!
-
-=== FizzBuzz ===
-
-(((FizzBuzz (exercise))))(((loop)))(((conditional execution)))Write a
-program that uses `console.log` to print all the numbers from 1 to
-100, with two exceptions. For numbers divisible by 3, print `"Fizz"`
-instead of the number, and for numbers divisible by 5 (and not 3),
-print `"Buzz"` instead.
-
-When you have that working, modify your program to print `"FizzBuzz"`
-for numbers that are divisible by both 3 and 5.
-
-(This is actually an ((interview question)) that has been claimed to
-weed out a significant percentage of programmer candidates. So if you
-solved it, you're now allowed to feel good about yourself.)
-
-ifdef::html_target[]
-[source,javascript]
-----
-// Your code here.
-----
-endif::html_target[]
-
-!!solution!!
-
-(((FizzBuzz (exercise))))(((remainder operator)))(((% operator)))Going
-over the numbers is clearly a looping job, and selecting what to print
-is a matter of conditional execution. Remember the trick of using the
-remainder (`%`) operator for checking whether a number is divisible by
-another number (has a remainder of zero).
-
-In the first version, there are three possible outcomes for every
-number, so you'll have to create an `if`/`else if`/`else` chain.
-
-(((|| operator)))(((if keyword,chaining)))The second version of the
-program has a straightforward solution and a clever one. The simple
-way is to add another “branch” to precisely test the given condition.
-For the clever method, build up a string containing the word or words
-to output, and print either this word or the number if there is no
-word, potentially by making elegant use of the `||` operator.
-
-!!solution!!
-
-=== Chess board ===
-
-(((chess board (exercise))))(((loop)))(((nesting,of loops)))(((newline
-character)))Write a program that creates a string that represents an
-8×8 grid, using newline characters to separate lines. At each position
-of the grid there is either a space or a “#” character. The characters
-should form a chess board.
-
-Passing this string to `console.log` should show something like this:
-
-----
-# # # #
- # # # #
-# # # #
- # # # #
-# # # #
- # # # #
-# # # #
- # # # #
-----
-
-When you have a program that generates this pattern, define a
-((variable)) `size = 8` and change the program so that it works for
-any `size`, outputting a grid of the given width and height.
-
-ifdef::html_target[]
-[source,javascript]
-----
-// Your code here.
-----
-endif::html_target[]
-
-!!solution!!
-
-(((chess board (exercise))))The string can be built by starting with
-an empty one (`""`) and repeatedly adding characters. A newline
-character is written `"\n"`.
-
-Use `console.log` to inspect the output of your program.
-
-(((nesting,of loops)))To work with two ((dimensions)), you will need a
-((loop)) inside of a loop. Put ((curly braces)) around the bodies of
-both loops to make it easy to see where they start and end. Try to
-properly indent these bodies. The order of the loops must follow the
-order in which we build up the string (line by line, left to right,
-top to bottom). So the outer loop handles the lines and the inner loop
-handles the characters on a line.
-
-(((counter variable)))(((remainder operator)))(((% operator)))You'll
-need two variables to track your progress. To know whether to put a
-space or a hash sign at a given position, you could test whether the
-sum of the two counters is even (`% 2`).
-
-Terminating a line by adding a newline character happens after the
-line has been built up, so do this after the inner loop but inside of
-the outer loop.
-
-!!solution!!
diff --git a/03_functions.md b/03_functions.md
new file mode 100644
index 000000000..9f5ce287f
--- /dev/null
+++ b/03_functions.md
@@ -0,0 +1,762 @@
+# Functions
+
+{{quote {author: "Donald Knuth", chapter: true}
+
+People think that computer science is the art of geniuses but the actual reality is the opposite, just many people doing things that build on each other, like a wall of mini stones.
+
+quote}}
+
+{{index "Knuth, Donald"}}
+
+{{figure {url: "img/chapter_picture_3.jpg", alt: "Illustration of fern leaves with a fractal shape, bees in the background", chapter: framed}}}
+
+{{index function, [code, "structure of"]}}
+
+Functions are one of the most central tools in JavaScript programming. The concept of wrapping a piece of program in a value has many uses. It gives us a way to structure larger programs, to reduce repetition, to associate names with subprograms, and to isolate these subprograms from each other.
+
+The most obvious application of functions is defining new ((vocabulary)). Creating new words in prose is usually bad style, but in programming, it is indispensable.
+
+{{index abstraction, vocabulary}}
+
+Typical adult English speakers have some 20,000 words in their vocabulary. Few programming languages come with 20,000 commands built in. And the vocabulary that _is_ available tends to be more precisely defined, and thus less flexible, than in human language. Therefore, we _have_ to introduce new words to avoid excessive verbosity.
+
+## Defining a function
+
+{{index "square example", [function, definition], [binding, definition]}}
+
+A function definition is a regular binding where the value of the binding is a function. For example, this code defines `square` to refer to a function that produces the square of a given number:
+
+```
+const square = function(x) {
+ return x * x;
+};
+
+console.log(square(12));
+// → 144
+```
+
+{{indexsee "curly braces", braces}}
+{{index [braces, "function body"], block, [syntax, function], "function keyword", [function, body], [function, "as value"], [parentheses, arguments]}}
+
+A function is created with an expression that starts with the keyword `function`. Functions have a set of _((parameter))s_ (in this case, only `x`) and a _body_, which contains the statements that are to be executed when the function is called. The body of a function created this way must always be wrapped in braces, even when it consists of only a single ((statement)).
+
+{{index "roundTo example"}}
+
+A function can have multiple parameters or no parameters at all. In the following example, `makeNoise` does not list any parameter names, whereas `roundTo` (which rounds `n` to the nearest multiple of `step`) lists two:
+
+```
+const makeNoise = function() {
+ console.log("Pling!");
+};
+
+makeNoise();
+// → Pling!
+
+const roundTo = function(n, step) {
+ let remainder = n % step;
+ return n - remainder + (remainder < step / 2 ? 0 : step);
+};
+
+console.log(roundTo(23, 10));
+// → 20
+```
+
+{{index "return value", "return keyword", undefined}}
+
+Some functions, such as `roundTo` and `square`, produce a value, and some don't, such as `makeNoise`, whose only result is a ((side effect)). A `return` statement determines the value the function returns. When control comes across such a statement, it immediately jumps out of the current function and gives the returned value to the code that called the function. A `return` keyword without an expression after it will cause the function to return `undefined`. Functions that don't have a `return` statement at all, such as `makeNoise`, similarly return `undefined`.
+
+{{index parameter, [function, application], [binding, "from parameter"]}}
+
+Parameters to a function behave like regular bindings, but their initial values are given by the _caller_ of the function, not the code in the function itself.
+
+## Bindings and scopes
+
+{{indexsee "top-level scope", "global scope"}}
+{{index "var keyword", "global scope", [binding, global], [binding, "scope of"]}}
+
+Each binding has a _((scope))_, which is the part of the program in which the binding is visible. For bindings defined outside of any function, block, or module (see [Chapter ?](modules)), the scope is the whole program—you can refer to such bindings wherever you want. These are called _global_.
+
+{{index "local scope", [binding, local]}}
+
+Bindings created for function ((parameter))s or declared inside a function can be referenced only in that function, so they are known as _local_ bindings. Every time the function is called, new instances of these bindings are created. This provides some isolation between functions—each function call acts in its own little world (its local environment) and can often be understood without knowing a lot about what's going on in the global environment.
+
+{{index "let keyword", "const keyword", "var keyword"}}
+
+Bindings declared with `let` and `const` are in fact local to the _((block))_ in which they are declared, so if you create one of those inside of a loop, the code before and after the loop cannot "see" it. In pre-2015 JavaScript, only functions created new scopes, so old-style bindings, created with the `var` keyword, are visible throughout the whole function in which they appear—or throughout the global scope, if they are not in a function.
+
+```
+let x = 10; // global
+if (true) {
+ let y = 20; // local to block
+ var z = 30; // also global
+}
+```
+
+{{index [binding, visibility]}}
+
+Each ((scope)) can "look out" into the scope around it, so `x` is visible inside the block in the example. The exception is when multiple bindings have the same name—in that case, code can see only the innermost one. For example, when the code inside the `halve` function refers to `n`, it is seeing its _own_ `n`, not the global `n`.
+
+```
+const halve = function(n) {
+ return n / 2;
+};
+
+let n = 10;
+console.log(halve(100));
+// → 50
+console.log(n);
+// → 10
+```
+
+{{id scoping}}
+
+## Nested scope
+
+{{index [nesting, "of functions"], [nesting, "of scope"], scope, "inner function", "lexical scoping"}}
+
+JavaScript distinguishes not just global and local bindings. Blocks and functions can be created inside other blocks and functions, producing multiple degrees of locality.
+
+{{index "landscape example"}}
+
+For example, this function—which outputs the ingredients needed to make a batch of hummus—has another function inside it:
+
+```
+const hummus = function(factor) {
+ const ingredient = function(amount, unit, name) {
+ let ingredientAmount = amount * factor;
+ if (ingredientAmount > 1) {
+ unit += "s";
+ }
+ console.log(`${ingredientAmount} ${unit} ${name}`);
+ };
+ ingredient(1, "can", "chickpeas");
+ ingredient(0.25, "cup", "tahini");
+ ingredient(0.25, "cup", "lemon juice");
+ ingredient(1, "clove", "garlic");
+ ingredient(2, "tablespoon", "olive oil");
+ ingredient(0.5, "teaspoon", "cumin");
+};
+```
+
+{{index [function, scope], scope}}
+
+The code inside the `ingredient` function can see the `factor` binding from the outer function, but its local bindings, such as `unit` or `ingredientAmount`, are not visible in the outer function.
+
+The set of bindings visible inside a block is determined by the place of that block in the program text. Each local scope can also see all the local scopes that contain it, and all scopes can see the global scope. This approach to binding visibility is called _((lexical scoping))_.
+
+## Functions as values
+
+{{index [function, "as value"], [binding, definition]}}
+
+A function binding usually simply acts as a name for a specific piece of the program. Such a binding is defined once and never changed. This makes it easy to confuse the function and its name.
+
+{{index [binding, assignment]}}
+
+But the two are different. A function value can do all the things that other values can do—you can use it in arbitrary ((expression))s, not just call it. It is possible to store a function value in a new binding, pass it as an argument to a function, and so on. Similarly, a binding that holds a function is still just a regular binding and can, if not constant, be assigned a new value, like so:
+
+```{test: no}
+let launchMissiles = function() {
+ missileSystem.launch("now");
+};
+if (safeMode) {
+ launchMissiles = function() {/* do nothing */};
+}
+```
+
+{{index [function, "higher-order"]}}
+
+In [Chapter ?](higher_order), we'll discuss the interesting things that we can do by passing function values to other functions.
+
+## Declaration notation
+
+{{index [syntax, function], "function keyword", "square example", [function, definition], [function, declaration]}}
+
+There is a slightly shorter way to create a function binding. When the `function` keyword is used at the start of a statement, it works differently:
+
+```{test: wrap}
+function square(x) {
+ return x * x;
+}
+```
+
+{{index future, "execution order"}}
+
+This is a function _declaration_. The statement defines the binding `square` and points it at the given function. It is slightly easier to write and doesn't require a semicolon after the function.
+
+There is one subtlety with this form of function definition.
+
+```
+console.log("The future says:", future());
+
+function future() {
+ return "You'll never have flying cars";
+}
+```
+
+The preceding code works, even though the function is defined _below_ the code that uses it. Function declarations are not part of the regular top-to-bottom flow of control. They are conceptually moved to the top of their scope and can be used by all the code in that scope. This is sometimes useful because it offers the freedom to order code in a way that seems the clearest, without worrying about having to define all functions before they are used.
+
+## Arrow functions
+
+{{index function, "arrow function"}}
+
+There's a third notation for functions, which looks very different from the others. Instead of the `function` keyword, it uses an arrow (`=>`) made up of an equal sign and a greater-than character (not to be confused with the greater-than-or-equal operator, which is written `>=`):
+
+```{test: wrap}
+const roundTo = (n, step) => {
+ let remainder = n % step;
+ return n - remainder + (remainder < step / 2 ? 0 : step);
+};
+```
+
+{{index [function, body]}}
+
+The arrow comes _after_ the list of parameters and is followed by the function's body. It expresses something like "this input (the ((parameter))s) produces this result (the body)".
+
+{{index [braces, "function body"], "square example", [parentheses, arguments]}}
+
+When there is only one parameter name, you can omit the parentheses around the parameter list. If the body is a single expression rather than a ((block)) in braces, that expression will be returned from the function. So, these two definitions of `square` do the same thing:
+
+```
+const square1 = (x) => { return x * x; };
+const square2 = x => x * x;
+```
+
+{{index [parentheses, arguments]}}
+
+When an arrow function has no parameters at all, its parameter list is just an empty set of parentheses.
+
+```
+const horn = () => {
+ console.log("Toot");
+};
+```
+
+{{index verbosity}}
+
+There's no deep reason to have both arrow functions and `function` expressions in the language. Apart from a minor detail, which we'll discuss in [Chapter ?](object), they do the same thing. Arrow functions were added in 2015, mostly to make it possible to write small function expressions in a less verbose way. We'll use them often in [Chapter ?](higher_order).
+
+{{id stack}}
+
+## The call stack
+
+{{indexsee stack, "call stack"}}
+{{index "call stack", [function, application]}}
+
+The way control flows through functions is somewhat involved. Let's take a closer look at it. Here is a simple program that makes a few function calls:
+
+```
+function greet(who) {
+ console.log("Hello " + who);
+}
+greet("Harry");
+console.log("Bye");
+```
+
+{{index ["control flow", functions], "execution order", "console.log"}}
+
+A run through this program goes roughly like this: the call to `greet` causes control to jump to the start of that function (line 2). The function calls `console.log`, which takes control, does its job, and then returns control to line 2. There, it reaches the end of the `greet` function, so it returns to the place that called it—line 4. The line after that calls `console.log` again. After that returns, the program reaches its end.
+
+We could show the flow of control schematically like this:
+
+```{lang: null}
+not in function
+ in greet
+ in console.log
+ in greet
+not in function
+ in console.log
+not in function
+```
+
+{{index "return keyword", [memory, call stack]}}
+
+Because a function has to jump back to the place that called it when it returns, the computer must remember the context from which the call happened. In one case, `console.log` has to return to the `greet` function when it is done. In the other case, it returns to the end of the program.
+
+The place where the computer stores this context is the _((call stack))_. Every time a function is called, the current context is stored on top of this stack. When a function returns, it removes the top context from the stack and uses that context to continue execution.
+
+{{index "infinite loop", "stack overflow", recursion}}
+
+Storing this stack requires space in the computer's memory. When the stack grows too big, the computer will fail with a message like "out of stack space" or "too much recursion". The following code illustrates this by asking the computer a really hard question that causes an infinite back-and-forth between two functions. Or rather, it _would_ be infinite, if the computer had an infinite stack. As it is, we will run out of space, or "blow the stack".
+
+```{test: no}
+function chicken() {
+ return egg();
+}
+function egg() {
+ return chicken();
+}
+console.log(chicken() + " came first.");
+// → ??
+```
+
+## Optional Arguments
+
+{{index argument, [function, application]}}
+
+The following code is allowed and executes without any problem:
+
+```
+function square(x) { return x * x; }
+console.log(square(4, true, "hedgehog"));
+// → 16
+```
+
+We defined `square` with only one ((parameter)). Yet when we call it with three, the language doesn't complain. It ignores the extra arguments and computes the square of the first one.
+
+{{index undefined}}
+
+JavaScript is extremely broad-minded about the number of arguments you can pass to a function. If you pass too many, the extra ones are ignored. If you pass too few, the missing parameters are assigned the value `undefined`.
+
+The downside of this is that it is possible—likely, even—that you'll accidentally pass the wrong number of arguments to functions. And no one will tell you about it. The upside is that you can use this behavior to allow a function to be called with different numbers of arguments. For example, this `minus` function tries to imitate the `-` operator by acting on either one or two arguments:
+
+```
+function minus(a, b) {
+ if (b === undefined) return -a;
+ else return a - b;
+}
+
+console.log(minus(10));
+// → -10
+console.log(minus(10, 5));
+// → 5
+```
+
+{{id roundTo}}
+{{index "optional argument", "default value", parameter, ["= operator", "for default value"] "roundTo example"}}
+
+If you write an `=` operator after a parameter, followed by an expression, the value of that expression will replace the argument when it is not given. For example, this version of `roundTo` makes its second argument optional. If you don't provide it or pass the value `undefined`, it will default to one:
+
+```{test: wrap}
+function roundTo(n, step = 1) {
+ let remainder = n % step;
+ return n - remainder + (remainder < step / 2 ? 0 : step);
+};
+
+console.log(roundTo(4.5));
+// → 5
+console.log(roundTo(4.5, 2));
+// → 4
+```
+
+{{index "console.log"}}
+
+The [next chapter](data#rest_parameters) will introduce a way in which a function body can get at the whole list of arguments it was passed. This is helpful because it allows a function to accept any number of arguments. For example, `console.log` does this, outputting all the values it is given:
+
+```
+console.log("C", "O", 2);
+// → C O 2
+```
+
+## Closure
+
+{{index "call stack", "local binding", [function, "as value"], scope}}
+
+The ability to treat functions as values, combined with the fact that local bindings are re-created every time a function is called, brings up an interesting question: What happens to local bindings when the function call that created them is no longer active?
+
+The following code shows an example of this. It defines a function, `wrapValue`, that creates a local binding. It then returns a function that accesses and returns this local binding.
+
+```
+function wrapValue(n) {
+ let local = n;
+ return () => local;
+}
+
+let wrap1 = wrapValue(1);
+let wrap2 = wrapValue(2);
+console.log(wrap1());
+// → 1
+console.log(wrap2());
+// → 2
+```
+
+This is allowed and works as you'd hope—both instances of the binding can still be accessed. This situation is a good demonstration of the fact that local bindings are created anew for every call, and different calls don't affect each other's local bindings.
+
+This feature—being able to reference a specific instance of a local binding in an enclosing scope—is called _((closure))_. A function that references bindings from local scopes around it is called _a_ closure. This behavior not only frees you from having to worry about the lifetimes of bindings but also makes it possible to use function values in some creative ways.
+
+{{index "multiplier function"}}
+
+With a slight change, we can turn the previous example into a way to create functions that multiply by an arbitrary amount.
+
+```
+function multiplier(factor) {
+ return number => number * factor;
+}
+
+let twice = multiplier(2);
+console.log(twice(5));
+// → 10
+```
+
+{{index [binding, "from parameter"]}}
+
+The explicit `local` binding from the `wrapValue` example isn't really needed since a parameter is itself a local binding.
+
+{{index [function, "model of"]}}
+
+Thinking about programs like this takes some practice. A good mental model is to think of function values as containing both the code in their body and the environment in which they are created. When called, the function body sees the environment in which it was created, not the environment in which it is called.
+
+In the previous example, `multiplier` is called and creates an environment in which its `factor` parameter is bound to 2. The function value it returns, which is stored in `twice`, remembers this environment so that when that is called, it multiplies its argument by 2.
+
+## Recursion
+
+{{index "power example", "stack overflow", recursion, [function, application]}}
+
+It is perfectly okay for a function to call itself, as long as it doesn't do it so often that it overflows the stack. A function that calls itself is called _recursive_. Recursion allows some functions to be written in a different style. Take, for example, this `power` function, which does the same as the `**` (exponentiation) operator:
+
+```{test: wrap}
+function power(base, exponent) {
+ if (exponent == 0) {
+ return 1;
+ } else {
+ return base * power(base, exponent - 1);
+ }
+}
+
+console.log(power(2, 3));
+// → 8
+```
+
+{{index loop, readability, mathematics}}
+
+This is rather close to the way mathematicians define exponentiation and arguably describes the concept more clearly than the loop we used in [Chapter ?](program_structure). The function calls itself multiple times with ever smaller exponents to achieve the repeated multiplication.
+
+{{index [function, application], efficiency}}
+
+However, this implementation has one problem: in typical JavaScript implementations, it's about three times slower than a version using a `for` loop. Running through a simple loop is generally cheaper than calling a function multiple times.
+
+{{index optimization}}
+
+The dilemma of speed versus ((elegance)) is an interesting one. You can see it as a kind of continuum between human-friendliness and machine-friendliness. Almost any program can be made faster by making it bigger and more convoluted. The programmer has to find an appropriate balance.
+
+In the case of the `power` function, an inelegant (looping) version is still fairly simple and easy to read. It doesn't make much sense to replace it with a recursive function. Often, though, a program deals with such complex concepts that giving up some efficiency in order to make the program more straightforward is helpful.
+
+{{index profiling}}
+
+Worrying about efficiency can be a distraction. It's yet another factor that complicates program design, and when you're doing something that's already difficult, that extra thing to worry about can be paralyzing.
+
+{{index "premature optimization"}}
+
+Therefore, you should generally start by writing something that's correct and easy to understand. If you're worried that it's too slow—which it usually isn't since most code simply isn't executed often enough to take any significant amount of time—you can measure afterward and improve it if necessary.
+
+{{index "branching recursion"}}
+
+Recursion is not always just an inefficient alternative to looping. Some problems really are easier to solve with recursion than with loops. Most often these are problems that require exploring or processing several "branches", each of which might branch out again into even more branches.
+
+{{id recursive_puzzle}}
+{{index recursion, "number puzzle example"}}
+
+Consider this puzzle: by starting from the number 1 and repeatedly either adding 5 or multiplying by 3, an infinite set of numbers can be produced. How would you write a function that, given a number, tries to find a sequence of such additions and multiplications that produces that number? For example, the number 13 could be reached by first multiplying by 3 and then adding 5 twice, whereas the number 15 cannot be reached at all.
+
+Here is a recursive solution:
+
+```
+function findSolution(target) {
+ function find(current, history) {
+ if (current == target) {
+ return history;
+ } else if (current > target) {
+ return null;
+ } else {
+ return find(current + 5, `(${history} + 5)`) ??
+ find(current * 3, `(${history} * 3)`);
+ }
+ }
+ return find(1, "1");
+}
+
+console.log(findSolution(24));
+// → (((1 * 3) + 5) * 3)
+```
+
+Note that this program doesn't necessarily find the _shortest_ sequence of operations. It is satisfied when it finds any sequence at all.
+
+It's okay if you don't see how this code works right away. Let's work through it since it makes for a great exercise in recursive thinking.
+
+The inner function `find` does the actual recursing. It takes two ((argument))s: the current number and a string that records how we reached this number. If it finds a solution, it returns a string that shows how to get to the target. If it can find no solution starting from this number, it returns `null`.
+
+{{index null, "?? operator", "short-circuit evaluation"}}
+
+To do this, the function performs one of three actions. If the current number is the target number, the current history is a way to reach that target, so it is returned. If the current number is greater than the target, there's no sense in further exploring this branch because both adding and multiplying will only make the number bigger, so it returns `null`. Finally, if we're still below the target number, the function tries both possible paths that start from the current number by calling itself twice, once for addition and once for multiplication. If the first call returns something that is not `null`, it is returned. Otherwise, the second call is returned, regardless of whether it produces a string or `null`.
+
+{{index "call stack"}}
+
+To better understand how this function produces the effect we're looking for, let's look at all the calls to `find` that are made when searching for a solution for the number 13:
+
+```{lang: null}
+find(1, "1")
+ find(6, "(1 + 5)")
+ find(11, "((1 + 5) + 5)")
+ find(16, "(((1 + 5) + 5) + 5)")
+ too big
+ find(33, "(((1 + 5) + 5) * 3)")
+ too big
+ find(18, "((1 + 5) * 3)")
+ too big
+ find(3, "(1 * 3)")
+ find(8, "((1 * 3) + 5)")
+ find(13, "(((1 * 3) + 5) + 5)")
+ found!
+```
+
+The indentation indicates the depth of the call stack. The first time `find` is called, the function starts by calling itself to explore the solution that starts with `(1 + 5)`. That call will further recurse to explore _every_ continued solution that yields a number less than or equal to the target number. Since it doesn't find one that hits the target, it returns `null` back to the first call. There the `??` operator causes the call that explores `(1 * 3)` to happen. This search has more luck—its first recursive call, through yet _another_ recursive call, hits upon the target number. That innermost call returns a string, and each of the `??` operators in the intermediate calls passes that string along, ultimately returning the solution.
+
+## Growing functions
+
+{{index [function, definition]}}
+
+There are two more or less natural ways for functions to be introduced into programs.
+
+{{index repetition}}
+
+The first occurs when you find yourself writing similar code multiple times. You'd prefer not to do that, as having more code means more space for mistakes to hide and more material to read for people trying to understand the program. So you take the repeated functionality, find a good name for it, and put it into a function.
+
+The second way is that you find you need some functionality that you haven't written yet and that sounds like it deserves its own function. You start by naming the function, and then write its body. You might even start writing code that uses the function before you actually define the function itself.
+
+{{index [function, naming], [binding, naming]}}
+
+How difficult it is to find a good name for a function is a good indication of how clear a concept it is that you're trying to wrap. Let's go through an example.
+
+{{index "farm example"}}
+
+We want to write a program that prints two numbers: the numbers of cows and chickens on a farm, with the words `Cows` and `Chickens` after them and zeros padded before both numbers so that they are always three digits long:
+
+```{lang: null}
+007 Cows
+011 Chickens
+```
+
+This asks for a function of two arguments—the number of cows and the number of chickens. Let's get coding.
+
+```
+function printFarmInventory(cows, chickens) {
+ let cowString = String(cows);
+ while (cowString.length < 3) {
+ cowString = "0" + cowString;
+ }
+ console.log(`${cowString} Cows`);
+ let chickenString = String(chickens);
+ while (chickenString.length < 3) {
+ chickenString = "0" + chickenString;
+ }
+ console.log(`${chickenString} Chickens`);
+}
+printFarmInventory(7, 11);
+```
+
+{{index ["length property", "for string"], "while loop"}}
+
+Writing `.length` after a string expression will give us the length of that string. Thus, the `while` loops keep adding zeros in front of the number strings until they are at least three characters long.
+
+Mission accomplished! But just as we are about to send the farmer the code (along with a hefty invoice), she calls and tells us she's also started keeping pigs, and couldn't we please extend the software to also print pigs?
+
+{{index "copy-paste programming"}}
+
+We sure can. But just as we're in the process of copying and pasting those four lines one more time, we stop and reconsider. There has to be a better way. Here's a first attempt:
+
+```
+function printZeroPaddedWithLabel(number, label) {
+ let numberString = String(number);
+ while (numberString.length < 3) {
+ numberString = "0" + numberString;
+ }
+ console.log(`${numberString} ${label}`);
+}
+
+function printFarmInventory(cows, chickens, pigs) {
+ printZeroPaddedWithLabel(cows, "Cows");
+ printZeroPaddedWithLabel(chickens, "Chickens");
+ printZeroPaddedWithLabel(pigs, "Pigs");
+}
+
+printFarmInventory(7, 11, 3);
+```
+
+{{index [function, naming]}}
+
+It works! But that name, `printZeroPaddedWithLabel`, is a little awkward. It conflates three things—printing, zero-padding, and adding a label—into a single function.
+
+{{index "zeroPad function"}}
+
+Instead of lifting out the repeated part of our program wholesale, let's try to pick out a single _concept_:
+
+```
+function zeroPad(number, width) {
+ let string = String(number);
+ while (string.length < width) {
+ string = "0" + string;
+ }
+ return string;
+}
+
+function printFarmInventory(cows, chickens, pigs) {
+ console.log(`${zeroPad(cows, 3)} Cows`);
+ console.log(`${zeroPad(chickens, 3)} Chickens`);
+ console.log(`${zeroPad(pigs, 3)} Pigs`);
+}
+
+printFarmInventory(7, 16, 3);
+```
+
+{{index readability, "pure function"}}
+
+A function with a nice, obvious name like `zeroPad` makes it easier for someone who reads the code to figure out what it does. Such a function is also useful in more situations than just this specific program. For example, you could use it to help print nicely aligned tables of numbers.
+
+{{index [interface, design]}}
+
+How smart and versatile _should_ our function be? We could write anything, from a terribly simple function that can only pad a number to be three characters wide to a complicated generalized number-formatting system that handles fractional numbers, negative numbers, alignment of decimal dots, padding with different characters, and so on.
+
+A useful principle is to refrain from adding cleverness unless you are absolutely sure you're going to need it. It can be tempting to write general "((framework))s" for every bit of functionality you come across. Resist that urge. You won't get any real work done—you'll be too busy writing code that you never use.
+
+{{id pure}}
+## Functions and side effects
+
+{{index "side effect", "pure function", [function, purity]}}
+
+Functions can be roughly divided into those that are called for their side effects and those that are called for their return value (though it is also possible to both have side effects and return a value).
+
+{{index reuse}}
+
+The first helper function in the ((farm example)), `printZeroPaddedWithLabel`, is called for its side effect: it prints a line. The second version, `zeroPad`, is called for its return value. It is no coincidence that the second is useful in more situations than the first. Functions that create values are easier to combine in new ways than functions that directly perform side effects.
+
+{{index substitution}}
+
+A _pure_ function is a specific kind of value-producing function that not only has no side effects but also doesn't rely on side effects from other code—for example, it doesn't read global bindings whose value might change. A pure function has the pleasant property that, when called with the same arguments, it always produces the same value (and doesn't do anything else). A call to such a function can be substituted by its return value without changing the meaning of the code. When you are not sure that a pure function is working correctly, you can test it by simply calling it and know that if it works in that context, it will work in any context. Nonpure functions tend to require more scaffolding to test.
+
+{{index optimization, "console.log"}}
+
+Still, there's no need to feel bad when writing functions that are not pure. Side effects are often useful. There's no way to write a pure version of `console.log`, for example, and `console.log` is good to have. Some operations are also easier to express in an efficient way when we use side effects.
+
+## Summary
+
+This chapter taught you how to write your own functions. The `function` keyword, when used as an expression, can create a function value. When used as a statement, it can be used to declare a binding and give it a function as its value. Arrow functions are yet another way to create functions.
+
+```
+// Define f to hold a function value
+const f = function(a) {
+ console.log(a + 2);
+};
+
+// Declare g to be a function
+function g(a, b) {
+ return a * b * 3.5;
+}
+
+// A less verbose function value
+let h = a => a % 3;
+```
+
+A key part of understanding functions is understanding scopes. Each block creates a new scope. Parameters and bindings declared in a given scope are local and not visible from the outside. Bindings declared with `var` behave differently—they end up in the nearest function scope or the global scope.
+
+Separating the tasks your program performs into different functions is helpful. You won't have to repeat yourself as much, and functions can help organize a program by grouping code into pieces that do specific things.
+
+## Exercises
+
+### Minimum
+
+{{index "Math object", "minimum (exercise)", "Math.min function", minimum}}
+
+The [previous chapter](program_structure#return_values) introduced the standard function `Math.min` that returns its smallest argument. We can write a function like that ourselves now. Define the function `min` that takes two arguments and returns their minimum.
+
+{{if interactive
+
+```{test: no}
+// Your code here.
+
+console.log(min(0, 10));
+// → 0
+console.log(min(0, -10));
+// → -10
+```
+if}}
+
+{{hint
+
+{{index "minimum (exercise)"}}
+
+If you have trouble putting braces and parentheses in the right place to get a valid function definition, start by copying one of the examples in this chapter and modifying it.
+
+{{index "return keyword"}}
+
+A function may contain multiple `return` statements.
+
+hint}}
+
+### Recursion
+
+{{index recursion, "isEven (exercise)", "even number"}}
+
+We've seen that we can use `%` (the remainder operator) to test whether a number is even or odd by using `% 2` to see whether it's divisible by two. Here's another way to define whether a positive whole number is even or odd:
+
+- Zero is even.
+
+- One is odd.
+
+- For any other number _N_, its evenness is the same as _N_ - 2.
+
+Define a recursive function `isEven` corresponding to this description. The function should accept a single parameter (a positive, whole number) and return a Boolean.
+
+{{index "stack overflow"}}
+
+Test it on 50 and 75. See how it behaves on -1. Why? Can you think of a way to fix this?
+
+{{if interactive
+
+```{test: no}
+// Your code here.
+
+console.log(isEven(50));
+// → true
+console.log(isEven(75));
+// → false
+console.log(isEven(-1));
+// → ??
+```
+
+if}}
+
+{{hint
+
+{{index "isEven (exercise)", ["if keyword", chaining], recursion}}
+
+Your function will likely look somewhat similar to the inner `find` function in the recursive `findSolution` [example](functions#recursive_puzzle) in this chapter, with an `if`/`else if`/`else` chain that tests which of the three cases applies. The final `else`, corresponding to the third case, makes the recursive call. Each of the branches should contain a `return` statement or in some other way arrange for a specific value to be returned.
+
+{{index "stack overflow"}}
+
+When given a negative number, the function will recurse again and again, passing itself an ever more negative number, thus getting further and further away from returning a result. It will eventually run out of stack space and abort.
+
+hint}}
+
+### Bean counting
+
+{{index "bean counting (exercise)", [string, indexing], "zero-based counting", ["length property", "for string"]}}
+
+You can get the *N*th character, or letter, from a string by writing `[N]` after the string (for example, `string[2]`). The resulting value will be a string containing only one character (for example, `"b"`). The first character has position 0, which causes the last one to be found at position `string.length - 1`. In other words, a two-character string has length 2, and its characters have positions 0 and 1.
+
+Write a function called `countBs` that takes a string as its only argument and returns a number that indicates how many uppercase B characters there are in the string.
+
+Next, write a function called `countChar` that behaves like `countBs`, except it takes a second argument that indicates the character that is to be counted (rather than counting only uppercase B characters). Rewrite `countBs` to make use of this new function.
+
+{{if interactive
+
+```{test: no}
+// Your code here.
+
+console.log(countBs("BOB"));
+// → 2
+console.log(countChar("kakkerlak", "k"));
+// → 4
+```
+
+if}}
+
+{{hint
+
+{{index "bean counting (exercise)", ["length property", "for string"], "counter variable"}}
+
+Your function will need a ((loop)) that looks at every character in the string. It can run an index from zero to one below its length (`< string.length`). If the character at the current position is the same as the one the function is looking for, it adds 1 to a counter variable. Once the loop has finished, the counter can be returned.
+
+{{index "local binding"}}
+
+Take care to make all the bindings used in the function _local_ to the function by properly declaring them with the `let` or `const` keyword.
+
+hint}}
diff --git a/03_functions.txt b/03_functions.txt
deleted file mode 100644
index 64096bfd6..000000000
--- a/03_functions.txt
+++ /dev/null
@@ -1,1036 +0,0 @@
-:chap_num: 3
-:prev_link: 02_program_structure
-:next_link: 04_data
-
-= Functions =
-
-[chapterquote="true"]
-[quote, Donald Knuth]
-____
-People think that computer science is the art of
-geniuses but the actual reality is the opposite, just many people
-doing things that build on each other, like a wall of mini stones.
-____
-
-(((Knuth+++,+++ Donald)))(((function)))(((code structure)))You've seen function values, such
-as `alert`, and how to call them. Functions are the bread and butter
-of JavaScript programming. The concept of wrapping a piece of program
-in a value has many uses. It is a tool to structure larger programs,
-to reduce repetition, to associate names with subprograms, and to
-isolate these subprograms from each other.
-
-(((human language)))The most obvious application of functions is
-defining new ((vocabulary)). Creating new words in regular,
-human-language prose is usually bad style. But in programming, it is
-indispensable.
-
-(((abstraction)))Typical adult English speakers have some 20,000 words
-in their vocabulary. Few programming languages come with 20,000
-commands built in. And the vocabulary that _is_ available tends to be
-more precisely defined, and thus less flexible, than in human
-language. Therefore, we usually _have_ to add some of our own
-vocabulary to avoid repeating ourselves too much.
-
-== Defining a function ==
-
-(((square)))(((function,definition)))A function definition is just a
-regular ((variable)) definition where the value given to the variable
-happens to be a function. For example, the following code defines the
-variable `square` to refer to a function that produces the square of a
-given number:
-
-[source,javascript]
-----
-var square = function(x) {
- return x * x;
-};
-
-console.log(square(12));
-// → 144
-----
-
-indexsee:[braces,curly braces]
-
-(((curly braces)))(((block)))(((syntax)))(((function
-keyword)))(((function,body)))(((function,as value)))A function is
-created by an expression that starts with the keyword `function`.
-Functions have a set of _((parameter))s_ (in this case, only `x`) and
-a _body_, which contains the statements that are to be executed when
-the function is called. The function body must always be wrapped in
-braces, even when it consists of only a single ((statement)) (as
-in the previous example).
-
-(((power example)))A function can have multiple parameters or no
-parameters at all. In the following example, `makeNoise` does not list
-any parameter names, whereas `power` lists two:
-
-[source,javascript]
-----
-var makeNoise = function() {
- console.log("Pling!");
-};
-
-makeNoise();
-// → Pling!
-
-var power = function(base, exponent) {
- var result = 1;
- for (var count = 0; count < exponent; count++)
- result *= base;
- return result;
-};
-
-console.log(power(2, 10));
-// → 1024
-----
-
-(((return value)))(((return keyword)))(((undefined)))Some functions
-produce a value, such as `power` and `square`, and some don't, such as
-`makeNoise`, which produces only a ((side effect)). A `return`
-statement determines the value the function returns. When control
-comes across such a statement, it immediately jumps out of the current
-function and gives the returned value to the code that called the
-function. The `return` keyword without an expression after it will
-cause the function to return `undefined`.
-
-== Parameters and scopes ==
-
-(((function,application)))(((variable,from parameter)))The
-((parameter))s to a function behave like regular variables, but their
-initial values are given by the _caller_ of the function, not the code
-in the function itself.
-
-(((function,scope)))(((scope)))(((local variable)))An
-important property of functions is that the variables created inside
-of them, including their parameters, are _local_ to the function. This
-means, for example, that the `result` variable in the `power` example
-will be newly created every time the function is called, and these
-separate incarnations do not interfere with each other.
-
-indexsee:[top-level scope,global scope]
-
-(((var keyword)))(((global scope)))(((variable,global)))This
-“localness” of variables applies only to the parameters and variables
-declared with the `var` keyword inside the function body. Variables
-declared outside of any function are called _global_, because they are
-visible throughout the program. It is possible to access such
-variables from inside a function, as long as you haven't declared a
-local variable with the same name.
-
-(((variable,assignment)))The following code demonstrates this. It
-defines and calls two functions that both assign a value to the
-variable `x`. The first one declares the variable as local and thus
-changes only the local variable. The second does not declare `x`
-locally, so references to `x` inside of it refer to the global
-variable `x` defined at the top of the example.
-
-[source,javascript]
-----
-var x = "outside";
-
-var f1 = function() {
- var x = "inside f1";
-};
-f1();
-console.log(x);
-// → outside
-
-var f2 = function() {
- x = "inside f2";
-};
-f2();
-console.log(x);
-// → inside f2
-----
-
-(((variable,naming)))(((scope)))(((global scope)))(((code,structure
-of)))This behavior helps prevent accidental interference between
-functions. If all variables were shared by the whole program, it'd
-take a lot of effort to make sure no name is ever used for two
-different purposes. And if you _did_ reuse a variable name, you might
-see strange effects from unrelated code messing with the value of your
-variable. By treating function-local variables as existing only within
-the function, the language makes it possible to read and understand
-functions as small universes, without having to worry about all the
-code at once.
-
-[[scoping]]
-== Nested scope ==
-
-(((nesting,of functions)))(((nesting,of
-scope)))(((scope)))(((inner function)))(((lexical
-scoping)))JavaScript distinguishes not just between _global_ and
-_local_ variables. Functions can be created inside other functions,
-producing several degrees of locality.
-
-(((landscape example)))For example, this rather nonsensical function
-has two functions inside of it:
-
-[source,javascript]
-----
-var landscape = function() {
- var result = "";
- var flat = function(size) {
- for (var count = 0; count < size; count++)
- result += "_";
- };
- var mountain = function(size) {
- result += "/";
- for (var count = 0; count < size; count++)
- result += "'";
- result += "\\";
- };
-
- flat(3);
- mountain(4);
- flat(6);
- mountain(1);
- flat(1);
- return result;
-};
-
-console.log(landscape());
-// → ___/''''\______/'\_
-----
-
-(((function,scope)))(((scope)))The `flat` and `mountain` functions
-can “see” the variable called `result`, since they are inside the
-function that defines it. But they cannot see each other's `count`
-variables since they are outside each other's scope. The environment
-outside of the `landscape` function doesn't see any of the variables
-defined inside `landscape`.
-
-In short, each local scope can also see all the local scopes that
-contain it. The set of variables visible inside a function is
-determined by the place of that function in the program text. All
-variables from blocks _around_ a function's definition are
-visible—meaning both those in function bodies that enclose it and
-those at the top level of the program. This approach to variable
-visibility is called _((lexical scoping))_.
-
-((({} (block))))People who have experience with other programming
-languages might expect that any block of code between braces produces
-a new local environment. But in JavaScript, functions are the only
-things that create a new scope. You are allowed to use free-standing
-blocks.
-
-[source,javascript]
-----
-var something = 1;
-{
- var something = 2;
- // Do stuff with variable something...
-}
-// Outside of the block again...
-----
-
-But the `something` inside the block refers to the same variable as
-the one outside the block. In fact, although blocks like this are
-allowed, they are useful only to group the body of an `if` statement
-or a loop.
-
-(((let keyword)))(((ECMAScript 6)))If you find this odd, you're not
-alone. The next version of JavaScript will introduce a `let` keyword,
-which works like `var` but creates a variable that is local to the
-enclosing _block_, not the enclosing _function_.
-
-== Functions as values ==
-
-(((function,as value)))Function ((variable))s usually simply act as
-names for a specific piece of the program. Such a variable is defined
-once and never changed. This makes it easy to start confusing the
-function and its name.
-
-(((variable,assignment)))But the two are different. A function value
-can do all the things that other values can do—you can use it in
-arbitrary ((expression))s, not just call it. It is possible to store a
-function value in a new place, pass it as an argument to a function,
-and so on. Similarly, a variable that holds a function is still just a
-regular variable and can be assigned a new value, like so:
-
-// test: no
-
-[source,javascript]
-----
-var launchMissiles = function(value) {
- missileSystem.launch("now");
-};
-if (safeMode)
- launchMissiles = function(value) {/* do nothing */};
-----
-
-(((function,higher-order)))In
-link:05_higher_order.html#higher_order[Chapter 5], we will discuss the
-wonderful things that can be done by passing around function values to
-other functions.
-
-== Declaration notation ==
-
-(((syntax)))(((square example)))(((function
-keyword)))(((function,definition)))(((function,declaration)))There is
-a slightly shorter way to say “`var square = function…`”. The
-`function` keyword can also be used at the start of a statement, as in
-the following:
-
-[source,javascript]
-----
-function square(x) {
- return x * x;
-}
-----
-
-(((future)))(((execution order)))This is a function _declaration_. The
-statement defines the variable `square` and points it at the given
-function. So far so good. There is one subtlety with this form of
-function definition, however.
-
-[source,javascript]
-----
-console.log("The future says:", future());
-
-function future() {
- return "We STILL have no flying cars.";
-}
-----
-
-This code works, even though the function is defined _below_ the code
-that uses it. This is because function declarations are not part of
-the regular top-to-bottom flow of control. They are conceptually moved
-to the top of their scope and can be used by all the code in that
-scope. This is sometimes useful because it gives us the freedom to
-order code in a way that seems meaningful, without worrying about
-having to define all functions above their first use.
-
-(((function,declaration)))What happens when you put such a function
-definition inside a conditional (`if`) block or a loop? Well, don't do
-that. Different JavaScript platforms in different browsers have
-traditionally done different things in that situation, and the latest
-((standard)) actually forbids it. If you want your programs to behave
-consistently, only use this form of function-defining statements in
-the outermost block of a function or program.
-
-[source,javascript]
-----
-function example() {
- function a() {} // Okay
- if (something) {
- function b() {} // Danger!
- }
-}
-----
-
-[[stack]]
-== The call stack ==
-
-indexsee:[stack,call stack]
-
-(((call stack)))(((function,application)))It will be helpful to take a
-closer look at the way control flows through functions. Here is a
-simple program that makes a few function calls:
-
-[source,javascript]
-----
-function greet(who) {
- console.log("Hello " + who);
-}
-greet("Harry");
-console.log("Bye");
-----
-
-(((control flow)))(((execution order)))(((console.log)))A run through
-this program goes roughly like this: the call to `greet` causes
-control to jump to the start of that function (line 2). It calls
-`console.log` (a built-in browser function), which takes control, does
-its job, and then returns control to line 2. Then it reaches the end
-of the `greet` function, so it returns to the place that called it, at
-line 4. The line after that calls `console.log` again.
-
-We could show the flow of control schematically like this:
-
-----
-top
- greet
- console.log
- greet
-top
- console.log
-top
-----
-
-(((return keyword)))(((memory)))Because a function has to jump back to
-the place of the call when it returns, the computer must remember the
-context from which the function was called. In one case, `console.log`
-had to jump back to the `greet` function. In the other case, it jumps
-back to the end of the program.
-
-The place where the computer stores this context is the _((call
-stack))_. Every time a function is called, the current context is put
-on top of this “stack”. When the function returns, it takes the top
-context from the stack and uses it to continue execution.
-
-(((infinite loop)))(((stack overflow)))(((recursion)))Storing this
-stack requires space in the computer's memory. When the stack grows
-too big, the computer will fail with a message like “out of stack
-space” or “too much recursion”. The following code illustrates this by
-asking the computer a really hard question, which causes an infinite
-back-and-forth between two functions. Rather, it _would_ be infinite,
-if the computer had an infinite stack. As it is, we will run out of
-space, or “blow the stack”.
-
-// test: no
-
-[source,javascript]
-----
-function chicken() {
- return egg();
-}
-function egg() {
- return chicken();
-}
-console.log(chicken() + " came first.");
-// → ??
-----
-
-== Optional Arguments ==
-
-(((argument)))(((function,application)))The following code is allowed
-and executes without any problem:
-
-[source,javascript]
-----
-alert("Hello", "Good Evening", "How do you do?");
-----
-
-(((alert function)))The function `alert` officially accepts only one
-argument. Yet when you call it like this, it doesn't complain. It
-simply ignores the other arguments and shows you “Hello”.
-
-(((undefined)))(((parameter)))JavaScript is extremely broad-minded
-about the number of arguments you pass to a function. If you pass too
-many, the extra ones are ignored. If you pass too few, the missing
-parameters simply get assigned the value `undefined`.
-
-The downside of this is that it is possible—likely, even—that you'll
-accidentally pass the wrong number of arguments to functions and no
-one will tell you about it.
-
-[[power]]
-(((power example)))(((optional argument)))The
-upside is that this behavior can be used to have a function take
-“optional” arguments. For example, the following version of `power`
-can be called either with two arguments or with a single argument, in
-which case the exponent is assumed to be two, and the function behaves
-like `square`.
-
-// test: wrap
-
-[source,javascript]
-----
-function power(base, exponent) {
- if (exponent == undefined)
- exponent = 2;
- var result = 1;
- for (var count = 0; count < exponent; count++)
- result *= base;
- return result;
-}
-
-console.log(power(4));
-// → 16
-console.log(power(4, 3));
-// → 64
-----
-
-(((console.log)))In the link:04_data.html#arguments_object[next
-chapter], we will see a way in which a function body can get at the
-exact list of arguments that were passed. This is helpful because it
-makes it possible for a function to accept any number of arguments.
-For example, `console.log` makes use of this—it outputs all of the
-values it is given.
-
-[source,javascript]
-----
-console.log("R", 2, "D", 2);
-// → R 2 D 2
-----
-
-== Closure ==
-
-(((call stack)))(((local variable)))(((function,as
-value)))(((closure)))(((scope)))The ability to treat functions as
-values, combined with the fact that local variables are “re-created”
-every time a function is called, brings up an interesting question.
-What happens to local variables when the function call that created
-them is no longer active?
-
-The following code shows an example of this. It defines a function,
-`wrapValue` that creates a local variable. It then returns a function
-that accesses and returns this local variable.
-
-[source,javascript]
-----
-function wrapValue(n) {
- var localVariable = n;
- return function() { return localVariable; };
-}
-
-var wrap1 = wrapValue(1);
-var wrap2 = wrapValue(2);
-console.log(wrap1());
-// → 1
-console.log(wrap2());
-// → 2
-----
-
-This is allowed and works as you'd hope—the variable can still be
-accessed. In fact, multiple instances of the variable can be alive at
-the same time, which is another good illustration of the concept that
-local variables really are re-created for every call—different calls
-can't trample on one another's local variables.
-
-This feature—being able to reference a specific instance of local
-variables in an enclosing function—is called _closure_. A function
-that “closes over” some local variables is called _a_ closure. This
-behavior not only frees you from having to worry about lifetimes of
-variables but also allows for some creative use of function values.
-
-(((multiplier function)))With a slight change, we can turn the
-previous example into a way to create functions that multiply by an
-arbitrary amount.
-
-[source,javascript]
-----
-function multiplier(factor) {
- return function(number) {
- return number * factor;
- };
-}
-
-var twice = multiplier(2);
-console.log(twice(5));
-// → 10
-----
-
-(((variable,from parameter)))The explicit `localVariable` from the
-`wrapValue` example isn't needed since a parameter is itself a local
-variable.
-
-(((function,model of)))Thinking about programs like this takes some
-practice. A good mental model is to think of the `function` keyword as
-“freezing” the code in its body and wrapping it into a package (the
-function value). So when you read `return function(...) {...}`, think
-of it as returning a handle to a piece of computation, frozen for
-later use.
-
-In the example, `multiplier` returns a frozen chunk of code that gets
-stored in the `twice` variable. The last line then calls the value in
-this variable, causing the frozen code (`return number * factor;`) to
-be activated. It still has access to the `factor` variable from the
-`multiplier` call that created it, and in addition it gets access to
-the argument passed when unfreezing it, 5, through its `number`
-parameter.
-
-== Recursion ==
-
-(((power example)))(((stack
-overflow)))(((recursion)))(((function,application)))It is perfectly
-okay for a function to call itself, as long as it takes care not to
-overflow the stack. A function that calls itself is called
-_recursive_. Recursion allows some functions to be written in a
-different style. Take, for example, this alternative implementation of
-`power`:
-
-// test: wrap
-
-[source,javascript]
-----
-function power(base, exponent) {
- if (exponent == 0)
- return 1;
- else
- return base * power(base, exponent - 1);
-}
-
-console.log(power(2, 3));
-// → 8
-----
-
-(((loop)))(((readability)))(((mathematics)))This is rather
-close to the way mathematicians define exponentiation and arguably
-describes the concept in a more elegant way than the looping variant
-does. The function calls itself multiple times with different
-arguments to achieve the repeated multiplication.
-
-(((function,application)))(((efficiency)))But this implementation has
-one important problem: in typical JavaScript implementations, it's
-about 10 times slower than the looping version. Running through a
-simple loop is a lot cheaper than calling a function multiple times.
-
-(((optimization)))The dilemma of speed versus ((elegance)) is an
-interesting one. You can see it as a kind of continuum between
-human-friendliness and machine-friendliness. Almost any program can be
-made faster by making it bigger and more convoluted. The programmer
-must decide on an appropriate balance.
-
-In the case of the link:03_functions.html#power[earlier] `power`
-function, the inelegant (looping) version is still fairly simple and
-easy to read. It doesn't make much sense to replace it with the
-recursive version. Often, though, a program deals with such complex
-concepts that giving up some efficiency in order to make the program
-more straightforward becomes an attractive choice.
-
-(((profiling)))The basic rule, which has been repeated by many
-programmers and with which I wholeheartedly agree, is to not worry
-about efficiency until you know for sure that the program is too slow.
-If it is, find out which parts are taking up the most time, and start
-exchanging elegance for efficiency in those parts.
-
-Of course, this rule doesn't mean one should start ignoring
-performance altogether. In many cases, like the `power` function, not
-much simplicity is gained from the “elegant” approach. And sometimes
-an experienced programmer can see right away that a simple approach is
-never going to be fast enough.
-
-(((premature optimization)))The reason I'm stressing this is that
-surprisingly many beginning programmers focus fanatically on
-efficiency, even in the smallest details. The result is bigger, more
-complicated, and often less correct programs, that take longer to
-write than their more straightforward equivalents and that usually run
-only marginally faster.
-
-(((branching recursion)))But recursion is not always just a
-less-efficient alternative to looping. Some problems are much easier
-to solve with recursion than with loops. Most often these are problems
-that require exploring or processing several “branches”, each of which
-might branch out again into more branches.
-
-[[recursive_puzzle]]
-
-(((recursion)))(((number puzzle example)))Consider this puzzle: by
-starting from the number 1 and repeatedly either adding 5 or
-multiplying by 3, an infinite amount of new numbers can be produced.
-How would you write a function that, given a number, tries to find a
-sequence of such additions and multiplications that produce that
-number? For example, the number 13 could be reached by first
-multiplying by 3 and then adding 5 twice, whereas the number 15 cannot
-be reached at all.
-
-Here is a recursive solution:
-
-[source,javascript]
-----
-function findSolution(target) {
- function find(start, history) {
- if (start == target)
- return history;
- else if (start > target)
- return null;
- else
- return find(start + 5, "(" + history + " + 5)") ||
- find(start * 3, "(" + history + " * 3)");
- }
- return find(1, "1");
-}
-
-console.log(findSolution(24));
-// → (((1 * 3) + 5) * 3)
-----
-
-Note that this program doesn't necessarily find the _shortest_
-sequence of operations. It is satisfied when it finds any sequence at
-all.
-
-I don't necessarily expect you to see how it works right away. But
-let's work through it, since it makes for a great exercise in
-recursive thinking.
-
-The inner function `find` does the actual recursing. It takes two
-((argument))s—the current number and a string that records how we
-reached this number—and returns either a string that shows how to get
-to the target or `null`.
-
-(((null)))(((|| operator)))(((short-circuit evaluation)))To do this, the
-function performs one of three actions. If the current number is the
-target number, the current history is a way to reach that target, so
-it is simply returned. If the current number is greater than the
-target, there's no sense in further exploring this history since both
-adding and multiplying will only make the number bigger. And finally,
-if we're still below the target, the function tries both possible
-paths that start from the current number, by calling itself twice,
-once for each of the allowed next steps. If the first call returns
-something that is not `null`, it is returned. Otherwise, the second
-call is returned—regardless of whether it produces a string or `null`.
-
-(((call stack)))To better understand how this function produces the
-effect we're looking for, let's look at all the calls to `find` that
-are made when searching for a solution for the number 13.
-
-----
-find(1, "1")
- find(6, "(1 + 5)")
- find(11, "((1 + 5) + 5)")
- find(16, "(((1 + 5) + 5) + 5)")
- too big
- find(33, "(((1 + 5) + 5) * 3)")
- too big
- find(18, "((1 + 5) * 3)")
- too big
- find(3, "(1 * 3)")
- find(8, "((1 * 3) + 5)")
- find(13, "(((1 * 3) + 5) + 5)")
- found!
-----
-
-The indentation suggests the depth of the call stack. The first call
-to `find` calls itself twice to explore the solutions that start with
-`(1 + 5)` and `(1 * 3)`. The first call tries to find a solution that
-starts with `(1 + 5)` and, using recursion, explores _every_ solution
-that yields a number smaller than or equal to the target number. Since
-it doesn't find a solution that hits the target, it returns `null`
-back to the first call. There the `||` operator causes the call that
-explores `(1 * 3)` to happen. This search has more luck because its
-first recursive call, through yet _another_ recursive call, hits upon
-the target number, 13. This innermost recursive call returns a string,
-and each of the `||` operators in the intermediate calls pass that
-string along, ultimately returning our solution.
-
-== Growing functions ==
-
-(((function,definition)))There are two more or less natural ways for
-functions to be introduced into programs.
-
-(((repetition)))The first is that you find yourself writing very
-similar code multiple times. We want to avoid doing that since having
-more code means more space for mistakes to hide and more material to
-read for people trying to understand the program. So we take the
-repeated functionality, find a good name for it, and put it into a
-function.
-
-The second way is that you find you need some functionality that you
-haven't written yet and that sounds like it deserves its own function.
-You'll start by naming the function, and you'll then write its body.
-You might even start writing code that uses the function before you
-actually define the function itself.
-
-(((function,naming)))(((variable,naming)))How difficult it is to find
-a good name for a function is a good indication of how clear a concept
-it is that you're trying to wrap. Let's go through an example.
-
-(((farm example)))We want to write a program that prints two numbers,
-the numbers of cows and chickens on a farm, with the words `Cows` and
-`Chickens` after them, and zeroes padded before both numbers so that
-they are always three digits long.
-
-----
-007 Cows
-011 Chickens
-----
-
-That clearly asks for a function of two arguments. Let's get coding.
-
-[source,javascript]
-----
-function printFarmInventory(cows, chickens) {
- var cowString = String(cows);
- while (cowString.length < 3)
- cowString = "0" + cowString;
- console.log(cowString + " Cows");
- var chickenString = String(chickens);
- while (chickenString.length < 3)
- chickenString = "0" + chickenString;
- console.log(chickenString + " Chickens");
-}
-printFarmInventory(7, 11);
-----
-
-(((length property,for strings)))(((while loop)))Adding `.length`
-after a string value will give us the length of that string. Thus, the
-`while` loops keep adding zeroes in front of the number strings until
-they are at least three characters long.
-
-Mission accomplished! But just as we are about to send the farmer the
-code (along with a hefty invoice, of course), he calls and tells us
-he's also started keeping pigs, and couldn't we please extend the
-software to also print pigs?
-
-(((copy-paste programming)))We sure can. But just as we're in the
-process of copying and pasting those four lines one more time, we stop
-and reconsider. There has to be a better way. Here's a first attempt:
-
-[source,javascript]
-----
-function printZeroPaddedWithLabel(number, label) {
- var numberString = String(number);
- while (numberString.length < 3)
- numberString = "0" + numberString;
- console.log(numberString + " " + label);
-}
-
-function printFarmInventory(cows, chickens, pigs) {
- printZeroPaddedWithLabel(cows, "Cows");
- printZeroPaddedWithLabel(chickens, "Chickens");
- printZeroPaddedWithLabel(pigs, "Pigs");
-}
-
-printFarmInventory(7, 11, 3);
-----
-
-(((function,naming)))It works! But that name,
-`printZeroPaddedWithLabel`, is a little awkward. It conflates three
-things—printing, zero-padding, and adding a label—into a single
-function.
-
-(((zeroPad function)))Instead of lifting out the repeated part of our
-program wholesale, let's try to pick out a single _concept_.
-
-[source,javascript]
-----
-function zeroPad(number, width) {
- var string = String(number);
- while (string.length < width)
- string = "0" + string;
- return string;
-}
-
-function printFarmInventory(cows, chickens, pigs) {
- console.log(zeroPad(cows, 3) + " Cows");
- console.log(zeroPad(chickens, 3) + " Chickens");
- console.log(zeroPad(pigs, 3) + " Pigs");
-}
-
-printFarmInventory(7, 16, 3);
-----
-
-(((readability)))(((pure function)))A function with a nice, obvious
-name like `zeroPad` makes it easier for someone who reads the code to
-figure out what it does. And it is useful in more situations than just
-this specific program. For example, you could use it to help print
-nicely aligned tables of numbers.
-
-(((interface,design)))How smart and versatile should our function be?
-We could write anything from a terribly simple function that simply
-pads a number so that it's three characters wide to a complicated
-generalized number-formatting system that handles fractional numbers,
-negative numbers, alignment of dots, padding with different
-characters, and so on.
-
-A useful principle is not to add cleverness unless you are absolutely
-sure you're going to need it. It can be tempting to write general
-“((framework))s” for every little bit of functionality you come
-across. Resist that urge. You won't get any real work done, and you'll
-end up writing a lot of code that no one will ever use.
-
-[[pure]]
-== Functions and side effects ==
-
-(((side effect)))(((pure function)))(((function,purity)))Functions can
-be roughly divided into those that are called for their side effects
-and those that are called for their return value. (Though it is
-definitely also possible to have both side effects and return a
-value.)
-
-(((reuse)))The first helper function in the ((farm example)),
-`printZeroPaddedWithLabel`, is called for its side effect: it prints a
-line. The second version, `zeroPad`, is called for its return value.
-It is no coincidence that the second is useful in more situations than
-the first. Functions that create values are easier to combine in new
-ways than functions that directly perform side effects.
-
-(((substitution)))A _pure_ function is a specific kind of
-value-producing function that not only has no side effects but also
-doesn't rely on side effects from other code—for example, it doesn't
-read global variables that are occasionally changed by other code. A
-pure function has the pleasant property that, when called with the
-same arguments, it always produces the same value (and doesn't do
-anything else). This makes it easy to reason about. A call to such a
-function can be mentally substituted by its result, without changing
-the meaning of the code. When you are not sure that a pure function is
-working correctly, you can test it by simply calling it, and know that
-if it works in that context, it will work in any context. Nonpure
-functions might return different values based on all kinds of factors
-and have side effects that might be hard to test and think about.
-
-(((optimization)))(((console.log)))Still, there's no need to feel bad
-when writing functions that are not pure or to wage a holy war to
-purge them from your code. Side effects are often useful. There'd be
-no way to write a pure version of `console.log`, for example, and
-`console.log` is certainly useful. Some operations are also easier to
-express in an efficient way when we use side effects, so computing
-speed can be a reason to avoid purity.
-
-== Summary ==
-
-This chapter taught you how to write your own functions. The
-`function` keyword, when used as an expression, can create a function
-value. When used as a statement, it can be used to declare a variable
-and give it a function as its value.
-
-[source,javascript]
-----
-// Create a function value f
-var f = function(a) {
- console.log(a + 2);
-};
-
-// Declare g to be a function
-function g(a, b) {
- return a * b * 3.5;
-}
-----
-
-A key aspect in understanding functions is understanding local scopes.
-Parameters and variables declared inside a function are local to the
-function, re-created every time the function is called, and not visible
-from the outside. Functions declared inside another function have
-access to the outer function's local scope.
-
-Separating the different tasks your program performs into different
-functions is helpful. You won't have to repeat yourself as much, and
-functions can make a program more readable by grouping code into
-conceptual chunks, in the same way that chapters and sections help
-organize regular text.
-
-== Exercises ==
-
-=== Minimum ===
-
-(((Math object)))(((minimum (exercise))))(((Math.min
-function)))(((minimum)))The
-link:02_program_structure.html#return_values[previous chapter]
-introduced the standard function `Math.min` that returns its smallest
-argument. We can do that ourselves now. Write a function `min` that
-takes two arguments and returns their minimum.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-console.log(min(0, 10));
-// → 0
-console.log(min(0, -10));
-// → -10
-----
-endif::html_target[]
-
-!!solution!!
-
-(((minimum (exercise))))If you have trouble putting braces and
-parentheses in the right place to get a valid function definition,
-start by copying one of the examples in this chapter and modifying it.
-
-(((return keyword)))A function may contain multiple `return`
-statements.
-
-!!solution!!
-
-=== Recursion ===
-
-(((recursion)))(((isEven (exercise))))(((even number)))We've seen
-that `%` (the remainder operator) can be used to test whether a number
-is even or odd by using `% 2` to check whether it's divisible by two.
-Here's another way to define whether a positive whole number is even
-or odd:
-
-- Zero is even.
-
-- One is odd.
-
-- For any other number N, its evenness is the same as N - 2.
-
-Define a recursive function `isEven` corresponding to this
-description. The function should accept a `number` parameter and
-return a Boolean.
-
-(((stack overflow)))Test it on 50 and 75. See how it behaves on -1.
-Why? Can you think of a way to fix this?
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-console.log(isEven(50));
-// → true
-console.log(isEven(75));
-// → false
-console.log(isEven(-1));
-// → ??
-----
-endif::html_target[]
-
-!!solution!!
-
-(((isEven (exercise))))(((if keyword,chaining)))(((recursion)))Your
-function will likely look somewhat similar to the inner `find`
-function in the recursive `findSolution`
-link:03_functions.html#recursive_puzzle[example] in this chapter, with
-an `if`/`else if`/`else` chain that tests which of the three cases
-applies. The final `else`, corresponding to the third case, makes the
-recursive call. Each of the branches should contain a `return`
-statement or in some other way arrange for a specific value to be
-returned.
-
-(((stack overflow)))When given a negative number, the function will
-recurse again and again, passing itself an ever more negative number,
-thus getting further and further away from returning a result. It will
-eventually run out of stack space and abort.
-
-!!solution!!
-
-=== Bean counting ===
-
-(((bean counting (exercise))))(((charAt
-method)))(((string,indexing)))(((zero-based counting)))You can get the
-Nth character, or letter, from a string by writing
-`"string".charAt(N)`, similar to how you get its length with
-`"s".length`. The returned value will be a string containing only one
-character (for example, `"b"`). The first character has position zero,
-which causes the last one to be found at position `string.length - 1`.
-In other words, a two-character string has length 2, and its
-characters have positions 0 and 1.
-
-Write a function `countBs` that takes a string as its only argument
-and returns a number that indicates how many uppercase “B” characters
-are in the string.
-
-Next, write a function called `countChar` that behaves like `countBs`,
-except it takes a second argument that indicates the character that is
-to be counted (rather than counting only uppercase "B" characters).
-Rewrite `countBs` to make use of this new function.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-console.log(countBs("BBC"));
-// → 2
-console.log(countChar("kakkerlak", "k"));
-// → 4
-----
-endif::html_target[]
-
-!!solution!!
-
-(((bean counting (exercise))))(((length property,for
-strings)))(((counter variable)))A ((loop)) in your function will have
-to look at every character in the string by running an index from zero
-to one below its length (`< string.length`). If the character at the
-current position is the same as the one the function is looking for,
-it adds 1 to a counter variable. Once the loop has finished, the
-counter can be returned.
-
-(((local variable)))Take care to make all the variables used in the
-function _local_ to the function by using the `var` keyword.
-
-!!solution!!
diff --git a/04_data.md b/04_data.md
new file mode 100644
index 000000000..51541b8d2
--- /dev/null
+++ b/04_data.md
@@ -0,0 +1,1190 @@
+{{meta {load_files: ["code/journal.js", "code/chapter/04_data.js"], zip: "node/html"}}}
+
+# Data Structures: Objects and Arrays
+
+{{quote {author: "Charles Babbage", title: "Passages from the Life of a Philosopher (1864)", chapter: true}
+
+On two occasions I have been asked, 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' [...] I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.
+
+quote}}
+
+{{index "Babbage, Charles"}}
+
+{{figure {url: "img/chapter_picture_4.jpg", alt: "Illustration of a squirrel next to a pile of books and a pair of glasses. A moon and stars are visible in the background.", chapter: framed}}}
+
+{{index object, "data structure"}}
+
+Numbers, Booleans, and strings are the atoms from which ((data)) structures are built. Many types of information require more than one atom, though. _Objects_ allow us to group values—including other objects—to build more complex structures.
+
+The programs we have built so far have been limited by the fact that they were operating only on simple data types. After learning the basics of data structures in this chapter, you'll know enough to start writing useful programs.
+
+The chapter will work through a more or less realistic programming example, introducing concepts as they apply to the problem at hand. The example code will often build on functions and bindings introduced earlier in the book.
+
+{{if book
+
+The online coding ((sandbox)) for the book ([_https://eloquentjavascript.net/code_](https://eloquentjavascript.net/code)) provides a way to run code in the context of a particular chapter. If you decide to work through the examples in another environment, be sure to first download the full code for this chapter from the sandbox page.
+
+if}}
+
+## The weresquirrel
+
+{{index "weresquirrel example", lycanthropy}}
+
+Every now and then, usually between 8 p.m. and 10 p.m., ((Jacques)) finds himself transforming into a small furry rodent with a bushy tail.
+
+On one hand, Jacques is quite glad that he doesn't have classic lycanthropy. Turning into a squirrel does cause fewer problems than turning into a wolf. Instead of having to worry about accidentally eating the neighbor (_that_ would be awkward), he worries about being eaten by the neighbor's cat. After two occasions of waking up on a precariously thin branch in the crown of an oak, naked and disoriented, he has taken to locking the doors and windows of his room at night and putting a few walnuts on the floor to keep himself busy.
+
+But Jacques would prefer to get rid of his condition entirely. The irregular occurrences of the transformation make him suspect that they might be triggered by something. For a while, he believed that it happened only on days when he had been near oak trees. However, avoiding oak trees did not solve the problem.
+
+{{index journal}}
+
+Switching to a more scientific approach, Jacques has started keeping a daily log of everything he does on a given day and whether he changed form. With this data he hopes to narrow down the conditions that trigger the transformations.
+
+The first thing he needs is a data structure to store this information.
+
+## Datasets
+
+{{index ["data structure", collection], [memory, organization]}}
+
+To work with a chunk of digital data, we first have to find a way to represent it in our machine's memory. Say, for example, that we want to represent a ((collection)) of the numbers 2, 3, 5, 7, and 11.
+
+{{index string}}
+
+We could get creative with strings—after all, strings can have any length, so we can put a lot of data into them—and use `"2 3 5 7 11"` as our representation. But this is awkward. We'd have to somehow extract the digits and convert them back to numbers to access them.
+
+{{index [array, creation], "[] (array)"}}
+
+Fortunately, JavaScript provides a data type specifically for storing sequences of values. It is called an _array_ and is written as a list of values between ((square brackets)), separated by commas.
+
+```
+let listOfNumbers = [2, 3, 5, 7, 11];
+console.log(listOfNumbers[2]);
+// → 5
+console.log(listOfNumbers[0]);
+// → 2
+console.log(listOfNumbers[2 - 1]);
+// → 3
+```
+
+{{index "[] (subscript)", [array, indexing]}}
+
+The notation for getting at the elements inside an array also uses ((square brackets)). A pair of square brackets immediately after an expression, with another expression inside of them, will look up the element in the left-hand expression that corresponds to the _((index))_ given by the expression in the brackets.
+
+{{id array_indexing}}
+{{index "zero-based counting"}}
+
+The first index of an array is zero, not one, so the first element is retrieved with `listOfNumbers[0]`. Zero-based counting has a long tradition in technology and in certain ways makes a lot of sense, but it takes some getting used to. Think of the index as the number of items to skip, counting from the start of the array.
+
+{{id properties}}
+
+## Properties
+
+{{index "Math object", "Math.max function", ["length property", "for string"], [object, property], "period character", [property, access]}}
+
+We've seen a few expressions like `myString.length` (to get the length of a string) and `Math.max` (the maximum function) in past chapters. These expressions access a _property_ of some value. In the first case, we access the `length` property of the value in `myString`. In the second, we access the property named `max` in the `Math` object (which is a collection of mathematics-related constants and functions).
+
+{{index [property, access], null, undefined}}
+
+Almost all JavaScript values have properties. The exceptions are `null` and `undefined`. If you try to access a property on one of these nonvalues, you get an error:
+
+```{test: no}
+null.length;
+// → TypeError: null has no properties
+```
+
+{{indexsee "dot character", "period character"}}
+{{index "[] (subscript)", "period character", "square brackets", "computed property", [property, access]}}
+
+The two main ways to access properties in JavaScript are with a dot and with square brackets. Both `value.x` and `value[x]` access a property on `value`—but not necessarily the same property. The difference is in how `x` is interpreted. When using a dot, the word after the dot is the literal name of the property. When using square brackets, the expression between the brackets is _evaluated_ to get the property name. Whereas `value.x` fetches the property of `value` named "x", `value[x]` takes the value of the variable named `x` and uses that, converted to a string, as the property name.
+
+If you know that the property in which you are interested is called _color_, you say `value.color`. If you want to extract the property named by the value held in the binding `i`, you say `value[i]`. Property names are strings. They can be any string, but the dot notation works only with names that look like valid binding names—starting with a letter or underscore, and containing only letters, numbers, and underscores. If you want to access a property named _2_ or _John Doe_, you must use square brackets: `value[2]` or `value["John Doe"]`.
+
+The elements in an ((array)) are stored as the array's properties, using numbers as property names. Because you can't use the dot notation with numbers and usually want to use a binding that holds the index anyway, you have to use the bracket notation to get at them.
+
+{{index ["length property", "for array"], [array, "length of"]}}
+
+Just like strings, arrays have a `length` property that tells us how many elements the array has.
+
+{{id methods}}
+
+## Methods
+
+{{index [function, "as property"], method, string}}
+
+Both string and array values contain, in addition to the `length` property, a number of properties that hold function values.
+
+```
+let doh = "Doh";
+console.log(typeof doh.toUpperCase);
+// → function
+console.log(doh.toUpperCase());
+// → DOH
+```
+
+{{index "case conversion", "toUpperCase method", "toLowerCase method"}}
+
+Every string has a `toUpperCase` property. When called, it will return a copy of the string in which all letters have been converted to uppercase. There is also `toLowerCase`, going the other way.
+
+{{index "this binding"}}
+
+Interestingly, even though the call to `toUpperCase` does not pass any arguments, the function somehow has access to the string `"Doh"`, the value whose property we called. You'll find out how this works in [Chapter ?](object#obj_methods).
+
+Properties that contain functions are generally called _methods_ of the value they belong to, as in "`toUpperCase` is a method of a string".
+
+{{id array_methods}}
+
+This example demonstrates two methods you can use to manipulate arrays.
+
+```
+let sequence = [1, 2, 3];
+sequence.push(4);
+sequence.push(5);
+console.log(sequence);
+// → [1, 2, 3, 4, 5]
+console.log(sequence.pop());
+// → 5
+console.log(sequence);
+// → [1, 2, 3, 4]
+```
+
+{{index collection, array, "push method", "pop method"}}
+
+The `push` method adds values to the end of an array. The `pop` method does the opposite, removing the last value in the array and returning it.
+
+{{index ["data structure", stack]}}
+
+These somewhat silly names are the traditional terms for operations on a _((stack))_. A stack, in programming, is a data structure that allows you to push values into it and pop them out again in the opposite order so that the thing that was added last is removed first. Stacks are common in programming—you might remember the function ((call stack)) from [the previous chapter](functions#stack), which is an instance of the same idea.
+
+## Objects
+
+{{index journal, "weresquirrel example", array, record}}
+
+Back to the weresquirrel. A set of daily log entries can be represented as an array, but the entries do not consist of just a number or a string—each entry needs to store a list of activities and a Boolean value that indicates whether Jacques turned into a squirrel or not. Ideally, we would like to group these together into a single value and then put those grouped values into an array of log entries.
+
+{{index [syntax, object], [property, definition], [braces, object], "{} (object)"}}
+
+Values of the type _((object))_ are arbitrary collections of properties. One way to create an object is by using braces as an expression.
+
+```
+let day1 = {
+ squirrel: false,
+ events: ["work", "touched tree", "pizza", "running"]
+};
+console.log(day1.squirrel);
+// → false
+console.log(day1.wolf);
+// → undefined
+day1.wolf = false;
+console.log(day1.wolf);
+// → false
+```
+
+{{index [quoting, "of object properties"], "colon character"}}
+
+Inside the braces, you write a list of properties separated by commas. Each property has a name followed by a colon and a value. When an object is written over multiple lines, indenting it as shown in this example helps with readability. Properties whose names aren't valid binding names or valid numbers must be quoted:
+
+```
+let descriptions = {
+ work: "Went to work",
+ "touched tree": "Touched a tree"
+};
+```
+
+{{index [braces, object]}}
+
+This means that braces have _two_ meanings in JavaScript. At the start of a ((statement)), they begin a ((block)) of statements. In any other position, they describe an object. Fortunately, it is rarely useful to start a statement with an object in braces, so the ambiguity between these two is not much of a problem. The one case where this does come up is when you want to return an object from a shorthand arrow function—you can't write `n => {prop: n}` since the braces will be interpreted as a function body. Instead, you have to put a set of parentheses around the object to make it clear that it is an expression.
+
+{{index undefined}}
+
+Reading a property that doesn't exist will give you the value `undefined`.
+
+{{index [property, assignment], mutability, "= operator"}}
+
+It is possible to assign a value to a property expression with the `=` operator. This will replace the property's value if it already existed or create a new property on the object if it didn't.
+
+{{index "tentacle (analogy)", [property, "model of"], [binding, "model of"]}}
+
+To briefly return to our tentacle model of ((binding))s—property bindings are similar. They _grasp_ values, but other bindings and properties might be holding onto those same values. You can think of objects as octopuses with any number of tentacles, each of which has a name written on it.
+
+{{index "delete operator", [property, deletion]}}
+
+The `delete` operator cuts off a tentacle from such an octopus. It is a unary operator that, when applied to an object property, will remove the named property from the object. This is not a common thing to do, but it is possible.
+
+```
+let anObject = {left: 1, right: 2};
+console.log(anObject.left);
+// → 1
+delete anObject.left;
+console.log(anObject.left);
+// → undefined
+console.log("left" in anObject);
+// → false
+console.log("right" in anObject);
+// → true
+```
+
+{{index "in operator", [property, "testing for"], object}}
+
+The binary `in` operator, when applied to a string and an object, tells you whether that object has a property with that name. The difference between setting a property to `undefined` and actually deleting it is that in the first case, the object still _has_ the property (it just doesn't have a very interesting value), whereas in the second case, the property is no longer present and `in` will return `false`.
+
+{{index "Object.keys function"}}
+
+To find out what properties an object has, you can use the `Object.keys` function. Give the function an object and it will return an array of strings—the object's property names:
+
+```
+console.log(Object.keys({x: 0, y: 0, z: 2}));
+// → ["x", "y", "z"]
+```
+
+There's an `Object.assign` function that copies all properties from one object into another:
+
+```
+let objectA = {a: 1, b: 2};
+Object.assign(objectA, {b: 3, c: 4});
+console.log(objectA);
+// → {a: 1, b: 3, c: 4}
+```
+
+{{index array, collection}}
+
+Arrays, then, are just a kind of object specialized for storing sequences of things. If you evaluate `typeof []`, it produces `"object"`. You can visualize arrays as long, flat octopuses with all their tentacles in a neat row, labeled with numbers.
+
+{{index journal, "weresquirrel example"}}
+
+Jacques will represent the journal that Jacques keeps as an array of objects:
+
+```{test: wrap}
+let journal = [
+ {events: ["work", "touched tree", "pizza",
+ "running", "television"],
+ squirrel: false},
+ {events: ["work", "ice cream", "cauliflower",
+ "lasagna", "touched tree", "brushed teeth"],
+ squirrel: false},
+ {events: ["weekend", "cycling", "break", "peanuts",
+ "beer"],
+ squirrel: true},
+ /* And so on... */
+];
+```
+
+## Mutability
+
+We will get to actual programming soon, but first, there's one more piece of theory to understand.
+
+{{index mutability, "side effect", number, string, Boolean, [object, mutability]}}
+
+We saw that object values can be modified. The types of values discussed in earlier chapters, such as numbers, strings, and Booleans, are all _((immutable))_—it is impossible to change values of those types. You can combine them and derive new values from them, but when you take a specific string value, that value will always remain the same. The text inside it cannot be changed. If you have a string that contains `"cat"`, it is not possible for other code to change a character in your string to make it spell `"rat"`.
+
+Objects work differently. You _can_ change their properties, causing a single object value to have different content at different times.
+
+{{index [object, identity], identity, [memory, organization], mutability}}
+
+When we have two numbers, 120 and 120, we can consider them precisely the same number, whether or not they refer to the same physical bits. With objects, there is a difference between having two references to the same object and having two different objects that contain the same properties. Consider the following code:
+
+```
+let object1 = {value: 10};
+let object2 = object1;
+let object3 = {value: 10};
+
+console.log(object1 == object2);
+// → true
+console.log(object1 == object3);
+// → false
+
+object1.value = 15;
+console.log(object2.value);
+// → 15
+console.log(object3.value);
+// → 10
+```
+
+{{index "tentacle (analogy)", [binding, "model of"]}}
+
+The `object1` and `object2` bindings grasp the _same_ object, which is why changing `object1` also changes the value of `object2`. They are said to have the same _identity_. The binding `object3` points to a different object, which initially contains the same properties as `object1` but lives a separate life.
+
+{{index "const keyword", "let keyword", [binding, "as state"]}}
+
+Bindings can also be changeable or constant, but this is separate from the way their values behave. Even though number values don't change, you can use a `let` binding to keep track of a changing number by changing the value at which the binding points. Similarly, though a `const` binding to an object can itself not be changed and will continue to point at the same object, the _contents_ of that object might change.
+
+```{test: no}
+const score = {visitors: 0, home: 0};
+// This is okay
+score.visitors = 1;
+// This isn't allowed
+score = {visitors: 1, home: 1};
+```
+
+{{index "== operator", [comparison, "of objects"], "deep comparison"}}
+
+When you compare objects with JavaScript's `==` operator, it compares by identity: it will produce `true` only if both objects are precisely the same value. Comparing different objects will return `false`, even if they have identical properties. There is no "deep" comparison operation built into JavaScript that compares objects by contents, but it is possible to write it yourself (which is one of the [exercises](data#exercise_deep_compare) at the end of this chapter).
+
+## The lycanthrope's log
+
+{{index "weresquirrel example", lycanthropy, "addEntry function"}}
+
+Jacques starts up his JavaScript interpreter and sets up the environment he needs to keep his ((journal)):
+
+```{includeCode: true}
+let journal = [];
+
+function addEntry(events, squirrel) {
+ journal.push({events, squirrel});
+}
+```
+
+{{index [braces, object], "{} (object)", [property, definition]}}
+
+Note that the object added to the journal looks a little odd. Instead of declaring properties like `events: events`, it just gives a property name: `events`. This is shorthand that means the same thing—if a property name in brace notation isn't followed by a value, its value is taken from the binding with the same name.
+
+Every evening at 10 p.m.—or sometimes the next morning, after climbing down from the top shelf of his bookcase—Jacques records the day:
+
+```
+addEntry(["work", "touched tree", "pizza", "running",
+ "television"], false);
+addEntry(["work", "ice cream", "cauliflower", "lasagna",
+ "touched tree", "brushed teeth"], false);
+addEntry(["weekend", "cycling", "break", "peanuts",
+ "beer"], true);
+```
+
+Once he has enough data points, he intends to use statistics to find out which of these events may be related to the squirrelifications.
+
+{{index correlation}}
+
+_Correlation_ is a measure of ((dependence)) between statistical variables. A statistical variable is not quite the same as a programming variable. In statistics you typically have a set of _measurements_, and each variable is measured for every measurement. Correlation between variables is usually expressed as a value that ranges from -1 to 1. Zero correlation means the variables are not related. A correlation of 1 indicates that the two are perfectly related—if you know one, you also know the other. Negative 1 also means that the variables are perfectly related but are opposites—when one is true, the other is false.
+
+{{index "phi coefficient"}}
+
+To compute the measure of correlation between two Boolean variables, we can use the _phi coefficient_ (_ϕ_). This is a formula whose input is a ((frequency table)) containing the number of times the different combinations of the variables were observed. The output of the formula is a number between -1 and 1 that describes the correlation.
+
+We could take the event of eating ((pizza)) and put that in a frequency table like this, where each number indicates the number of times that combination occurred in our measurements.
+
+{{figure {url: "img/pizza-squirrel.svg", alt: "A two-by-two table showing the pizza variable on the horizontal, and the squirrel variable on the vertical axis. Each cell show how many time that combination occurred. In 76 cases, neither happened. In 9 cases, only pizza was true. In 4 cases only squirrel was true. And in one case both occurred.", width: "7cm"}}}
+
+If we call that table _n_, we can compute _ϕ_ using the following formula:
+
+{{if html
+
+
ϕ =
n11n00 − n10n01
√n1•n0•n•1n•0
+
+if}}
+
+{{if tex
+
+[\begin{equation}\varphi = \frac{n_{11}n_{00}-n_{10}n_{01}}{\sqrt{n_{1\bullet}n_{0\bullet}n_{\bullet1}n_{\bullet0}}}\end{equation}]{latex}
+
+if}}
+
+(If at this point you're putting the book down to focus on a terrible flashback to 10th grade math class—hold on! I do not intend to torture you with endless pages of cryptic notation—it's just this one formula for now. And even with this one, all we do is turn it into JavaScript.)
+
+The notation [_n_~01~]{if html}[[$n_{01}$]{latex}]{if tex} indicates the number of measurements where the first variable (squirrelness) is false (0) and the second variable (pizza) is true (1). In the pizza table, [_n_~01~]{if html}[[$n_{01}$]{latex}]{if tex} is 9.
+
+The value [_n_~1•~]{if html}[[$n_{1\bullet}$]{latex}]{if tex} refers to the sum of all measurements where the first variable is true, which is 5 in the example table. Likewise, [_n_~•0~]{if html}[[$n_{\bullet0}$]{latex}]{if tex} refers to the sum of the measurements where the second variable is false.
+
+{{index correlation, "phi coefficient"}}
+
+So for the pizza table, the part above the division line (the dividend) would be 1×76−4×9 = 40, and the part below it (the divisor) would be the square root of 5×85×10×80, or [√340,000]{if html}[[$\sqrt{340,000}$]{latex}]{if tex}. This comes out to _ϕ_ ≈ 0.069, which is tiny. Eating ((pizza)) does not appear to have influence on the transformations.
+
+## Computing correlation
+
+{{index [array, "as table"], [nesting, "of arrays"]}}
+
+We can represent a two-by-two ((table)) in JavaScript with a four-element array (`[76, 9, 4, 1]`). We could also use other representations, such as an array containing two two-element arrays (`[[76, 9], [4, 1]]`) or an object with property names like `"11"` and `"01"`, but the flat array is simple and makes the expressions that access the table pleasantly short. We'll interpret the indices to the array as two-((bit)) ((binary number))s, where the leftmost (most significant) digit refers to the squirrel variable and the rightmost (least significant) digit refers to the event variable. For example, the binary number `10` refers to the case where Jacques did turn into a squirrel, but the event (say, "pizza") didn't occur. This happened four times. And since binary `10` is 2 in decimal notation, we will store this number at index 2 of the array.
+
+{{index "phi coefficient", "phi function"}}
+
+{{id phi_function}}
+
+This is the function that computes the _ϕ_ coefficient from such an array:
+
+```{includeCode: strip_log, test: clip}
+function phi(table) {
+ return (table[3] * table[0] - table[2] * table[1]) /
+ Math.sqrt((table[2] + table[3]) *
+ (table[0] + table[1]) *
+ (table[1] + table[3]) *
+ (table[0] + table[2]));
+}
+
+console.log(phi([76, 9, 4, 1]));
+// → 0.068599434
+```
+
+{{index "square root", "Math.sqrt function"}}
+
+This is a direct translation of the _ϕ_ formula into JavaScript. `Math.sqrt` is the square root function, as provided by the `Math` object in a standard JavaScript environment. We have to add two fields from the table to get fields like [n~1•~]{if html}[[$n_{1\bullet}$]{latex}]{if tex} because the sums of rows or columns are not stored directly in our data structure.
+
+{{index "JOURNAL dataset"}}
+
+Jacques keeps his journal for three months. The resulting ((dataset)) is available in the [coding sandbox](https://eloquentjavascript.net/code#4) for this chapter[ ([_https://eloquentjavascript.net/code#4_](https://eloquentjavascript.net/code#4))]{if book}, where it is stored in the `JOURNAL` binding, and in a downloadable [file](https://eloquentjavascript.net/code/journal.js).
+
+{{index "tableFor function"}}
+
+To extract a two-by-two ((table)) for a specific event from the journal, we must loop over all the entries and tally how many times the event occurs in relation to squirrel transformations:
+
+```{includeCode: strip_log}
+function tableFor(event, journal) {
+ let table = [0, 0, 0, 0];
+ for (let i = 0; i < journal.length; i++) {
+ let entry = journal[i], index = 0;
+ if (entry.events.includes(event)) index += 1;
+ if (entry.squirrel) index += 2;
+ table[index] += 1;
+ }
+ return table;
+}
+
+console.log(tableFor("pizza", JOURNAL));
+// → [76, 9, 4, 1]
+```
+
+{{index [array, searching], "includes method"}}
+
+Arrays have an `includes` method that checks whether a given value exists in the array. The function uses that to determine whether the event name it is interested in is part of the event list for a given day.
+
+{{index [array, indexing]}}
+
+The body of the loop in `tableFor` figures out which box in the table each journal entry falls into by checking whether the entry contains the specific event it's interested in and whether the event happens alongside a squirrel incident. The loop then adds one to the correct box in the table.
+
+We now have the tools we need to compute individual ((correlation))s. The only step remaining is to find a correlation for every type of event that was recorded and see whether anything stands out.
+
+{{id for_of_loop}}
+
+## Array loops
+
+{{index "for loop", loop, [array, iteration]}}
+
+In the `tableFor` function, there's a loop like this:
+
+```
+for (let i = 0; i < JOURNAL.length; i++) {
+ let entry = JOURNAL[i];
+ // Do something with entry
+}
+```
+
+This kind of loop is common in classical JavaScript—going over arrays one element at a time is something that comes up a lot, and to do that you'd run a counter over the length of the array and pick out each element in turn.
+
+There is a simpler way to write such loops in modern JavaScript:
+
+```
+for (let entry of JOURNAL) {
+ console.log(`${entry.events.length} events.`);
+}
+```
+
+{{index "for/of loop"}}
+
+When a `for` loop uses the word `of` after its variable definition, it will loop over the elements of the value given after `of`. This works not only for arrays but also for strings and some other data structures. We'll discuss _how_ it works in [Chapter ?](object).
+
+{{id analysis}}
+
+## The final analysis
+
+{{index journal, "weresquirrel example", "journalEvents function"}}
+
+We need to compute a correlation for every type of event that occurs in the dataset. To do that, we first need to _find_ every type of event.
+
+{{index "includes method", "push method"}}
+
+```{includeCode: "strip_log"}
+function journalEvents(journal) {
+ let events = [];
+ for (let entry of journal) {
+ for (let event of entry.events) {
+ if (!events.includes(event)) {
+ events.push(event);
+ }
+ }
+ }
+ return events;
+}
+
+console.log(journalEvents(JOURNAL));
+// → ["carrot", "exercise", "weekend", "bread", …]
+```
+
+By adding any event names that aren't already in it to the `events` array, the function collects every type of event.
+
+Using that function, we can see all the ((correlation))s:
+
+```{test: no}
+for (let event of journalEvents(JOURNAL)) {
+ console.log(event + ":", phi(tableFor(event, JOURNAL)));
+}
+// → carrot: 0.0140970969
+// → exercise: 0.0685994341
+// → weekend: 0.1371988681
+// → bread: -0.0757554019
+// → pudding: -0.0648203724
+// And so on...
+```
+
+Most correlations seem to lie close to zero. Eating carrots, bread, or pudding apparently does not trigger squirrel-lycanthropy. The transformations _do_ seem to occur somewhat more often on weekends. Let's filter the results to show only correlations greater than 0.1 or less than -0.1:
+
+```{test: no, startCode: true}
+for (let event of journalEvents(JOURNAL)) {
+ let correlation = phi(tableFor(event, JOURNAL));
+ if (correlation > 0.1 || correlation < -0.1) {
+ console.log(event + ":", correlation);
+ }
+}
+// → weekend: 0.1371988681
+// → brushed teeth: -0.3805211953
+// → candy: 0.1296407447
+// → work: -0.1371988681
+// → spaghetti: 0.2425356250
+// → reading: 0.1106828054
+// → peanuts: 0.5902679812
+```
+
+Aha! There are two factors with a ((correlation)) clearly stronger than the others. Eating ((peanuts)) has a strong positive effect on the chance of turning into a squirrel, whereas brushing teeth has a significant negative effect.
+
+Interesting. Let's try something.
+
+```
+for (let entry of JOURNAL) {
+ if (entry.events.includes("peanuts") &&
+ !entry.events.includes("brushed teeth")) {
+ entry.events.push("peanut teeth");
+ }
+}
+console.log(phi(tableFor("peanut teeth", JOURNAL)));
+// → 1
+```
+
+That's a strong result. The phenomenon occurs precisely when Jacques eats ((peanuts)) and fails to brush his teeth. If only he weren't such a slob about dental hygiene, he'd never even have noticed his affliction.
+
+Knowing this, Jacques stops eating peanuts altogether and finds that his transformations stop.
+
+{{index "weresquirrel example"}}
+
+But it takes only a few months for him to notice that something is missing from this entirely human way of living. Without his feral adventures, Jacques hardly feels alive at all. He decides he'd rather be a full-time wild animal. After building a beautiful little tree house in the forest and equipping it with a peanut butter dispenser and a ten-year supply of peanut butter, he changes form one last time, and lives the short and energetic life of a squirrel.
+
+## Further arrayology
+
+{{index [array, methods], [method, array]}}
+
+Before finishing the chapter, I want to introduce you to a few more object-related concepts. I'll start with some generally useful array methods.
+
+{{index "push method", "pop method", "shift method", "unshift method"}}
+
+We saw `push` and `pop`, which add and remove elements at the end of an array, [earlier](data#array_methods) in this chapter. The corresponding methods for adding and removing things at the start of an array are called `unshift` and `shift`.
+
+```
+let todoList = [];
+function remember(task) {
+ todoList.push(task);
+}
+function getTask() {
+ return todoList.shift();
+}
+function rememberUrgently(task) {
+ todoList.unshift(task);
+}
+```
+
+{{index "task management example"}}
+
+This program manages a queue of tasks. You add tasks to the end of the queue by calling `remember("groceries")`, and when you're ready to do something, you call `getTask()` to get (and remove) the front item from the queue. The `rememberUrgently` function also adds a task but adds it to the front instead of the back of the queue.
+
+{{index [array, searching], "indexOf method", "lastIndexOf method"}}
+
+To search for a specific value, arrays provide an `indexOf` method. The method searches through the array from the start to the end and returns the index at which the requested value was found—or -1 if it wasn't found. To search from the end instead of the start, there's a similar method called `lastIndexOf`:
+
+```
+console.log([1, 2, 3, 2, 1].indexOf(2));
+// → 1
+console.log([1, 2, 3, 2, 1].lastIndexOf(2));
+// → 3
+```
+
+Both `indexOf` and `lastIndexOf` take an optional second argument that indicates where to start searching.
+
+{{index "slice method", [array, indexing]}}
+
+Another fundamental array method is `slice`, which takes start and end indices and returns an array that has only the elements between them. The start index is inclusive and the end index is exclusive.
+
+```
+console.log([0, 1, 2, 3, 4].slice(2, 4));
+// → [2, 3]
+console.log([0, 1, 2, 3, 4].slice(2));
+// → [2, 3, 4]
+```
+
+{{index [string, indexing]}}
+
+When the end index is not given, `slice` will take all of the elements after the start index. You can also omit the start index to copy the entire array.
+
+{{index concatenation, "concat method"}}
+
+The `concat` method can be used to append arrays together to create a new array, similar to what the `+` operator does for strings.
+
+The following example shows both `concat` and `slice` in action. It takes an array and an index and returns a new array that is a copy of the original array with the element at the given index removed:
+
+```
+function remove(array, index) {
+ return array.slice(0, index)
+ .concat(array.slice(index + 1));
+}
+console.log(remove(["a", "b", "c", "d", "e"], 2));
+// → ["a", "b", "d", "e"]
+```
+
+If you pass `concat` an argument that is not an array, that value will be added to the new array as if it were a one-element array.
+
+## Strings and their properties
+
+{{index [string, properties]}}
+
+We can read properties like `length` and `toUpperCase` from string values. But if we try to add a new property, it doesn't stick.
+
+```
+let kim = "Kim";
+kim.age = 88;
+console.log(kim.age);
+// → undefined
+```
+
+Values of type string, number, and Boolean are not objects, and though the language doesn't complain if you try to set new properties on them, it doesn't actually store those properties. As mentioned earlier, such values are immutable and cannot be changed.
+
+{{index [string, methods], "slice method", "indexOf method", [string, searching]}}
+
+But these types do have built-in properties. Every string value has a number of methods. Some very useful ones are `slice` and `indexOf`, which resemble the array methods of the same name:
+
+```
+console.log("coconuts".slice(4, 7));
+// → nut
+console.log("coconut".indexOf("u"));
+// → 5
+```
+
+One difference is that a string's `indexOf` can search for a string containing more than one character, whereas the corresponding array method looks only for a single element:
+
+```
+console.log("one two three".indexOf("ee"));
+// → 11
+```
+
+{{index [whitespace, trimming], "trim method"}}
+
+The `trim` method removes whitespace (spaces, newlines, tabs, and similar characters) from the start and end of a string:
+
+```
+console.log(" okay \n ".trim());
+// → okay
+```
+
+{{id padStart}}
+
+The `zeroPad` function from the [previous chapter](functions) also exists as a method. It is called `padStart` and takes the desired length and padding character as arguments:
+
+```
+console.log(String(6).padStart(3, "0"));
+// → 006
+```
+
+{{id split}}
+
+{{index "split method"}}
+
+You can split a string on every occurrence of another string with `split` and join it again with `join`:
+
+```
+let sentence = "Secretarybirds specialize in stomping";
+let words = sentence.split(" ");
+console.log(words);
+// → ["Secretarybirds", "specialize", "in", "stomping"]
+console.log(words.join(". "));
+// → Secretarybirds. specialize. in. stomping
+```
+
+{{index "repeat method"}}
+
+A string can be repeated with the `repeat` method, which creates a new string containing multiple copies of the original string, glued together:
+
+```
+console.log("LA".repeat(3));
+// → LALALA
+```
+
+{{index ["length property", "for string"], [string, indexing]}}
+
+We have already seen the string type's `length` property. Accessing the individual characters in a string looks like accessing array elements (with a complication that we'll discuss in [Chapter ?](higher_order#code_units)).
+
+```
+let string = "abc";
+console.log(string.length);
+// → 3
+console.log(string[1]);
+// → b
+```
+
+{{id rest_parameters}}
+
+## Rest parameters
+
+{{index "Math.max function", "period character", "max example", spread, [array, "of rest arguments"]}}
+
+It can be useful for a function to accept any number of ((argument))s. For example, `Math.max` computes the maximum of _all_ the arguments it is given. To write such a function, you put three dots before the function's last ((parameter)), like this:
+
+```{includeCode: strip_log}
+function max(...numbers) {
+ let result = -Infinity;
+ for (let number of numbers) {
+ if (number > result) result = number;
+ }
+ return result;
+}
+console.log(max(4, 1, 9, -2));
+// → 9
+```
+
+When such a function is called, the _((rest parameter))_ is bound to an array containing all further arguments. If there are other parameters before it, their values aren't part of that array. When, as in `max`, it is the only parameter, it will hold all arguments.
+
+{{index [function, application]}}
+
+You can use a similar three-dot notation to _call_ a function with an array of arguments.
+
+```
+let numbers = [5, 1, 7];
+console.log(max(...numbers));
+// → 7
+```
+
+This "((spread))s" out the array into the function call, passing its elements as separate arguments. It is possible to include an array like that along with other arguments, as in `max(9, ...numbers, 2)`.
+
+{{index "[] (array)"}}
+
+Square bracket array notation similarly allows the triple-dot operator to spread another array into the new array:
+
+```
+let words = ["never", "fully"];
+console.log(["will", ...words, "understand"]);
+// → ["will", "never", "fully", "understand"]
+```
+
+{{index "{} (object)"}}
+
+This works even in curly brace objects, where it adds all properties from another object. If a property is added multiple times, the last value to be added wins:
+
+```
+let coordinates = {x: 10, y: 0};
+console.log({...coordinates, y: 5, z: 1});
+// → {x: 10, y: 5, z: 1}
+```
+
+## The Math object
+
+{{index "Math object", "Math.min function", "Math.max function", "Math.sqrt function", minimum, maximum, "square root"}}
+
+As we've seen, `Math` is a grab bag of number-related utility functions such as `Math.max` (maximum), `Math.min` (minimum), and `Math.sqrt` (square root).
+
+{{index namespace, [object, property]}}
+
+{{id namespace_pollution}}
+
+The `Math` object is used as a container to group a bunch of related functionality. There is only one `Math` object, and it is almost never useful as a value. Rather, it provides a _namespace_ so that all these functions and values do not have to be global bindings.
+
+{{index [binding, naming]}}
+
+Having too many global bindings "pollutes" the namespace. The more names have been taken, the more likely you are to accidentally overwrite the value of some existing binding. For example, it's not unlikely you'll want to name something `max` in one of your programs. Since JavaScript's built-in `max` function is tucked safely inside the `Math` object, you don't have to worry about overwriting it.
+
+{{index "let keyword", "const keyword"}}
+
+Many languages will stop you, or at least warn you, when you are defining a binding with a name that is already taken. JavaScript does this for bindings you declared with `let` or `const` but—perversely—not for standard bindings nor for bindings declared with `var` or `function`.
+
+{{index "Math.cos function", "Math.sin function", "Math.tan function", "Math.acos function", "Math.asin function", "Math.atan function", "Math.PI constant", cosine, sine, tangent, "PI constant", pi}}
+
+Back to the `Math` object. If you need to do ((trigonometry)), `Math` can help. It contains `cos` (cosine), `sin` (sine), and `tan` (tangent), as well as their inverse functions, `acos`, `asin`, and `atan`, respectively. The number π (pi)—or at least the closest approximation that fits in a JavaScript number—is available as `Math.PI`. There is an old programming tradition of writing the names of ((constant)) values in all caps.
+
+```{test: no}
+function randomPointOnCircle(radius) {
+ let angle = Math.random() * 2 * Math.PI;
+ return {x: radius * Math.cos(angle),
+ y: radius * Math.sin(angle)};
+}
+console.log(randomPointOnCircle(2));
+// → {x: 0.3667, y: 1.966}
+```
+
+If you're not familiar with sines and cosines, don't worry. I'll explain them when they are used in [Chapter ?](dom#sin_cos).
+
+{{index "Math.random function", "random number"}}
+
+The previous example used `Math.random`. This is a function that returns a new pseudorandom number between 0 (inclusive) and 1 (exclusive) every time you call it:
+
+```{test: no}
+console.log(Math.random());
+// → 0.36993729369714856
+console.log(Math.random());
+// → 0.727367032552138
+console.log(Math.random());
+// → 0.40180766698904335
+```
+
+{{index "pseudorandom number", "random number"}}
+
+Though computers are deterministic machines—they always react the same way if given the same input—it is possible to have them produce numbers that appear random. To do that, the machine keeps some hidden value, and whenever you ask for a new random number, it performs complicated computations on this hidden value to create a new value. It stores a new value and returns some number derived from it. That way, it can produce ever new, hard-to-predict numbers in a way that _seems_ random.
+
+{{index rounding, "Math.floor function"}}
+
+If we want a whole random number instead of a fractional one, we can use `Math.floor` (which rounds down to the nearest whole number) on the result of `Math.random`:
+
+```{test: no}
+console.log(Math.floor(Math.random() * 10));
+// → 2
+```
+
+Multiplying the random number by 10 gives us a number greater than or equal to 0 and below 10. Since `Math.floor` rounds down, this expression will produce, with equal chance, any number from 0 through 9.
+
+{{index "Math.ceil function", "Math.round function", "Math.abs function", "absolute value"}}
+
+There are also the functions `Math.ceil` (for "ceiling", which rounds up to a whole number), `Math.round` (to the nearest whole number), and `Math.abs`, which takes the absolute value of a number, meaning it negates negative values but leaves positive ones as they are.
+
+## Destructuring
+
+{{index "phi function"}}
+
+Let's return to the `phi` function for a moment.
+
+```{test: wrap}
+function phi(table) {
+ return (table[3] * table[0] - table[2] * table[1]) /
+ Math.sqrt((table[2] + table[3]) *
+ (table[0] + table[1]) *
+ (table[1] + table[3]) *
+ (table[0] + table[2]));
+}
+```
+
+{{index "destructuring binding", parameter}}
+
+One reason this function is awkward to read is that we have a binding pointing at our array, but we'd much prefer to have bindings for the _elements_ of the array—that is, `let n00 = table[0]` and so on. Fortunately, there is a succinct way to do this in JavaScript:
+
+```
+function phi([n00, n01, n10, n11]) {
+ return (n11 * n00 - n10 * n01) /
+ Math.sqrt((n10 + n11) * (n00 + n01) *
+ (n01 + n11) * (n00 + n10));
+}
+```
+
+{{index "let keyword", "var keyword", "const keyword", [binding, destructuring]}}
+
+This also works for bindings created with `let`, `var`, or `const`. If you know that the value you are binding is an array, you can use ((square brackets)) to "look inside" of the value, binding its contents.
+
+{{index [object, property], [braces, object]}}
+
+A similar trick works for objects, using braces instead of square brackets.
+
+```
+let {name} = {name: "Faraji", age: 23};
+console.log(name);
+// → Faraji
+```
+
+{{index null, undefined}}
+
+Note that if you try to destructure `null` or `undefined`, you get an error, much as you would if you directly try to access a property of those values.
+
+## Optional property access
+
+{{index "optional chaining", "period character"}}
+
+When you aren't sure whether a given value produces an object, but still want to read a property from it when it does, you can use a variant of the dot notation: `object?.property`.
+
+```
+function city(object) {
+ return object.address?.city;
+}
+console.log(city({address: {city: "Toronto"}}));
+// → Toronto
+console.log(city({name: "Vera"}));
+// → undefined
+```
+
+The expression `a?.b` means the same as `a.b` when `a` isn't null or undefined. When it is, it evaluates to `undefined`. This can be convenient when, as in the example, you aren't sure that a given property exists or when a variable might hold an undefined value.
+
+A similar notation can be used with square bracket access, and even with function calls, by putting `?.` in front of the parentheses or brackets:
+
+```
+console.log("string".notAMethod?.());
+// → undefined
+console.log({}.arrayProp?.[0]);
+// → undefined
+```
+
+## JSON
+
+{{index [array, representation], [object, representation], "data format", [memory, organization]}}
+
+Because properties grasp their value rather than contain it, objects and arrays are stored in the computer's memory as sequences of bits holding the _((address))es_—the place in memory—of their contents. An array with another array inside of it consists of (at least) one memory region for the inner array and another for the outer array, containing (among other things) a number that represents the address of the inner array.
+
+If you want to save data in a file for later or send it to another computer over the network, you have to somehow convert these tangles of memory addresses to a description that can be stored or sent. You _could_ send over your entire computer memory along with the address of the value you're interested in, I suppose, but that doesn't seem like the best approach.
+
+{{indexsee "JavaScript Object Notation", JSON}}
+
+{{index serialization, "World Wide Web"}}
+
+What we can do is _serialize_ the data. That means it is converted into a flat description. A popular serialization format is called _((JSON))_ (pronounced "Jason"), which stands for JavaScript Object Notation. It is widely used as a data storage and communication format on the web, even with languages other than JavaScript.
+
+{{index [array, notation], [object, creation], [quoting, "in JSON"], comment}}
+
+JSON looks similar to JavaScript's way of writing arrays and objects, with a few restrictions. All property names have to be surrounded by double quotes, and only simple data expressions are allowed—no function calls, bindings, or anything that involves actual computation. Comments are not allowed in JSON.
+
+A journal entry might look like this when represented as JSON data:
+
+```{lang: "json"}
+{
+ "squirrel": false,
+ "events": ["work", "touched tree", "pizza", "running"]
+}
+```
+
+{{index "JSON.stringify function", "JSON.parse function", serialization, deserialization, parsing}}
+
+JavaScript gives us the functions `JSON.stringify` and `JSON.parse` to convert data to and from this format. The first takes a JavaScript value and returns a JSON-encoded string. The second takes such a string and converts it to the value it encodes:
+
+```
+let string = JSON.stringify({squirrel: false,
+ events: ["weekend"]});
+console.log(string);
+// → {"squirrel":false,"events":["weekend"]}
+console.log(JSON.parse(string).events);
+// → ["weekend"]
+```
+
+## Summary
+
+Objects and arrays provide ways to group several values into a single value. This allows us to put a bunch of related things in a bag and run around with the bag instead of wrapping our arms around all of the individual things and trying to hold on to them separately.
+
+Most values in JavaScript have properties, with the exceptions being `null` and `undefined`. Properties are accessed using `value.prop` or `value["prop"]`. Objects tend to use names for their properties and store more or less a fixed set of them. Arrays, on the other hand, usually contain varying amounts of conceptually identical values and use numbers (starting from 0) as the names of their properties.
+
+There _are_ some named properties in arrays, such as `length` and a number of methods. Methods are functions that live in properties and (usually) act on the value of which they are a property.
+
+You can iterate over arrays using a special kind of `for` loop: `for (let element of array)`.
+
+## Exercises
+
+### The sum of a range
+
+{{index "summing (exercise)"}}
+
+The [introduction](intro) of this book alluded to the following as a nice way to compute the sum of a range of numbers:
+
+```{test: no}
+console.log(sum(range(1, 10)));
+```
+
+{{index "range function", "sum function"}}
+
+Write a `range` function that takes two arguments, `start` and `end`, and returns an array containing all the numbers from `start` up to and including `end`.
+
+Next, write a `sum` function that takes an array of numbers and returns the sum of these numbers. Run the example program and see whether it does indeed return 55.
+
+{{index "optional argument"}}
+
+As a bonus assignment, modify your `range` function to take an optional third argument that indicates the "step" value used when building the array. If no step is given, the elements should go up by increments of one, corresponding to the old behavior. The function call `range(1, 10, 2)` should return `[1, 3, 5, 7, 9]`. Make sure this also works with negative step values so that `range(5, 2, -1)` produces `[5, 4, 3, 2]`.
+
+{{if interactive
+
+```{test: no}
+// Your code here.
+
+console.log(range(1, 10));
+// → [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+console.log(range(5, 2, -1));
+// → [5, 4, 3, 2]
+console.log(sum(range(1, 10)));
+// → 55
+```
+
+if}}
+
+{{hint
+
+{{index "summing (exercise)", [array, creation], "square brackets"}}
+
+Building up an array is most easily done by first initializing a binding to `[]` (a fresh, empty array) and repeatedly calling its `push` method to add a value. Don't forget to return the array at the end of the function.
+
+{{index [array, indexing], comparison}}
+
+Since the end boundary is inclusive, you'll need to use the `<=` operator rather than `<` to check for the end of your loop.
+
+The step parameter can be an optional parameter that defaults (using the `=` operator) to 1.
+
+{{index "range function", "for loop"}}
+
+Having `range` understand negative step values is probably best done by writing two separate loops—one for counting up and one for counting down—because the comparison that checks whether the loop is finished needs to be `>=` rather than `<=` when counting downward.
+
+It might also be worthwhile to use a different default step, namely, -1, when the end of the range is smaller than the start. That way, `range(5, 2)` returns something meaningful rather than getting stuck in an ((infinite loop)). It is possible to refer to previous parameters in the default value of a parameter.
+
+hint}}
+
+### Reversing an array
+
+{{index "reversing (exercise)", "reverse method", [array, methods]}}
+
+Arrays have a `reverse` method that changes the array by inverting the order in which its elements appear. For this exercise, write two functions, `reverseArray` and `reverseArrayInPlace`. The first, `reverseArray`, should take an array as its argument and produce a _new_ array that has the same elements in the inverse order. The second, `reverseArrayInPlace`, should do what the `reverse` method does: _modify_ the array given as its argument by reversing its elements. Neither may use the standard `reverse` method.
+
+{{index efficiency, "pure function", "side effect"}}
+
+Thinking back to the notes about side effects and pure functions in the [previous chapter](functions#pure), which variant do you expect to be useful in more situations? Which one runs faster?
+
+{{if interactive
+
+```{test: no}
+// Your code here.
+
+let myArray = ["A", "B", "C"];
+console.log(reverseArray(myArray));
+// → ["C", "B", "A"];
+console.log(myArray);
+// → ["A", "B", "C"];
+let arrayValue = [1, 2, 3, 4, 5];
+reverseArrayInPlace(arrayValue);
+console.log(arrayValue);
+// → [5, 4, 3, 2, 1]
+```
+
+if}}
+
+{{hint
+
+{{index "reversing (exercise)"}}
+
+There are two obvious ways to implement `reverseArray`. The first is to simply go over the input array from front to back and use the `unshift` method on the new array to insert each element at its start. The second is to loop over the input array backward and use the `push` method. Iterating over an array backward requires a (somewhat awkward) `for` specification, like `(let i = array.length - 1; i >= 0; i--)`.
+
+{{index "slice method"}}
+
+Reversing the array in place is harder. You have to be careful not to overwrite elements that you will later need. Using `reverseArray` or otherwise copying the whole array (`array.slice()` is a good way to copy an array) works but is cheating.
+
+The trick is to _swap_ the first and last elements, then the second and second-to-last, and so on. You can do this by looping over half the length of the array (use `Math.floor` to round down—you don't need to touch the middle element in an array with an odd number of elements) and swapping the element at position `i` with the one at position `array.length - 1 - i`. You can use a local binding to briefly hold onto one of the elements, overwrite that one with its mirror image, and then put the value from the local binding in the place where the mirror image used to be.
+
+hint}}
+
+{{id list}}
+
+### A list
+
+{{index ["data structure", list], "list (exercise)", "linked list", array, collection}}
+
+As generic blobs of values, objects can be used to build all sorts of data structures. A common data structure is the _list_ (not to be confused with arrays). A list is a nested set of objects, with the first object holding a reference to the second, the second to the third, and so on:
+
+```{includeCode: true}
+let list = {
+ value: 1,
+ rest: {
+ value: 2,
+ rest: {
+ value: 3,
+ rest: null
+ }
+ }
+};
+```
+
+The resulting objects form a chain, as shown in the following diagram:
+
+{{figure {url: "img/linked-list.svg", alt: "A diagram showing the memory structure of a linked list. There are 3 cells, each with a value field holding a number, and a 'rest' field with an arrow to the rest of the list. The first cell's arrow points at the second cell, the second cell's arrow at the last cell, and the last cell's 'rest' field holds null.",width: "8cm"}}}
+
+{{index "structure sharing", [memory, structure sharing]}}
+
+A nice thing about lists is that they can share parts of their structure. For example, if I create two new values `{value: 0, rest: list}` and `{value: -1, rest: list}` (with `list` referring to the binding defined earlier), they are both independent lists, but they share the structure that makes up their last three elements. The original list is also still a valid three-element list.
+
+Write a function `arrayToList` that builds up a list structure like the one shown when given `[1, 2, 3]` as argument. Also write a `listToArray` function that produces an array from a list. Add the helper functions `prepend`, which takes an element and a list and creates a new list that adds the element to the front of the input list, and `nth`, which takes a list and a number and returns the element at the given position in the list (with zero referring to the first element) or `undefined` when there is no such element.
+
+{{index recursion}}
+
+If you haven't already, also write a recursive version of `nth`.
+
+{{if interactive
+
+```{test: no}
+// Your code here.
+
+console.log(arrayToList([10, 20]));
+// → {value: 10, rest: {value: 20, rest: null}}
+console.log(listToArray(arrayToList([10, 20, 30])));
+// → [10, 20, 30]
+console.log(prepend(10, prepend(20, null)));
+// → {value: 10, rest: {value: 20, rest: null}}
+console.log(nth(arrayToList([10, 20, 30]), 1));
+// → 20
+```
+
+if}}
+
+{{hint
+
+{{index "list (exercise)", "linked list"}}
+
+Building up a list is easier when done back to front. So `arrayToList` could iterate over the array backward (see the previous exercise) and, for each element, add an object to the list. You can use a local binding to hold the part of the list that was built so far and use an assignment like `list = {value: X, rest: list}` to add an element.
+
+{{index "for loop"}}
+
+To run over a list (in `listToArray` and `nth`), a `for` loop specification like this can be used:
+
+```
+for (let node = list; node; node = node.rest) {}
+```
+
+Can you see how that works? Every iteration of the loop, `node` points to the current sublist, and the body can read its `value` property to get the current element. At the end of an iteration, `node` moves to the next sublist. When that is `null`, we have reached the end of the list, and the loop is finished.
+
+{{index recursion}}
+
+The recursive version of `nth` will, similarly, look at an ever smaller part of the "tail" of the list and at the same time count down the index until it reaches zero, at which point it can return the `value` property of the node it is looking at. To get the zeroth element of a list, you simply take the `value` property of its head node. To get element _N_ + 1, you take the *N*th element of the list that's in this list's `rest` property.
+
+hint}}
+
+{{id exercise_deep_compare}}
+
+### Deep comparison
+
+{{index "deep comparison (exercise)", [comparison, deep], "deep comparison", "== operator"}}
+
+The `==` operator compares objects by identity, but sometimes you'd prefer to compare the values of their actual properties.
+
+Write a function `deepEqual` that takes two values and returns `true` only if they are the same value or are objects with the same properties, where the values of the properties are equal when compared with a recursive call to `deepEqual`.
+
+{{index null, "=== operator", "typeof operator"}}
+
+To find out whether values should be compared directly (using the `===` operator for that) or have their properties compared, you can use the `typeof` operator. If it produces `"object"` for both values, you should do a deep comparison. But you have to take one silly exception into account: because of a historical accident, `typeof null` also produces `"object"`.
+
+{{index "Object.keys function"}}
+
+The `Object.keys` function will be useful when you need to go over the properties of objects to compare them.
+
+{{if interactive
+
+```{test: no}
+// Your code here.
+
+let obj = {here: {is: "an"}, object: 2};
+console.log(deepEqual(obj, obj));
+// → true
+console.log(deepEqual(obj, {here: 1, object: 2}));
+// → false
+console.log(deepEqual(obj, {here: {is: "an"}, object: 2}));
+// → true
+```
+
+if}}
+
+{{hint
+
+{{index "deep comparison (exercise)", [comparison, deep], "typeof operator", "=== operator"}}
+
+Your test for whether you are dealing with a real object will look something like `typeof x == "object" && x != null`. Be careful to compare properties only when _both_ arguments are objects. In all other cases you can just immediately return the result of applying `===`.
+
+{{index "Object.keys function"}}
+
+Use `Object.keys` to go over the properties. You need to test whether both objects have the same set of property names and whether those properties have identical values. One way to do that is to ensure that both objects have the same number of properties (the lengths of the property lists are the same). And then, when looping over one of the object's properties to compare them, always first make sure the other actually has a property by that name. If they have the same number of properties and all properties in one also exist in the other, they have the same set of property names.
+
+{{index "return value"}}
+
+Returning the correct value from the function is best done by immediately returning `false` when a mismatch is found and returning `true` at the end of the function.
+
+hint}}
diff --git a/04_data.txt b/04_data.txt
deleted file mode 100644
index 37c189409..000000000
--- a/04_data.txt
+++ /dev/null
@@ -1,1459 +0,0 @@
-:chap_num: 4
-:prev_link: 03_functions
-:next_link: 05_higher_order
-:load_files: ["code/jacques_journal.js", "code/chapter/04_data.js"]
-
-= Data Structures: Objects and Arrays =
-
-[chapterquote="true"]
-[quote, Charles Babbage, Passages from the Life of a Philosopher (1864)]
-____
-On two occasions I have been asked, ‘Pray,
-Mr. Babbage, if you put into the machine wrong figures, will the right
-answers come out?’ [...] I am not able rightly to apprehend the kind
-of confusion of ideas that could provoke such a question.
-____
-
-(((Babbage+++,+++ Charles)))(((object)))(((data structure)))Numbers, Booleans, and strings are the
-bricks that ((data)) structures are built from. But you can't make
-much of a house out of a single brick. _Objects_ allow us to group
-values—including other objects—together and thus build more complex
-structures.
-
-The programs we have built so far have been seriously hampered by the
-fact that they were operating only on simple data types. This chapter
-will add a basic understanding of data structures to your toolkit. By
-the end of it, you'll know enough to start writing some useful
-programs.
-
-The chapter will work through a more or less realistic programming
-example, introducing concepts as they apply to the problem at hand.
-The example code will often build on functions and variables that were
-introduced earlier in the text.
-
-ifdef::tex_target[]
-
-(((sandbox)))The online coding sandbox for the book
-(http://eloquentjavascript.net/code[_eloquentjavascript.net/code_])
-provides a way to run code in the context of a specific chapter. If
-you decide to work through the examples in another environment, be
-sure to first download the full code from this chapter from the
-sandbox page.
-
-endif::tex_target[]
-
-== The weresquirrel ==
-
-(((weresquirrel example)))(((lycanthropy)))Every now and then, usually
-between eight and ten in the evening, ((Jacques)) finds himself
-transforming into a small furry rodent with a bushy tail.
-
-On one hand, Jacques is quite glad that he doesn't have classic
-lycanthropy. Turning into a squirrel tends to cause fewer problems
-than turning into a wolf. Instead of having to worry about
-accidentally eating the neighbor (_that_ would be awkward), he worries
-about being eaten by the neighbor's cat. After two occasions where he
-woke up on a precariously thin branch in the crown of an oak, naked
-and disoriented, he has taken to locking the doors and windows of his
-room at night and putting a few walnuts on the floor to keep himself
-busy.
-
-image::img/weresquirrel.svg[alt="The weresquirrel"]
-
-That takes care of the cat and oak problems. But Jacques still suffers
-from his condition. The irregular occurrences of the transformation
-make him suspect that they might be triggered by something.
-For a while, he believed that it happened only on days when he
-had touched trees. So he stopped touching trees entirely and even
-avoided going near them. But the problem persisted.
-
-(((journal)))Switching to a more scientific approach, Jacques intends
-to start keeping a daily log of everything he did that day and whether
-he changed form. With this data he hopes to narrow down the conditions
-that trigger the transformations.
-
-The first thing he does is design a data structure to store this
-information.
-
-== Data sets ==
-
-(((data structure)))To work with a chunk of digital data, we'll first
-have to find a way to represent it in our machine's ((memory)). Say,
-as a simple example, that we want to represent a ((collection)) of
-numbers: 2, 3, 5, 7, and 11.
-
-(((string)))We could get creative with strings—after all, strings
-can be any length, so we can put a lot of data into them—and use `"2 3
-5 7 11"` as our representation. But this is awkward. You'd have to
-somehow extract the digits and convert them back to numbers to access
-them.
-
-(((array,creation)))((([] (array))))Fortunately, JavaScript
-provides a data type specifically for storing sequences of values. It
-is called an _array_ and is written as a list of values between
-((square brackets)), separated by commas.
-
-[source,javascript]
-----
-var listOfNumbers = [2, 3, 5, 7, 11];
-console.log(listOfNumbers[1]);
-// → 3
-console.log(listOfNumbers[1 - 1]);
-// → 2
-----
-
-((([] (subscript))))(((array,indexing)))The notation for getting
-at the elements inside an array also uses ((square brackets)). A pair
-of square brackets immediately after an expression, with another
-expression inside of them, will look up the element in the left-hand
-expression that corresponds to the _((index))_ given by the expression
-in the brackets.
-
-[[array_indexing]]
-The first index of an array is zero, not one. So the first element can
-be read with `listOfNumbers[0]`. If you don't have a programming
-background, this convention might take some getting used to. But
-((zero-based counting)) has a long tradition in technology, and as
-long as this convention is followed consistently (which it is, in
-JavaScript), it works well.
-
-[[properties]]
-== Properties ==
-
-(((Math object)))(((Math.max function)))(((length property,for
-strings)))(((object,property)))(((period character)))We've seen a few
-suspicious-looking expressions like `myString.length` (to get the
-length of a string) and `Math.max` (the maximum function) in past
-examples. These are expressions that access a _((property))_ of some
-value. In the first case, we access the `length` property of the value
-in `myString`. In the second, we access the property named `max` in
-the `Math` object (which is a collection of mathematics-related values
-and functions).
-
-(((property)))(((null)))(((undefined)))Almost all JavaScript values
-have properties. The exceptions are `null` and `undefined`. If you try
-to access a property on one of these nonvalues, you get an error.
-
-// test: no
-
-[source,javascript]
-----
-null.length;
-// → TypeError: Cannot read property 'length' of null
-----
-
-indexsee:[dot character,period character]
-
-((([] (subscript))))(((period character)))(((square
-brackets)))(((computed property)))The two most common ways to access
-properties in JavaScript are with a dot and with square brackets. Both
-`value.x` and `value[x]` access a ((property)) on ++value++—but not
-necessarily the same property. The difference is in how `x` is
-interpreted. When using a dot, the part after the dot must be a valid
-variable name, and it directly names the property. When using square
-brackets, the expression between the brackets is _evaluated_ to get
-the property name. Whereas `value.x` fetches the property of `value`
-named “x”, `value[x]` tries to evaluate the expression `x` and uses
-the result as the property name.
-
-So if you know that the property you are interested in is called
-“length”, you say `value.length`. If you want to extract the property
-named by the value held in the variable `i`, you say `value[i]`. And
-because property names can be any string, if you want to access a
-property named “0” or “John Doe”, you must use square brackets:
-`value[0]` or `value["John Doe"]`. This is the case even though you
-know the precise name of the property in advance, because neither "0"
-nor "John Doe" is a valid variable name and so cannot be accessed
-through dot notation.
-
-(((array)))(((length property,for arrays)))(((array,length
-of)))The elements in an array are stored in properties. Because the
-names of these properties are numbers and we often need to get their
-name from a variable, we have to use the bracket syntax to access
-them. The `length` property of an array tells us how many elements it
-contains. This property name is a valid variable name, and we know its
-name in advance, so to find the length of an array, you typically
-write `array.length` because it is easier to write than
-`array["length"]`.
-
-[[methods]]
-== Methods ==
-
-(((function,as property)))(((method)))(((string)))Both string and
-array objects contain, in addition to the `length` property, a number
-of properties that refer to function values.
-
-[source,javascript]
-----
-var doh = "Doh";
-console.log(typeof doh.toUpperCase);
-// → function
-console.log(doh.toUpperCase());
-// → DOH
-----
-
-(((case conversion)))(((toUpperCase method)))(((toLowerCase
-method)))Every string has a `toUpperCase` property. When called, it
-will return a copy of the string, in which all letters have been
-converted to uppercase. There is also `toLowerCase`. You can guess
-what that does.
-
-(((this)))Interestingly, even though the call to `toUpperCase` does
-not pass any arguments, the function somehow has access to the string
-`"Doh"`, the value whose property we called. How this works is
-described in link:06_object.html#obj_methods[Chapter 6].
-
-Properties that contain functions are generally called _methods_ of
-the value they belong to. As in, “`toUpperCase` is a method of a
-string”.
-
-[[array_methods]]
-(((collection)))(((array)))(((string)))(((push
-method)))(((pop method)))(((join method)))This example demonstrates
-some methods that array objects have:
-
-[source,javascript]
-----
-var mack = [];
-mack.push("Mack");
-mack.push("the", "Knife");
-console.log(mack);
-// → ["Mack", "the", "Knife"]
-console.log(mack.join(" "));
-// → Mack the Knife
-console.log(mack.pop());
-// → Knife
-console.log(mack);
-// → ["Mack", "the"]
-----
-
-The `push` method can be used to add values to the end of an array.
-The `pop` method does the opposite: it removes the value at the end of
-the array and returns it. An array of strings can be flattened to a
-single string with the `join` method. The argument given to `join`
-determines the text that is glued between the array's elements.
-
-== Objects ==
-
-(((journal)))(((weresquirrel example)))(((array)))(((record)))Back to the weresquirrel. A set of daily log
-entries can be represented as an array. But the entries do not consist
-of just a number or a string—each entry needs to store a list of
-activities and a Boolean value that indicates whether Jacques turned
-into a squirrel. Ideally, we would like to group these values together
-into a single value and then put these grouped values into an array of
-log entries.
-
-(((syntax)))(((object)))(((property)))(((curly braces)))((({}
-(object))))Values of the type _object_ are arbitrary collections of
-properties, and we can add or remove these properties as we please.
-One way to create an object is by using a curly brace notation.
-
-[source,javascript]
-----
-var day1 = {
- squirrel: false,
- events: ["work", "touched tree", "pizza", "running",
- "television"]
-};
-console.log(day1.squirrel);
-// → false
-console.log(day1.wolf);
-// → undefined
-day1.wolf = false;
-console.log(day1.wolf);
-// → false
-----
-
-(((quoting,of object properties)))(((colon character)))Inside the
-curly braces, we can give a list of properties separated by commas.
-Each property is written as a name, followed by a colon, followed by
-an expression that provides a value for the property. Spaces and line
-breaks are not significant. When an object spans multiple lines,
-indenting it like in the previous example improves readability.
-Properties whose names are not valid variable names or valid numbers
-have to be quoted.
-
-[source,javascript]
-----
-var descriptions = {
- work: "Went to work",
- "touched tree": "Touched a tree"
-};
-----
-
-This means that ((curly braces)) have _two_ meanings in JavaScript. At
-the start of a statement, they start a block of statements. In any
-other position, they describe an object. Fortunately, it is almost
-never useful to start a statement with a curly-brace object, and in
-typical programs, there is no ambiguity between these two uses.
-
-(((undefined)))Reading a property that doesn't exist will produce the
-value `undefined`, as happens the first time we try to read the `wolf`
-property in the previous example.
-
-(((property,assignment)))(((mutability)))(((= operator)))It is
-possible to assign a value to a property expression with the `=`
-operator. This will replace the property's value if it already existed
-or create a new property on the object if it didn't.
-
-(((tentacle (analogy))))(((property,model of)))To briefly return to
-our tentacle model of ((variable)) bindings—property bindings are
-similar. They _grasp_ values, but other variables and properties might
-be holding onto those same values. You may think of objects as
-octopuses with any number of tentacles, each of which has a name
-inscribed on it.
-
-image::img/octopus-object.jpg[alt="Artist's representation of an object"]
-
-(((delete operator)))(((property,deletion)))The `delete` operator cuts
-off a leg from such an octopus. It is a unary operator that, when
-applied to a property access expression, will remove the named
-property from the object. This is not a common thing to do, but it is
-possible.
-
-[source,javascript]
-----
-var anObject = {left: 1, right: 2};
-console.log(anObject.left);
-// → 1
-delete anObject.left;
-console.log(anObject.left);
-// → undefined
-console.log("left" in anObject);
-// → false
-console.log("right" in anObject);
-// → true
-----
-
-(((in operator)))(((property,testing for)))(((object)))The binary
-`in` operator, when applied to a string and an object, returns a
-Boolean value that indicates whether that object has that property.
-The difference between setting a property to `undefined` and actually
-deleting it is that, in the first case, the object still _has_ the
-property (it just doesn't have a very interesting value), whereas in
-the second case the property is no longer present and `in` will return
-`false`.
-
-(((array)))(((collection)))Arrays, then, are just a kind of
-object specialized for storing sequences of things. If you evaluate
-`typeof [1, 2]`, this produces `"object"`. You can see them as long,
-flat octopuses with all their arms in a neat row, labeled with
-numbers.
-
-image::img/octopus-array.jpg[alt="Artist's representation of an array"]
-
-(((journal)))(((weresquirrel example)))So we can represent Jacques’
-journal as an array of objects.
-
-[source,javascript]
-----
-var journal = [
- {events: ["work", "touched tree", "pizza",
- "running", "television"],
- squirrel: false},
- {events: ["work", "ice cream", "cauliflower",
- "lasagna", "touched tree", "brushed teeth"],
- squirrel: false},
- {events: ["weekend", "cycling", "break",
- "peanuts", "beer"],
- squirrel: true},
- /* and so on... */
-];
-----
-
-== Mutability ==
-
-We will get to actual programming _real_ soon now. But first, there's
-one last piece of theory to understand.
-
-(((mutability)))(((side effect)))(((number)))(((string)))(((Boolean)))(((object)))We've seen that object
-values can be modified. The types of values discussed in earlier
-chapters, such as numbers, strings, and Booleans, are all
-__immutable__—it is impossible to change an existing value of those
-types. You can combine them and derive new values from them, but when
-you take a specific string value, that value will always remain the
-same. The text inside it cannot be changed. If you have reference to a
-string that contains `"cat"`, it is not possible for other code to
-change a character in _that_ string to make it spell `"rat"`.
-
-With objects, on the other hand, the content of a value _can_ be
-modified by changing its properties.
-
-(((object,identity)))(((identitiy)))(((memory)))When we have two
-numbers, 120 and 120, we can consider them precisely the same number,
-whether or not they refer to the same physical bits. But with objects,
-there is a difference between having two references to the same object
-and having two different objects that contain the same properties.
-Consider the following code:
-
-[source,javascript]
-----
-var object1 = {value: 10};
-var object2 = object1;
-var object3 = {value: 10};
-
-console.log(object1 == object2);
-// → true
-console.log(object1 == object3);
-// → false
-
-object1.value = 15;
-console.log(object2.value);
-// → 15
-console.log(object3.value);
-// → 10
-----
-
-(((tentacle (analogy))))(((variable,model of)))The `object1` and
-`object2` variables grasp the _same_ object, which is why changing
-`object1` also changes the value of `object2`. The variable `object3`
-points to a different object, which initially contains the same
-properties as `object1` but lives a separate life.
-
-(((== operator)))(((comparison,of objects)))(((deep
-comparison)))JavaScript's `==` operator, when comparing objects, will
-return `true` only if both objects are precisely the same value.
-Comparing different objects will return `false`, even if they have
-identical contents. There is no “deep” comparison operation built into
-JavaScript, which looks at object's contents, but it is possible to
-write it yourself (which will be one of the
-link:04_data.html#exercise_deep_compare[exercises] at the end of this
-chapter).
-
-== The lycanthrope's log ==
-
-(((weresquirrel example)))(((lycanthropy)))(((addEntry function)))So
-Jacques starts up his JavaScript interpreter and sets up the
-environment he needs to keep his ((journal)).
-
-// include_code
-
-[source,javascript]
-----
-var journal = [];
-
-function addEntry(events, didITurnIntoASquirrel) {
- journal.push({
- events: events,
- squirrel: didITurnIntoASquirrel
- });
-}
-----
-
-And then, every evening at ten—or sometimes the next morning, after
-climbing down from the top shelf of his bookcase—he records the day.
-
-[source,javascript]
-----
-addEntry(["work", "touched tree", "pizza", "running",
- "television"], false);
-addEntry(["work", "ice cream", "cauliflower", "lasagna",
- "touched tree", "brushed teeth"], false);
-addEntry(["weekend", "cycling", "break", "peanuts",
- "beer"], true);
-----
-
-Once he has enough data points, he intends to compute the
-((correlation)) between his squirrelification and each of the day's
-events and ideally learn something useful from those correlations.
-
-(((correlation)))_Correlation_ is a measure of ((dependence)) between
-((variable))s (“variables” in the statistical sense, not the
-JavaScript sense). It is usually expressed as a coefficient that
-ranges from -1 to 1. Zero correlation means the variables are not
-related, whereas a correlation of one indicates that the two are
-perfectly related—if you know one, you also know the other. Negative
-one also means that the variables are perfectly related but that they
-are opposites—when one is true, the other is false.
-
-(((phi coefficient)))For binary (Boolean) variables, the _phi_
-coefficient (_ϕ_) provides a good measure of correlation and is
-relatively easy to compute. To compute _ϕ_, we need a ((table)) _n_
-that contains the number of times the various combinations of the two
-variables were observed. For example, we could take the event of
-eating ((pizza)) and put that in a table like this:
-
-image::img/pizza-squirrel.svg[alt="Eating pizza versus turning into a squirrel",width="7cm"]
-
-_ϕ_ can be computed using the following formula, where n refers to the table:
-
-ifdef::html_target[]
-
-++++
-
-
-
ϕ =
-
-
n11n00 - n10n01
-
√
- n1•n0•n•1n•0
-
-
-
-++++
-
-endif::html_target[]
-
-ifdef::tex_target[]
-
-pass:[\begin{equation}\phi = \frac{n_{11}n_{00}-n_{10}n_{01}}{\sqrt{n_{1\bullet}n_{0\bullet}n_{\bullet1}n_{\bullet0}}}\end{equation}]
-
-endif::tex_target[]
-
-The notation (!html _n_~01~!)(!tex pass:[$n_{01}$]!) indicates the
-number of measurements where the first measurement (pizza) is false
-(0) and the second measurement (squirrelness) is true (1). In this
-example, (!html _n_~01~!)(!tex pass:[$n_{01}$]!) is 4.
-
-The value (!html _n_~1•~!)(!tex pass:[$n_{1\bullet}$]!) refers to the
-sum of all measurements where the first variable is true, which is 10
-in the example table. Likewise, (!html _n_~•0~!)(!tex pass:[$n_{\bullet0}$]!)
-refers to the sum of the measurements where the squirrel variable is false.
-
-(((correlation)))(((phi coefficient)))So for the pizza table, the part
-above the division line (the dividend) would be 1×76 - 9×4 = 40, and
-the part below it (the divisor) would be the square root of
-10×80×5×85, or (!html √340000!)(!tex pass:[$\sqrt{340000}$]!). This
-comes out to _ϕ_ ≈ 0.069, which is tiny. Eating ((pizza)) does not
-appear to have influence on the transformations.
-
-== Computing correlation ==
-
-(((array,as table)))(((nesting,of arrays)))We can represent a
-two-by-two ((table)) in JavaScript with a four-element array (`[76, 9,
-4, 1]`). We could also use other representations, such as an array
-containing two two-element arrays (`[[76, 9], [4, 1]]`) or an object
-with property names like `"11"` and `"01"`, but the flat array is
-simple and makes the expressions that access the table pleasantly
-short. We'll interpret the indices to the array as two-((bit))
-((binary number)), where the rightmost digit refers to the squirrel
-variable and the leftmost digit refers to event variable. For example,
-the binary number `10` refers to the case where the event (say,
-"pizza") is true, but Jacques didn't turn into a squirrel. And since
-binary `10` is 2 in decimal notation, we will store this value in the
-array at index 2.
-
-(((phi coefficient)))(((phi function)))This is the function that
-computes the _ϕ_ coefficient from such an array:
-
-// test: clip
-// include_code strip_log
-
-[source,javascript]
-----
-function phi(table) {
- return (table[3] * table[0] - table[2] * table[1]) /
- Math.sqrt((table[2] + table[3]) *
- (table[0] + table[1]) *
- (table[1] + table[3]) *
- (table[0] + table[2]));
-}
-
-console.log(phi([76, 9, 4, 1]));
-// → 0.068599434
-----
-
-(((square root)))(((Math.sqrt function)))This is simply a direct
-translation of the _ϕ_ formula into JavaScript. `Math.sqrt` is the
-square root function, as provided by the `Math` object in a standard
-JavaScript environment. We have to sum two fields from the table to
-get fields like (!html n~1•~!)(!tex pass:[$n_{1\bullet}$]!) because
-the sums of rows or columns are not stored directly in our data
-structure.
-
-(((JOURNAL data set)))Jacques kept his journal for three months. The
-resulting ((data set)) is available in the coding sandbox for this
-chapter(!tex (http://eloquentjavascript.net/code[_eloquentjavascript.net/code_])!),
-where it is stored in the `JOURNAL` variable, and in a downloadable
-http://eloquentjavascript.net/code/jacques_journal.js[file].
-
-(((tableFor function)))(((hasEvent function)))To extract a two-by-two
-((table)) for a specific event from this journal, we must loop over
-all the entries and tally up how many times the event occurs in
-relation to squirrel transformations.
-
-// include_code strip_log
-
-[source,javascript]
-----
-function hasEvent(event, entry) {
- return entry.events.indexOf(event) != -1;
-}
-
-function tableFor(event, journal) {
- var table = [0, 0, 0, 0];
- for (var i = 0; i < journal.length; i++) {
- var entry = journal[i], index = 0;
- if (hasEvent(event, entry)) index += 1;
- if (entry.squirrel) index += 2;
- table[index] += 1;
- }
- return table;
-}
-
-console.log(tableFor("pizza", JOURNAL));
-// → [76, 9, 4, 1]
-----
-
-(((array,searching)))(((indexOf method)))The `hasEvent` function tests
-whether an entry contains a given event. Arrays have an `indexOf`
-method that tries to find a given value (in this case, the event name)
-in the array and returns the index at which it was found or -1 if it
-wasn't found. So if the call to `indexOf` doesn't return -1, then we
-know the event was found in the entry.
-
-(((array,indexing)))The body of the loop in `tableFor` figures
-out which box in the table each journal entry falls into by checking
-whether the entry contains the specific event it's interested in and
-whether the event happens alongside a squirrel incident. The loop then
-adds one to the number in the array that corresponds to this box on
-the table.
-
-We now have the tools we need to compute individual ((correlation))s.
-The only step remaining is to find a correlation for every type of
-event that was recorded and see whether anything stands out. But how
-should we store these correlations once we compute them?
-
-== Objects as maps ==
-
-(((weresquirrel example)))(((array)))One possible way is to store
-all the ((correlation))s in an array, using objects with `name` and
-`value` properties. But that makes looking up the correlation for a
-given event somewhat cumbersome: you'd have to loop over the whole
-array to find the object with the right `name`. We could wrap this
-lookup process in a function, but we would still be writing more code,
-and the computer would be doing more work than necessary.
-
-[[object_map]]
-(((object)))(((square brackets)))(((object,as map)))(((in
-operator)))A better way is to use object properties named after the
-event types. We can use the square bracket access notation to create
-and read the properties and can use the `in` operator to test whether
-a given property exists.
-
-[source,javascript]
-----
-var map = {};
-function storePhi(event, phi) {
- map[event] = phi;
-}
-
-storePhi("pizza", 0.069);
-storePhi("touched tree", -0.081);
-console.log("pizza" in map);
-// → true
-console.log(map["touched tree"]);
-// → -0.081
-----
-
-(((data structure)))A _((map))_ is a way to go from values in one
-domain (in this case, event names) to corresponding values in another
-domain (in this case, _ϕ_ coefficients).
-
-There are a few potential problems with using objects like this, which
-we will discuss in link:06_object.html#prototypes[Chapter 6], but for
-the time being, we won't worry about those.
-
-(((for/in loop)))(((for loop)))(((object,looping over)))What if
-we want to find all the events for which we have stored a coefficient?
-The properties don't form a predictable series, like they would in an
-array, so we can not use a normal `for` loop. JavaScript provides a
-loop construct specifically for going over the properties of an
-object. It looks a little like a normal `for` loop but distinguishes
-itself by the use of the word `in`.
-
-[source,javascript]
-----
-for (var event in map)
- console.log("The correlation for '" + event + "' is " + map[event]);
-// → The correlation for 'pizza' is 0.069
-// → The correlation for 'touched tree' is -0.081
-----
-
-[[analysis]]
-== The final analysis ==
-
-(((journal)))(((weresquirrel example)))(((gatherCorrelations
-function)))To find all the types of events that are present in the
-data set, we simply process each entry in turn and then loop over the
-events in that entry. We keep an object `phis` that has correlation
-coefficients for all the event types we have seen so far. Whenever we
-run across a type that isn't in the `phis` object yet, we compute its
-correlation and add it to the object.
-
-// test: clip
-// include_code strip_log
-
-[source,javascript]
-----
-function gatherCorrelations(journal) {
- var phis = {};
- for (var entry = 0; entry < journal.length; entry++) {
- var events = journal[entry].events;
- for (var i = 0; i < events.length; i++) {
- var event = events[i];
- if (!(event in phis))
- phis[event] = phi(tableFor(event, journal));
- }
- }
- return phis;
-}
-
-var correlations = gatherCorrelations(JOURNAL);
-console.log(correlations.pizza);
-// → 0.068599434
-----
-
-(((correlation)))Let's see what came out.
-
-// test: no
-
-[source,javascript]
-----
-for (var event in correlations)
- console.log(event + ": " + correlations[event]);
-// → carrot: 0.0140970969
-// → exercise: 0.0685994341
-// → weekend: 0.1371988681
-// → bread: -0.0757554019
-// → pudding: -0.0648203724
-// and so on...
-----
-
-(((for/in loop)))Most correlations seem to lie close to zero. Eating
-carrots, bread, or pudding apparently does not trigger
-squirrel-lycanthropy. It _does_ seem to occur somewhat more often on
-weekends, however. Let's filter the results to show only correlations
-greater than 0.1 or less than -0.1.
-
-// start_code
-// test: no
-
-[source,javascript]
-----
-for (var event in correlations) {
- var correlation = correlations[event];
- if (correlation > 0.1 || correlation < -0.1)
- console.log(event + ": " + correlation);
-}
-// → weekend: 0.1371988681
-// → brushed teeth: -0.3805211953
-// → candy: 0.1296407447
-// → work: -0.1371988681
-// → spaghetti: 0.2425356250
-// → reading: 0.1106828054
-// → peanuts: 0.5902679812
-----
-
-A-ha! There are two factors whose ((correlation)) is clearly stronger
-than the others. Eating ((peanuts)) has a strong positive effect on
-the chance of turning into a squirrel, whereas brushing his teeth has
-a significant negative effect.
-
-Interesting. Let's try something.
-
-// include_code strip_log
-
-[source,javascript]
-----
-for (var i = 0; i < JOURNAL.length; i++) {
- var entry = JOURNAL[i];
- if (hasEvent("peanuts", entry) &&
- !hasEvent("brushed teeth", entry))
- entry.events.push("peanut teeth");
-}
-console.log(phi(tableFor("peanut teeth", JOURNAL)));
-// → 1
-----
-
-Well, that's unmistakable! The phenomenon occurs precisely when
-Jacques eats ((peanuts)) and fails to brush his teeth. If only he
-weren't such a slob about dental hygiene, he'd have never even noticed
-his affliction.
-
-Knowing this, Jacques simply stops eating peanuts altogether and finds
-that this completely puts an end to his transformations.
-
-(((weresquirrel example)))All is well with Jacques for a while. But a
-few years later, he loses his ((job)) and is eventually forced to take
-employment with a ((circus)), where he performs as _The Incredible
-Squirrelman_ by stuffing his mouth with peanut butter before every
-show. One day, fed up with this pitiful existence, Jacques fails to
-change back into his human form, hops through a crack in the circus
-tent, and vanishes into the forest. He is never seen again.
-
-== Further arrayology ==
-
-(((array,methods)))(((method)))Before finishing up this chapter,
-I want to introduce you to a few more object-related concepts. We'll
-start by introducing some generally useful array methods.
-
-(((push method)))(((pop method)))(((shift method)))(((unshift
-method)))We saw `push` and `pop`, which add and remove elements at the
-end of an array, link:04_data.html#array_methods[earlier] in this
-chapter. The corresponding methods for adding and removing things at
-the start of an array are called `unshift` and `shift`.
-
-[source,javascript]
-----
-var todoList = [];
-function rememberTo(task) {
- todoList.push(task);
-}
-function whatIsNext() {
- return todoList.shift();
-}
-function urgentlyRememberTo(task) {
- todoList.unshift(task);
-}
-----
-
-(((task management example)))The previous program manages lists of
-tasks. You add tasks to the end of the list by calling
-`rememberTo("eat")`, and when you're ready to do something, you call
-`whatIsNext()` to get (and remove) the front item from the list. The
-`urgentlyRememberTo` function also adds a task but adds it to the
-front instead of the back of the list.
-
-(((array,searching)))(((indexOf method)))(((lastIndexOf
-method)))The `indexOf` method has a sibling called `lastIndexof`,
-which starts searching for the given element at the end of the array
-instead of the front.
-
-[source,javascript]
-----
-console.log([1, 2, 3, 2, 1].indexOf(2));
-// → 1
-console.log([1, 2, 3, 2, 1].lastIndexOf(2));
-// → 3
-----
-
-Both `indexOf` and `lastIndexOf` take an optional second argument that
-indicates where to start searching from.
-
-(((slice method)))(((array,indexing)))Another fundamental method
-is `slice`, which takes a start index and an end index and returns an
-array that has only the elements between those indices. The start
-index is inclusive, the end index exclusive.
-
-[source,javascript]
-----
-console.log([0, 1, 2, 3, 4].slice(2, 4));
-// → [2, 3]
-console.log([0, 1, 2, 3, 4].slice(2));
-// → [2, 3, 4]
-----
-
-(((string,indexing)))When the end index is not given, `slice`
-will take all of the elements after the start index. Strings also have
-a `slice` method, which has a similar effect.
-
-(((concatenation)))(((concat method)))The `concat` method can be used
-to glue arrays together, similar to what the `+` operator does for
-strings. The following example shows both `concat` and `slice` in
-action. It takes an array and an index, and it returns a new array
-which is a copy of the original array with the element at the given
-index removed.
-
-[source,javascript]
-----
-function remove(array, index) {
- return array.slice(0, index)
- .concat(array.slice(index + 1));
-}
-console.log(remove(["a", "b", "c", "d", "e"], 2));
-// → ["a", "b", "d", "e"]
-----
-
-== Strings and their properties ==
-
-(((string,properties)))We can read properties like `length` and
-`toUpperCase` from string values. But if you try to add a new
-property, it doesn't stick.
-
-[source,javascript]
-----
-var myString = "Fido";
-myString.myProperty = "value";
-console.log(myString.myProperty);
-// → undefined
-----
-
-Values of type string, number, and Boolean are not objects, and though
-the language doesn't complain if you try to set new properties on
-them, it doesn't actually store those properties. The values are
-immutable and cannot be changed.
-
-(((string,methods)))(((slice method)))(((indexOf
-method)))(((string,searching)))But these types do have some built-in
-properties. Every string value has a number of methods. The most
-useful ones are probably `slice` and `indexOf`, which resemble the
-array methods of the same name.
-
-[source,javascript]
-----
-console.log("coconuts".slice(4, 7));
-// → nut
-console.log("coconut".indexOf("u"));
-// → 5
-----
-
-One difference is that a string's `indexOf` can take a string
-containing more than one character, whereas the corresponding array
-method looks only for a single element.
-
-[source,javascript]
-----
-console.log("one two three".indexOf("ee"));
-// → 11
-----
-
-(((whitespace)))(((trim method)))(((trimLeft method)))(((trimRight
-method)))The `trim` method removes whitespace (spaces, newlines, tabs,
-and similar characters) from the start and end of a string. To trim
-only one side, the `trimLeft` and `trimRight` methods can be used.
-
-[source,javascript]
-----
-console.log(" okay \n ".trim());
-// → okay
-console.log("|" + " a ".trimLeft() + "|");
-// → |a |
-----
-
-(((length property,for strings)))(((charAt
-method)))(((string,indexing)))We have already seen the string type's
-`length` property. Accessing the individual characters in a string can
-be done with the `charAt` method but also by simply reading numeric
-properties, like you'd do for an array.
-
-[source,javascript]
-----
-var string = "abc";
-console.log(string.length);
-// → 3
-console.log(string.charAt(0));
-// → a
-console.log(string[1]);
-// → b
-----
-
-[[arguments_object]]
-== The arguments object ==
-
-(((arguments object)))(((length
-property)))(((parameter)))(((optional argument)))(((array-like
-object)))Whenever a function is called, a special variable named
-`arguments` is added to the environment in which the function body
-runs. This variable refers to an object that holds all of the
-arguments passed to the function. Remember that in JavaScript you are
-allowed to pass more (or fewer) arguments to a function than the
-number of parameters the function itself declares.
-
-[source,javascript]
-----
-function noArguments() {}
-noArguments(1, 2, 3); // This is okay
-function threeArguments(a, b, c) {}
-threeArguments(); // And so is this
-----
-
-(((length property)))The `arguments` object has a `length` property
-that tells us the number of arguments that were really passed to the
-function. It also has a property for each argument, named 0, 1, 2, and
-so on.
-
-indexsee:[pseudo array,array-like object]
-
-(((array,methods)))If that sounds a lot like an array to you,
-you're right, it _is_ a lot like an array. But this object,
-unfortunately, does not have any array methods (like `slice` or
-`indexOf`), so it is a little harder to use than a real array.
-
-[source,javascript]
-----
-function argumentCounter() {
- console.log("You gave me", arguments.length, "arguments.");
-}
-argumentCounter("Straw man", "Tautology", "Ad hominem");
-// → You gave me 3 arguments.
-----
-
-(((journal)))(((console.log)))(((variadic function)))Some functions
-can take any number of arguments, like `console.log`. These typically
-loop over the values in their `arguments` object. They can be used to
-create very pleasant interfaces. For example, remember how we created
-the entries to Jacques’ journal.
-
-[source,javascript]
-----
-addEntry(["work", "touched tree", "pizza", "running",
- "television"], false);
-----
-
-Since he is going to be calling this function a lot, we could create
-an alternative that is easier to call.
-
-[source,javascript]
-----
-function addEntry(squirrel) {
- var entry = {events: [], squirrel: squirrel};
- for (var i = 1; i < arguments.length; i++)
- entry.events.push(arguments[i]);
- journal.push(entry);
-}
-addEntry(true, "work", "touched tree", "pizza",
- "running", "television");
-----
-
-(((arguments object,indexing)))This version reads its first argument
-(`squirrel`) in the normal way and then goes over the rest of the
-arguments (the loop starts at index 1, skipping the first) to gather
-them into an array.
-
-== The Math object ==
-
-(((Math object)))(((Math.min function)))(((Math.max
-function)))(((Math.sqrt function)))(((minimum)))(((maximum)))(((square
-root)))As we've seen, `Math` is a grab-bag of number-related utility
-functions, such as `Math.max` (maximum), `Math.min` (minimum), and
-`Math.sqrt` (square root).
-
-[[namespace_pollution]]
-(((namespace)))(((namespace pollution)))(((object)))The
-`Math` object is used simply as a container to group a bunch of
-related functionality. There is only one `Math` object, and it is
-almost never useful as a value. Rather, it provides a _namespace_ so
-that all these functions and values do not have to be global
-variables.
-
-(((variable,naming)))Having too many global variables “pollutes” the
-namespace. The more names that have been taken, the more likely you
-are to accidentally overwrite the value of some variable. For example,
-it's not unlikely that you'll want to name something `max` in one of
-your programs. Since JavaScript's built-in `max` function is tucked
-safely inside the `Math` object, we don't have to worry about
-overwriting it.
-
-Many languages will stop you, or at least warn you, when you are
-defining a variable with a name that is already taken. JavaScript does
-neither, so be careful.
-
-(((Math.cos function)))(((Math.sin function)))(((Math.tan
-function)))(((Math.acos function)))(((Math.asin
-function)))(((Math.atan function)))(((Math.PI
-constant)))(((cosine)))(((sine)))(((tangent)))(((PI constant)))(((pi)))Back to
-the `Math` object. If you need to do ((trigonometry)), `Math` can
-help. It contains `cos` (cosine), `sin` (sine), and `tan` (tangent),
-as well as their inverse functions, `acos`, `asin`, and `atan`. The
-number π (pi)—or at least the closest approximation that fits in a
-JavaScript number—is available as `Math.PI`. (There is an old
-programming tradition of writing the names of ((constant)) values in
-all caps.)
-
-// test: no
-
-[source,javascript]
-----
-function randomPointOnCircle(radius) {
- var angle = Math.random() * 2 * Math.PI;
- return {x: radius * Math.cos(angle),
- y: radius * Math.sin(angle)};
-}
-console.log(randomPointOnCircle(2));
-// → {x: 0.3667, y: 1.966}
-----
-
-If sines and cosines are not something you are very familiar with,
-don't worry. When they are used in this book, in
-link:13_dom.html#sin_cos[Chapter 13], I'll explain them.
-
-(((Math.random function)))(((random number)))The previous example
-uses `Math.random`. This is a function that returns a new
-pseudo-random number between zero (inclusive) and one (exclusive)
-every time you call it.
-
-// test: no
-
-[source,javascript]
-----
-console.log(Math.random());
-// → 0.36993729369714856
-console.log(Math.random());
-// → 0.727367032552138
-console.log(Math.random());
-// → 0.40180766698904335
-----
-
-(((pseudo-random number)))(((random number)))Though computers are
-deterministic machines—they always react the same way if given the
-same input— it is possible to have them produce numbers that appear
-random. To do this, the machine keeps a number (or a bunch of numbers)
-in its internal state. Then, every time a random number is requested,
-it performs some complicated deterministic computations on this
-internal state and returns part of the result of those computations.
-The machine also uses the outcome to change its own internal state so
-that the next "random" number produced will be different.
-
-(((rounding)))(((Math.floor function)))If we want a whole random
-number instead of a fractional one, we can use `Math.floor` (which
-rounds down to the nearest whole number) on the result of
-`Math.random`.
-
-// test: no
-
-[source,javascript]
-----
-console.log(Math.floor(Math.random() * 10));
-// → 2
-----
-
-Multiplying the random number by ten gives us a number greater than or
-equal to zero, and below ten. Since `Math.floor` rounds down, this
-expression will produce, with equal chance, any number from 0 through
-9.
-
-(((Math.ceil function)))(((Math.round function)))There are also the
-functions `Math.ceil` (for "ceiling", which rounds up to a whole
-number) and `Math.round` (to the nearest whole number).
-
-== The global object ==
-
-(((global object)))(((window variable)))(((global
-scope)))(((scope)))(((object)))The global scope, the space in which
-global variables live, can also be approached as an object in
-JavaScript. Each global variable is present as a ((property)) of this
-object. In ((browser))s, the global scope object is stored in the
-`window` variable.
-
-// test: no
-
-[source,javascript]
-----
-var myVar = 10;
-console.log("myVar" in window);
-// → true
-console.log(window.myVar);
-// → 10
-----
-
-== Summary ==
-
-Objects and arrays (which are a specific kind of object) provide ways
-to group several values into a single value. Conceptually, this allows
-us to put a bunch of related things in a bag and run around with the
-bag, instead of trying to wrap our arms around all of the individual
-things and trying to hold on to them separately.
-
-Most values in JavaScript have properties, the exceptions being `null`
-and `undefined`. Properties are accessed using `value.propName` or
-`value["propName"]`. Objects tend to use names for their properties
-and store more or less a fixed set of them. Arrays, on the other hand,
-usually contain varying numbers of conceptually identical values and
-use numbers (starting from 0) as the names of their properties.
-
-There _are_ some named properties in arrays, such as `length` and a
-number of methods. Methods are functions that live in properties and
-(usually) act on the value they are a property of.
-
-Objects can also serve as maps, associating values with names. The `in`
-operator can be used to find out whether an object contains a property with
-a given name. The same keyword can also be used in a `for` loop
-(`for (var name in object)`) to loop over an object's properties.
-
-== Exercises ==
-
-=== The sum of a range ===
-
-(((summing (exercise))))The introduction of this book alluded to the
-following as a nice way to compute the sum of a range of numbers:
-
-// test: no
-
-[source,javascript]
-----
-console.log(sum(range(1, 10)));
-----
-
-(((range function)))(((sum function)))Write a `range` function that
-takes two arguments, `start` and `end`, and returns an array
-containing all the numbers from `start` up to (and including) `end`.
-
-Next, write a `sum` function that takes an array of numbers and
-returns the sum of these numbers. Run the previous program and see
-whether it does indeed return 55.
-
-(((optional argument)))As a bonus assignment, modify your `range`
-function to take an optional third argument that indicates the “step”
-value used to build up the array. If no step is given, the array
-elements go up by increments of one, corresponding to the old
-behavior. The function call `range(1, 10, 2)` should return `[1, 3, 5,
-7, 9]`. Make sure it also works with negative step values so that
-`range(5, 2, -1)` produces `[5, 4, 3, 2]`.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-console.log(sum(range(1, 10)));
-// → 55
-console.log(range(5, 2, -1));
-// → [5, 4, 3, 2]
-----
-endif::html_target[]
-
-!!solution!!
-
-(((summing (exercise))))(((array,creation)))(((square
-brackets)))Building up an array is most easily done by first
-initializing a variable to `[]` (a fresh, empty array) and repeatedly
-calling its `push` method to add a value. Don't forget to return the
-array at the end of the function.
-
-(((array,indexing)))(((comparison)))Since the end boundary is
-inclusive, you'll need to use the `<=` operator rather than simply `<`
-to check for the end of your loop.
-
-(((arguments object)))To check whether the optional step argument was
-given, either check `arguments.length` or compare the value of the
-argument to `undefined`. If it wasn't given, simply set it to its
-((default value)) (1) at the top of the function.
-
-(((range function)))(((for loop)))Having `range` understand negative
-step values is probably best done by writing two separate loops—one
-for counting up and one for counting down—because the comparison that
-checks whether the loop is finished needs to be `>=` rather than `<=`
-when counting downward.
-
-It might also be worthwhile to use a different default step, namely,
--1, when the end of the range is smaller than the start. That way,
-`range(5, 2)` returns something meaningful, rather than getting stuck
-in an ((infinite loop)).
-
-!!solution!!
-
-=== Reversing an array ===
-
-(((reversing (exercise))))(((reverse method)))(((array,methods)))Arrays have a method `reverse`, which changes the array
-by inverting the order in which its elements appear. For this
-exercise, write two functions, `reverseArray` and
-`reverseArrayInPlace`. The first, `reverseArray`, takes an array as
-argument and produces a _new_ array that has the same elements in the
-inverse order. The second, `reverseArrayInPlace`, does what the
-`reverse` method does: it modifies the array given as argument in
-order to reverse its elements.
-
-(((efficiency)))(((pure function)))(((side effect)))Thinking back to
-the notes about side effects and pure functions in the
-link:03_functions.html#pure[previous chapter], which variant do you
-expect to be useful in more situations? Which one is more efficient?
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-console.log(reverseArray(["A", "B", "C"]));
-// → ["C", "B", "A"];
-var arrayValue = [1, 2, 3, 4, 5];
-reverseArrayInPlace(arrayValue);
-console.log(arrayValue);
-// → [5, 4, 3, 2, 1]
-----
-endif::html_target[]
-
-!!solution!!
-
-(((reversing (exercise))))There are two obvious ways to implement
-`reverseArray`. The first is to simply go over the input array from
-front to back and use the `unshift` method on the new array to insert
-each element at its start. The second is to loop over the input array
-backward and use the `push` method. Iterating over an array backward
-requires a (somewhat awkward) `for` specification like `(var i =
-array.length - 1; i >= 0; i--)`.
-
-Reversing the array in place is harder. You have to be careful not to
-overwrite elements that you will later need. Using `reverseArray` or
-otherwise copying the whole array (`array.slice(0)` is a good way to
-copy an array) works but is cheating.
-
-The trick is to _swap_ the first and last elements, then the
-second and second-to-last, and so on. You can do this by looping
-over half the length of the array (use `Math.floor` to round down—you
-don't need to touch the middle element in an array with an odd
-length) and swapping the element at position `i` with the one at
-position `array.length - 1 - i`. You can use a local variable to
-briefly hold on to one of the elements, overwrite that one with its
-mirror image, and then put the value from the local variable in the
-place where the mirror image used to be.
-
-!!solution!!
-
-=== A list ===
-
-(((data structure)))(((list (exercise))))(((linked
-list)))(((object)))(((array)))(((collection)))Objects, as generic
-blobs of values, can be used to build all sorts of data structures. A
-common data structure is the _list_ (not to be confused with the
-array). A list is a nested set of objects, with the first object
-holding a reference to the second, the second to the third, and so on.
-
-// include_code
-
-[source,javascript]
-----
-var list = {
- value: 1,
- rest: {
- value: 2,
- rest: {
- value: 3,
- rest: null
- }
- }
-};
-----
-
-The resulting objects form a chain, like this:
-
-image::img/linked-list.svg[alt="A linked list",width="6cm"]
-
-(((structure sharing)))(((memory)))A nice thing about lists is that
-they can share parts of their structure. For example, if I create two
-new values `{value: 0, rest: list}` and `{value: -1, rest: list}`
-(with `list` referring to the variable defined earlier), they are both
-independent lists, but they share the structure that makes up their
-last three elements. In addition, the original list is also still a
-valid three-element list.
-
-Write a function `arrayToList` that builds up a data structure like
-the previous one when given `[1, 2, 3]` as argument, and write a
-`listToArray` function that produces an array from a list. Also write
-the helper functions `prepend`, which takes an element and a list and
-creates a new list that adds the element to the front of the input
-list, and `nth`, which takes a list and a number and returns the
-element at the given position in the list, or `undefined` when there
-is no such element.
-
-(((recursion)))If you haven't already, also write a recursive version
-of `nth`.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-console.log(arrayToList([10, 20]));
-// → {value: 10, rest: {value: 20, rest: null}}
-console.log(listToArray(arrayToList([10, 20, 30])));
-// → [10, 20, 30]
-console.log(prepend(10, prepend(20, null)));
-// → {value: 10, rest: {value: 20, rest: null}}
-console.log(nth(arrayToList([10, 20, 30]), 1));
-// → 20
-----
-endif::html_target[]
-
-!!solution!!
-
-(((list (exercise))))(((linked list)))Building up a list is best done
-back to front. So `arrayToList` could iterate over the array backward
-(see previous exercise) and, for each element, add an object to the
-list. You can use a local variable to hold the part of the list that
-was built so far and use a pattern like `list = {value: X, rest:
-list}` to add an element.
-
-(((for loop)))To run over a list (in `listToArray` and `nth`), a `for`
-loop specification like this can be used:
-
-[source,javascript]
-----
-for (var node = list; node; node = node.rest) {}
-----
-
-Can you see how that works? Every iteration of the loop, `node` points
-to the current sublist, and the body can read its `value` property to
-get the current element. At the end of an iteration, `node` moves to
-the next sublist. When that is null, we have reached the end of the
-list and the loop is finished.
-
-(((recursion)))The recursive version of `nth` will, similarly, look at
-an ever smaller part of the “tail” of the list and at the same time
-count down the index until it reaches zero, at which point it can
-return the `value` property of the node it is looking at. To get the
-zeroeth element of a list, you simply take the `value` property of its
-head node. To get element _N_ + 1, you take the __N__th element of the
-list that's in this list's `rest` property.
-
-!!solution!!
-
-[[exercise_deep_compare]]
-=== Deep comparison ===
-
-(((deep comparison (exercise))))(((comparison)))(((deep
-comparison)))(((== operator)))The `==` operator compares objects by
-identity. But sometimes, you would prefer to compare the values of
-their actual properties.
-
-Write a function, `deepEqual`, that takes two values and returns true
-only if they are the same value or are objects with the same
-properties whose values are also equal when compared with a recursive
-call to `deepEqual`.
-
-(((null)))(((=== operator)))(((typeof operator)))To find out whether
-to compare two things by identity (use the `===` operator for that) or
-by looking at their properties, you can use the `typeof` operator. If
-it produces `"object"` for both values, you should do a deep
-comparison. But you have to take one silly exception into account: by
-a historical accident, `typeof null` also produces `"object"`.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-var obj = {here: {is: "an"}, object: 2};
-console.log(deepEqual(obj, obj));
-// → true
-console.log(deepEqual(obj, {here: 1, object: 2}));
-// → false
-console.log(deepEqual(obj, {here: {is: "an"}, object: 2}));
-// → true
-----
-endif::html_target[]
-
-!!solution!!
-
-(((deep comparison (exercise))))(((typeof operator)))(((object)))(((=== operator)))Your test for whether you are dealing with a
-real object will look something like `typeof x == "object" && x !=
-null`. Be careful to compare properties only when _both_ arguments are
-objects. In all other cases you can just immediately return the result
-of applying `===`.
-
-(((for/in loop)))(((in operator)))Use a `for`/`in` loop to go over the
-properties. You need to test whether both objects have the same set of
-property names and whether those properties have identical values. The
-first test can be done by counting the properties in both objects and
-returning false if the numbers of properties are different. If they're
-the same, then go over the properties of one object, and for each of
-them, verify that the other object also has the property. The values
-of the properties are compared by a recursive call to `deepEqual`.
-
-(((return value)))Returning the correct value from the function is
-best done by immediately returning false when a mismatch is noticed
-and returning true at the end of the function.
-
-!!solution!!
diff --git a/05_higher_order.md b/05_higher_order.md
new file mode 100644
index 000000000..8a37a8dd8
--- /dev/null
+++ b/05_higher_order.md
@@ -0,0 +1,670 @@
+{{meta {load_files: ["code/scripts.js", "code/chapter/05_higher_order.js", "code/intro.js"], zip: "node/html"}}}
+
+# Higher-Order Functions
+
+{{quote {author: "C.A.R. Hoare", title: "1980 ACM Turing Award Lecture", chapter: true}
+
+{{index "Hoare, C.A.R."}}
+
+There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies.
+
+quote}}
+
+{{figure {url: "img/chapter_picture_5.jpg", alt: "Illustration showing letters and hieroglyphs from different scripts—Latin, Greek, Arabic, ancient Egyptian, and others", chapter: true}}}
+
+{{index "program size"}}
+
+A large program is a costly program, and not just because of the time it takes to build. Size almost always involves ((complexity)), and complexity confuses programmers. Confused programmers, in turn, introduce mistakes (_((bug))s_) into programs. A large program then provides a lot of space for these bugs to hide, making them hard to find.
+
+{{index "summing example"}}
+
+Let's briefly go back to the final two example programs in the introduction. The first is self contained and six lines long.
+
+```
+let total = 0, count = 1;
+while (count <= 10) {
+ total += count;
+ count += 1;
+}
+console.log(total);
+```
+
+The second relies on two external functions and is one line long.
+
+```
+console.log(sum(range(1, 10)));
+```
+
+Which one is more likely to contain a bug?
+
+{{index "program size"}}
+
+If we count the size of the definitions of `sum` and `range`, the second program is also big—even bigger than the first. But still, I'd argue that it is more likely to be correct.
+
+{{index [abstraction, "with higher-order functions"], "domain-specific language"}}
+
+This is because the solution is expressed in a ((vocabulary)) that corresponds to the problem being solved. Summing a range of numbers isn't about loops and counters. It is about ranges and sums.
+
+The definitions of this vocabulary (the functions `sum` and `range`) will still involve loops, counters, and other incidental details. But because they are expressing simpler concepts than the program as a whole, they are easier to get right.
+
+## Abstraction
+
+In the context of programming, these kinds of vocabularies are usually called _((abstraction))s_. Abstractions give us the ability to talk about problems at a higher (or more abstract) level, without getting sidetracked by uninteresting details.
+
+{{index "recipe analogy", "pea soup"}}
+
+As an analogy, compare these two recipes for pea soup. The first goes like this:
+
+{{quote
+
+Put 1 cup of dried peas per person into a container. Add water until the peas are well covered. Leave the peas in water for at least 12 hours. Take the peas out of the water and put them in a cooking pan. Add 4 cups of water per person. Cover the pan and keep the peas simmering for two hours. Take half an onion per person. Cut it into pieces with a knife. Add it to the peas. Take a stalk of celery per person. Cut it into pieces with a knife. Add it to the peas. Take a carrot per person. Cut it into pieces. With a knife! Add it to the peas. Cook for 10 more minutes.
+
+quote}}
+
+And this is the second recipe:
+
+{{quote
+
+Per person: 1 cup dried split peas, 4 cups of water, half a chopped onion, a stalk of celery, and a carrot.
+
+Soak peas for 12 hours. Simmer for 2 hours. Chop and add vegetables. Cook for 10 more minutes.
+
+quote}}
+
+{{index vocabulary}}
+
+The second is shorter and easier to interpret. But you do need to understand a few more cooking-related words such as _soak_, _simmer_, _chop_, and, I guess, _vegetable_.
+
+When programming, we can't rely on all the words we need to be waiting for us in the dictionary. Thus, we might fall into the pattern of the first recipe—work out the precise steps the computer has to perform, one by one, blind to the higher-level concepts they express.
+
+{{index abstraction}}
+
+It is a useful skill, in programming, to notice when you are working at too low a level of abstraction.
+
+## Abstracting repetition
+
+{{index [array, iteration]}}
+
+Plain functions, as we've seen them so far, are a good way to build abstractions. But sometimes they fall short.
+
+{{index "for loop"}}
+
+It is common for a program to do something a given number of times. You can write a `for` ((loop)) for that, like this:
+
+```
+for (let i = 0; i < 10; i++) {
+ console.log(i);
+}
+```
+
+Can we abstract "doing something _N_ times" as a function? Well, it's easy to write a function that calls `console.log` _N_ times.
+
+```
+function repeatLog(n) {
+ for (let i = 0; i < n; i++) {
+ console.log(i);
+ }
+}
+```
+
+{{index [function, "higher-order"], loop, [function, "as value"]}}
+
+{{indexsee "higher-order function", "function, higher-order"}}
+
+But what if we want to do something other than logging the numbers? Since "doing something" can be represented as a function and functions are just values, we can pass our action as a function value.
+
+```{includeCode: "top_lines: 5"}
+function repeat(n, action) {
+ for (let i = 0; i < n; i++) {
+ action(i);
+ }
+}
+
+repeat(3, console.log);
+// → 0
+// → 1
+// → 2
+```
+
+We don't have to pass a predefined function to `repeat`. Often, it is easier to create a function value on the spot instead.
+
+```
+let labels = [];
+repeat(5, i => {
+ labels.push(`Unit ${i + 1}`);
+});
+console.log(labels);
+// → ["Unit 1", "Unit 2", "Unit 3", "Unit 4", "Unit 5"]
+```
+
+{{index "loop body", [braces, body], [parentheses, arguments]}}
+
+This is structured a little like a `for` loop—it first describes the kind of loop and then provides a body. However, the body is now written as a function value, which is wrapped in the parentheses of the call to `repeat`. This is why it has to be closed with the closing brace _and_ closing parenthesis. In cases like this example, where the body is a single small expression, you could also omit the braces and write the loop on a single line.
+
+## Higher-order functions
+
+{{index [function, "higher-order"], [function, "as value"]}}
+
+Functions that operate on other functions, either by taking them as arguments or by returning them, are called _higher-order functions_. Since we have already seen that functions are regular values, there is nothing particularly remarkable about the fact that such functions exist. The term comes from ((mathematics)), where the distinction between functions and other values is taken more seriously.
+
+{{index abstraction}}
+
+Higher-order functions allow us to abstract over _actions_, not just values. They come in several forms. For example, we can have functions that create new functions.
+
+```
+function greaterThan(n) {
+ return m => m > n;
+}
+let greaterThan10 = greaterThan(10);
+console.log(greaterThan10(11));
+// → true
+```
+
+We can also have functions that change other functions.
+
+```
+function noisy(f) {
+ return (...args) => {
+ console.log("calling with", args);
+ let result = f(...args);
+ console.log("called with", args, ", returned", result);
+ return result;
+ };
+}
+noisy(Math.min)(3, 2, 1);
+// → calling with [3, 2, 1]
+// → called with [3, 2, 1] , returned 1
+```
+
+We can even write functions that provide new types of ((control flow)).
+
+```
+function unless(test, then) {
+ if (!test) then();
+}
+
+repeat(3, n => {
+ unless(n % 2 == 1, () => {
+ console.log(n, "is even");
+ });
+});
+// → 0 is even
+// → 2 is even
+```
+
+{{index [array, methods], [array, iteration], "forEach method"}}
+
+There is a built-in array method, `forEach`, that provides something like a `for`/`of` loop as a higher-order function.
+
+```
+["A", "B"].forEach(l => console.log(l));
+// → A
+// → B
+```
+
+{{id scripts}}
+
+## Script dataset
+
+One area where higher-order functions shine is data processing. To process data, we'll need some actual example data. This chapter will use a ((dataset)) about scripts—((writing system))s such as Latin, Cyrillic, or Arabic.
+
+Remember ((Unicode)), the system that assigns a number to each character in written language, from [Chapter ?](values#unicode)? Most of these characters are associated with a specific script. The standard contains 140 different scripts, of which 81 are still in use today and 59 are historic.
+
+Though I can fluently read only Latin characters, I appreciate the fact that people are writing texts in at least 80 other writing systems, many of which I wouldn't even recognize. For example, here's a sample of ((Tamil)) handwriting:
+
+{{figure {url: "img/tamil.png", alt: "A line of verse in Tamil handwriting. The characters are relatively simple, and neatly separated, yet completely different from Latin."}}}
+
+{{index "SCRIPTS dataset"}}
+
+The example ((dataset)) contains some pieces of information about the 140 scripts defined in Unicode. It is available in the [coding sandbox](https://eloquentjavascript.net/code#5) for this chapter[ ([_https://eloquentjavascript.net/code#5_](https://eloquentjavascript.net/code#5))]{if book} as the `SCRIPTS` binding. The binding contains an array of objects, each of which describes a script.
+
+
+```{lang: "json"}
+{
+ name: "Coptic",
+ ranges: [[994, 1008], [11392, 11508], [11513, 11520]],
+ direction: "ltr",
+ year: -200,
+ living: false,
+ link: "https://en.wikipedia.org/wiki/Coptic_alphabet"
+}
+```
+
+Such an object tells us the name of the script, the Unicode ranges assigned to it, the direction in which it is written, the (approximate) origin time, whether it is still in use, and a link to more information. The direction may be `"ltr"` for left to right, `"rtl"` for right to left (the way Arabic and Hebrew text are written), or `"ttb"` for top to bottom (as with Mongolian writing).
+
+{{index "slice method"}}
+
+The `ranges` property contains an array of Unicode character ((range))s, each of which is a two-element array containing a lower bound and an upper bound. Any character codes within these ranges are assigned to the script. The lower ((bound)) is inclusive (code 994 is a Coptic character) and the upper bound is noninclusive (code 1008 isn't).
+
+## Filtering arrays
+
+{{index [array, methods], [array, filtering], "filter method", [function, "higher-order"], "predicate function"}}
+
+If we want to find the scripts in the dataset that are still in use, the following function might be helpful. It filters out elements in an array that don't pass a test.
+
+```
+function filter(array, test) {
+ let passed = [];
+ for (let element of array) {
+ if (test(element)) {
+ passed.push(element);
+ }
+ }
+ return passed;
+}
+
+console.log(filter(SCRIPTS, script => script.living));
+// → [{name: "Adlam", …}, …]
+```
+
+{{index [function, "as value"], [function, application]}}
+
+The function uses the argument named `test`, a function value, to fill a "gap" in the computation—the process of deciding which elements to collect.
+
+{{index "filter method", "pure function", "side effect"}}
+
+Note how the `filter` function, rather than deleting elements from the existing array, builds up a new array with only the elements that pass the test. This function is _pure_. It does not modify the array it is given.
+
+Like `forEach`, `filter` is a ((standard)) array method. The example defined the function only to show what it does internally. From now on, we'll use it like this instead:
+
+```
+console.log(SCRIPTS.filter(s => s.direction == "ttb"));
+// → [{name: "Mongolian", …}, …]
+```
+
+{{id map}}
+
+## Transforming with map
+
+{{index [array, methods], "map method"}}
+
+Say we have an array of objects representing scripts, produced by filtering the `SCRIPTS` array somehow. We want an array of names instead, which is easier to inspect.
+
+{{index [function, "higher-order"]}}
+
+The `map` method transforms an array by applying a function to all of its elements and building a new array from the returned values. The new array will have the same length as the input array, but its content will have been _mapped_ to a new form by the function.
+
+```
+function map(array, transform) {
+ let mapped = [];
+ for (let element of array) {
+ mapped.push(transform(element));
+ }
+ return mapped;
+}
+
+let rtlScripts = SCRIPTS.filter(s => s.direction == "rtl");
+console.log(map(rtlScripts, s => s.name));
+// → ["Adlam", "Arabic", "Imperial Aramaic", …]
+```
+
+Like `forEach` and `filter`, `map` is a standard array method.
+
+## Summarizing with reduce
+
+{{index [array, methods], "summing example", "reduce method"}}
+
+Another common thing to do with arrays is to compute a single value from them. Our recurring example, summing a collection of numbers, is an instance of this. Another example is finding the script with the most characters.
+
+{{indexsee "fold", "reduce method"}}
+
+{{index [function, "higher-order"], "reduce method"}}
+
+The higher-order operation that represents this pattern is called _reduce_ (sometimes also called _fold_). It builds a value by repeatedly taking a single element from the array and combining it with the current value. When summing numbers, you'd start with the number zero and, for each element, add that to the sum.
+
+The parameters to `reduce` are, apart from the array, a combining function and a start value. This function is a little less straightforward than `filter` and `map`, so take a close look at it:
+
+```
+function reduce(array, combine, start) {
+ let current = start;
+ for (let element of array) {
+ current = combine(current, element);
+ }
+ return current;
+}
+
+console.log(reduce([1, 2, 3, 4], (a, b) => a + b, 0));
+// → 10
+```
+
+{{index "reduce method", "SCRIPTS dataset"}}
+
+The standard array method `reduce`, which of course corresponds to this function, has an added convenience. If your array contains at least one element, you are allowed to leave off the `start` argument. The method will take the first element of the array as its start value and start reducing at the second element.
+
+```
+console.log([1, 2, 3, 4].reduce((a, b) => a + b));
+// → 10
+```
+
+{{index maximum, "characterCount function"}}
+
+To use `reduce` (twice) to find the script with the most characters, we can write something like this:
+
+```
+function characterCount(script) {
+ return script.ranges.reduce((count, [from, to]) => {
+ return count + (to - from);
+ }, 0);
+}
+
+console.log(SCRIPTS.reduce((a, b) => {
+ return characterCount(a) < characterCount(b) ? b : a;
+}));
+// → {name: "Han", …}
+```
+
+The `characterCount` function reduces the ranges assigned to a script by summing their sizes. Note the use of destructuring in the parameter list of the reducer function. The second call to `reduce` then uses this to find the largest script by repeatedly comparing two scripts and returning the larger one.
+
+The Han script has more than 89,000 characters assigned to it in the Unicode standard, making it by far the biggest writing system in the dataset. Han is a script sometimes used for Chinese, Japanese, and Korean text. Those languages share a lot of characters, though they tend to write them differently. The (US-based) Unicode Consortium decided to treat them as a single writing system to save character codes. This is called _Han unification_ and still makes some people very angry.
+
+## Composability
+
+{{index loop, maximum}}
+
+Consider how we would have written the previous example (finding the biggest script) without higher-order functions. The code is not that much worse.
+
+```{test: no}
+let biggest = null;
+for (let script of SCRIPTS) {
+ if (biggest == null ||
+ characterCount(biggest) < characterCount(script)) {
+ biggest = script;
+ }
+}
+console.log(biggest);
+// → {name: "Han", …}
+```
+
+There are a few more bindings, and the program is four lines longer, but it is still very readable.
+
+{{index "average function", composability, [function, "higher-order"], "filter method", "map method", "reduce method"}}
+
+{{id average_function}}
+
+The abstractions these functions provide really shine when you need to _compose_ operations. As an example, let's write code that finds the average year of origin for living and dead scripts in the dataset.
+
+```
+function average(array) {
+ return array.reduce((a, b) => a + b) / array.length;
+}
+
+console.log(Math.round(average(
+ SCRIPTS.filter(s => s.living).map(s => s.year))));
+// → 1165
+console.log(Math.round(average(
+ SCRIPTS.filter(s => !s.living).map(s => s.year))));
+// → 204
+```
+
+As you can see, the dead scripts in Unicode are, on average, older than the living ones. This is not a terribly meaningful or surprising statistic. But I hope you'll agree that the code used to compute it isn't hard to read. You can see it as a pipeline: we start with all scripts, filter out the living (or dead) ones, take the years from those, average them, and round the result.
+
+You could definitely also write this computation as one big ((loop)).
+
+```
+let total = 0, count = 0;
+for (let script of SCRIPTS) {
+ if (script.living) {
+ total += script.year;
+ count += 1;
+ }
+}
+console.log(Math.round(total / count));
+// → 1165
+```
+
+However, it is harder to see what was being computed and how. And because intermediate results aren't represented as coherent values, it'd be a lot more work to extract something like `average` into a separate function.
+
+{{index efficiency, [array, creation]}}
+
+In terms of what the computer is actually doing, these two approaches are also quite different. The first will build up new arrays when running `filter` and `map`, whereas the second computes only some numbers, doing less work. You can usually afford the readable approach, but if you're processing huge arrays and doing so many times, the less abstract style might be worth the extra speed.
+
+## Strings and character codes
+
+{{index "SCRIPTS dataset"}}
+
+One interesting use of this dataset would be figuring out what script a piece of text is using. Let's go through a program that does this.
+
+Remember that each script has an array of character code ranges associated with it. Given a character code, we could use a function like this to find the corresponding script (if any):
+
+{{index "some method", "predicate function", [array, methods]}}
+
+```{includeCode: strip_log}
+function characterScript(code) {
+ for (let script of SCRIPTS) {
+ if (script.ranges.some(([from, to]) => {
+ return code >= from && code < to;
+ })) {
+ return script;
+ }
+ }
+ return null;
+}
+
+console.log(characterScript(121));
+// → {name: "Latin", …}
+```
+
+The `some` method is another higher-order function. It takes a test function and tells you whether that function returns true for any of the elements in the array.
+
+{{id code_units}}
+
+But how do we get the character codes in a string?
+
+In [Chapter ?](values) I mentioned that JavaScript ((string))s are encoded as a sequence of 16-bit numbers. These are called _((code unit))s_. A ((Unicode)) ((character)) code was initially supposed to fit within such a unit (which gives you a little over 65,000 characters). When it became clear that wasn't going to be enough, many people balked at the need to use more memory per character. To address these concerns, ((UTF-16)), the format also used by JavaScript strings, was invented. It describes most common characters using a single 16-bit code unit but uses a pair of two such units for others.
+
+{{index error}}
+
+UTF-16 is generally considered a bad idea today. It seems almost intentionally designed to invite mistakes. It's easy to write programs that pretend code units and characters are the same thing. And if your language doesn't use two-unit characters, that will appear to work just fine. But as soon as someone tries to use such a program with some less common ((Chinese characters)), it breaks. Fortunately, with the advent of ((emoji)), everybody has started using two-unit characters, and the burden of dealing with such problems is more fairly distributed.
+
+{{index [string, length], [string, indexing], "charCodeAt method"}}
+
+Unfortunately, obvious operations on JavaScript strings, such as getting their length through the `length` property and accessing their content using square brackets, deal only with code units.
+
+```{test: no}
+// Two emoji characters, horse and shoe
+let horseShoe = "🐴👟";
+console.log(horseShoe.length);
+// → 4
+console.log(horseShoe[0]);
+// → (Invalid half-character)
+console.log(horseShoe.charCodeAt(0));
+// → 55357 (Code of the half-character)
+console.log(horseShoe.codePointAt(0));
+// → 128052 (Actual code for horse emoji)
+```
+
+{{index "codePointAt method"}}
+
+JavaScript's `charCodeAt` method gives you a code unit, not a full character code. The `codePointAt` method, added later, does give a full Unicode character, so we could use that to get characters from a string. But the argument passed to `codePointAt` is still an index into the sequence of code units. To run over all characters in a string, we'd still need to deal with the question of whether a character takes up one or two code units.
+
+{{index "for/of loop", character}}
+
+In the [previous chapter](data#for_of_loop), I mentioned that a `for`/`of` loop can also be used on strings. Like `codePointAt`, this type of loop was introduced at a time when people were acutely aware of the problems with UTF-16. When you use it to loop over a string, it gives you real characters, not code units.
+
+```
+let roseDragon = "🌹🐉";
+for (let char of roseDragon) {
+ console.log(char);
+}
+// → 🌹
+// → 🐉
+```
+
+If you have a character (which will be a string of one or two code units), you can use `codePointAt(0)` to get its code.
+
+## Recognizing text
+
+{{index "SCRIPTS dataset", "countBy function", [array, counting]}}
+
+We have a `characterScript` function and a way to correctly loop over characters. The next step is to count the characters that belong to each script. The following counting abstraction will be useful there:
+
+```{includeCode: strip_log}
+function countBy(items, groupName) {
+ let counts = [];
+ for (let item of items) {
+ let name = groupName(item);
+ let known = counts.find(c => c.name == name);
+ if (!known) {
+ counts.push({name, count: 1});
+ } else {
+ known.count++;
+ }
+ }
+ return counts;
+}
+
+console.log(countBy([1, 2, 3, 4, 5], n => n > 2));
+// → [{name: false, count: 2}, {name: true, count: 3}]
+```
+
+The `countBy` function expects a collection (anything that we can loop over with `for`/`of`) and a function that computes a group name for a given element. It returns an array of objects, each of which names a group and tells you the number of elements that were found in that group.
+
+{{index "find method"}}
+
+It uses another array method, `find`, which goes over the elements in the array and returns the first one for which a function returns true. It returns `undefined` when it finds no such element.
+
+{{index "textScripts function", "Chinese characters"}}
+
+Using `countBy`, we can write the function that tells us which scripts are used in a piece of text.
+
+```{includeCode: strip_log, startCode: true}
+function textScripts(text) {
+ let scripts = countBy(text, char => {
+ let script = characterScript(char.codePointAt(0));
+ return script ? script.name : "none";
+ }).filter(({name}) => name != "none");
+
+ let total = scripts.reduce((n, {count}) => n + count, 0);
+ if (total == 0) return "No scripts found";
+
+ return scripts.map(({name, count}) => {
+ return `${Math.round(count * 100 / total)}% ${name}`;
+ }).join(", ");
+}
+
+console.log(textScripts('英国的狗说"woof", 俄罗斯的狗说"тяв"'));
+// → 61% Han, 22% Latin, 17% Cyrillic
+```
+
+{{index "characterScript function", "filter method"}}
+
+The function first counts the characters by name, using `characterScript` to assign them a name and falling back to the string `"none"` for characters that aren't part of any script. The `filter` call drops the entry for `"none"` from the resulting array, since we aren't interested in those characters.
+
+{{index "reduce method", "map method", "join method", [array, methods]}}
+
+To be able to compute ((percentage))s, we first need the total number of characters that belong to a script, which we can compute with `reduce`. If we find no such characters, the function returns a specific string. Otherwise, it transforms the counting entries into readable strings with `map` and then combines them with `join`.
+
+## Summary
+
+Being able to pass function values to other functions is a deeply useful aspect of JavaScript. It allows us to write functions that model computations with "gaps" in them. The code that calls these functions can fill in the gaps by providing function values.
+
+Arrays provide a number of useful higher-order methods. You can use `forEach` to loop over the elements in an array. The `filter` method returns a new array containing only the elements that pass the ((predicate function)). You can transform an array by putting each element through a function using `map`. You can use `reduce` to combine all the elements in an array into a single value. The `some` method tests whether any element matches a given predicate function, while `find` finds the first element that matches a predicate.
+
+## Exercises
+
+### Flattening
+
+{{index "flattening (exercise)", "reduce method", "concat method", [array, flattening]}}
+
+Use the `reduce` method in combination with the `concat` method to "flatten" an array of arrays into a single array that has all the elements of the original arrays.
+
+{{if interactive
+
+```{test: no}
+let arrays = [[1, 2, 3], [4, 5], [6]];
+// Your code here.
+// → [1, 2, 3, 4, 5, 6]
+```
+if}}
+
+### Your own loop
+
+{{index "your own loop (example)", "for loop"}}
+
+Write a higher-order function `loop` that provides something like a `for` loop statement. It should take a value, a test function, an update function, and a body function. Each iteration, it should first run the test function on the current loop value and stop if that returns `false`. It should then call the body function, giving it the current value, and finally call the update function to create a new value and start over from the beginning.
+
+When defining the function, you can use a regular loop to do the actual looping.
+
+{{if interactive
+
+```{test: no}
+// Your code here.
+
+loop(3, n => n > 0, n => n - 1, console.log);
+// → 3
+// → 2
+// → 1
+```
+
+if}}
+
+### Everything
+
+{{index "predicate function", "everything (exercise)", "every method", "some method", [array, methods], "&& operator", "|| operator"}}
+
+Arrays also have an `every` method analogous to the `some` method. This method returns `true` when the given function returns `true` for _every_ element in the array. In a way, `some` is a version of the `||` operator that acts on arrays, and `every` is like the `&&` operator.
+
+Implement `every` as a function that takes an array and a predicate function as parameters. Write two versions, one using a loop and one using the `some` method.
+
+{{if interactive
+
+```{test: no}
+function every(array, test) {
+ // Your code here.
+}
+
+console.log(every([1, 3, 5], n => n < 10));
+// → true
+console.log(every([2, 4, 16], n => n < 10));
+// → false
+console.log(every([], n => n < 10));
+// → true
+```
+
+if}}
+
+{{hint
+
+{{index "everything (exercise)", "short-circuit evaluation", "return keyword"}}
+
+Like the `&&` operator, the `every` method can stop evaluating further elements as soon as it has found one that doesn't match. So the loop-based version can jump out of the loop—with `break` or `return`—as soon as it runs into an element for which the predicate function returns `false`. If the loop runs to its end without finding such an element, we know that all elements matched and we should return `true`.
+
+To build `every` on top of `some`, we can apply _((De Morgan's laws))_, which state that `a && b` equals `!(!a || !b)`. This can be generalized to arrays, where all elements in the array match if there is no element in the array that does not match.
+
+hint}}
+
+### Dominant writing direction
+
+{{index "SCRIPTS dataset", "direction (writing)", "groupBy function", "dominant direction (exercise)"}}
+
+Write a function that computes the dominant writing direction in a string of text. Remember that each script object has a `direction` property that can be `"ltr"` (left to right), `"rtl"` (right to left), or `"ttb"` (top to bottom).
+
+{{index "characterScript function", "countBy function"}}
+
+The dominant direction is the direction of a majority of the characters that have a script associated with them. The `characterScript` and `countBy` functions defined earlier in the chapter are probably useful here.
+
+{{if interactive
+
+```{test: no}
+function dominantDirection(text) {
+ // Your code here.
+}
+
+console.log(dominantDirection("Hello!"));
+// → ltr
+console.log(dominantDirection("Hey, مساء الخير"));
+// → rtl
+```
+if}}
+
+{{hint
+
+{{index "dominant direction (exercise)", "textScripts function", "filter method", "characterScript function"}}
+
+Your solution might look a lot like the first half of the `textScripts` example. You again have to count characters by a criterion based on `characterScript` and then filter out the part of the result that refers to uninteresting (script-less) characters.
+
+{{index "reduce method"}}
+
+Finding the direction with the highest character count can be done with `reduce`. If it's not clear how, refer to the example earlier in the chapter, where `reduce` was used to find the script with the most characters.
+
+hint}}
diff --git a/05_higher_order.txt b/05_higher_order.txt
deleted file mode 100644
index 625fe4828..000000000
--- a/05_higher_order.txt
+++ /dev/null
@@ -1,1099 +0,0 @@
-:chap_num: 5
-:prev_link: 04_data
-:next_link: 06_object
-:load_files: ["code/ancestry.js", "code/chapter/05_higher_order.js", "js/intro.js"]
-
-= Higher-Order Functions =
-
-ifdef::html_target[]
-
-[chapterquote="true"]
-[quote, Master Yuan-Ma, The Book of Programming]
-____
-Tzu-li and Tzu-ssu were
-boasting about the size of their latest programs. ‘Two-hundred
-thousand lines,’ said Tzu-li, ‘not counting comments!’ Tzu-ssu
-responded, ‘Pssh, mine is almost a *million* lines already.’ Master
-Yuan-Ma said, ‘My best program has five hundred lines.’ Hearing this,
-Tzu-li and Tzu-ssu were enlightened.
-____
-
-endif::html_target[]
-
-[chapterquote="true"]
-[quote, C.A.R. Hoare, 1980 ACM Turing Award Lecture]
-____
-(((Hoare+++,+++ C.A.R.)))There are two ways of constructing a software
-design: One way is to make it so simple that there are obviously no
-deficiencies, and the other way is to make it so complicated that
-there are no obvious deficiencies.
-____
-
-(((program size)))A large program is a costly program, and not just
-because of the time it takes to build. Size almost always involves
-((complexity)), and complexity confuses programmers. Confused
-programmers, in turn, tend to introduce mistakes (_((bug))s_) into
-programs. A large program also provides a lot of space for these bugs
-to hide, making them hard to find.
-
-(((summing example)))Let us briefly go back to the final two example
-programs in the introduction. The first is self-contained, and six
-lines long.
-
-[source,javascript]
-----
-var total = 0, count = 1;
-while (count <= 10) {
- total += count;
- count += 1;
-}
-console.log(total);
-----
-
-The second relies on two external functions, and is one line long.
-
-[source,javascript]
-----
-console.log(sum(range(1, 10)));
-----
-
-Which one is more likely to contain a bug?
-
-(((program size)))If we count the size of the definitions of `sum` and
-`range`, the second program is also big—even bigger than the first.
-But still, I'd argue that it is more likely to be correct.
-
-(((abstraction)))(((domain-specific language)))It is is more likely to
-be correct because the solution is expressed in a ((vocabulary)) that
-corresponds to the problem that is being solved. Summing a range of
-numbers isn't about loops and counters, it is about ranges and sums.
-
-The definitions of this vocabulary (the functions `sum` and `range`)
-will still involve loops, counters, and other incidental details. But
-because they are expressing simpler concepts than the program as a
-whole, they are easier to get right.
-
-== Abstraction ==
-
-In the context of programming, these kinds of vocabularies are usually
-called _((abstraction))s_. Abstractions hide details, and give us the
-ability to talk about problems at a higher (or more abstract) level.
-
-(((recipe analogy)))(((pea soup)))As an analogy, compare these two
-recipes for pea soup:
-
-____
-Put 1 cup of dried peas per person into a container. Add water until
-the peas are well covered. Leave the peas in water for at least 12 hours.
-Take the peas out of the water and put them in a cooking pan. Add 4
-cups of water per person. Cover the pan and keep the peas
-simmering for two hours. Take half an onion per person. Cut it into
-pieces with a knife. Add it to the peas. Take a stalk of celery per
-person. Cut it into pieces with a knife. Add it to the peas. Take a
-carrot per person. Cut it into pieces. With a knife! Add it to the
-peas. Cook for 10 more minutes.
-____
-
-And the second recipe:
-____
-Per person: 1 cup dried split peas, half a chopped onion, a stalk of
-celery, and a carrot.
-
-Soak peas for 12 hours. Simmer for 2 hours in 4 cups of water
-(per person). Chop and add vegetables. Cook for 10 more minutes.
-____
-
-(((vocabulary)))The second is shorter, and easier to interpret. But
-you do need to understand a few more cooking-related words—“soak”,
-“simmer”, “chop”, and, I guess, “vegetables”.
-
-When programming, we can't rely on all the words we need to be waiting
-for us in the dictionary. Thus, you might fall into the pattern of the
-first recipe—work out the precise steps the computer has to perform,
-one by one, blind to the higher-level concepts that they express.
-
-(((abstraction)))It has to become second nature, for a programmer, to
-notice when a concept is begging to be abstracted into a new word.
-
-== Abstracting array traversal ==
-
-(((array)))Plain functions, as we've seen them so far, are a good
-way to build abstractions. But sometimes they fall short.
-
-(((for loop)))In the link:04_data.html#data[previous chapter], this
-type of `for` ((loop)) made several appearances:
-
-[source,javascript]
-----
-var array = [1, 2, 3];
-for (var i = 0; i < array.length; i++) {
- var current = array[i];
- console.log(current);
-}
-----
-
-(((length property,for
-array)))(((array,indexing)))(((readability)))It's trying to say: “For
-each element in the array, log it to the console”. But it uses a
-roundabout way that involves a counter variable `i`, a check against
-the array's length, and an extra variable declaration to pick out the
-current element. Apart from being a bit of an eyesore, this provides a
-lot of space for potential mistakes. We might accidentally reuse the
-`i` variable, or misspell `lenght`, or confuse the `i` and `current`
-variables, and so on.
-
-So let's try to abstract this into a function. Can you think of a way?
-
-Well, it's easy to write a function that goes over an array and calls
-`console.log` on every element:
-
-[source,javascript]
-----
-function logEach(array) {
- for (var i = 0; i < array.length; i++)
- console.log(array[i]);
-}
-----
-
-indexsee:[higher-order function,function+++,+++ higher-order]
-
-[[forEach]]
-(((function,higher-order)))(((loop)))(((array,traversal)))(((function,as value)))(((forEach method)))But what
-if we want to do something other than logging the elements? Since
-“doing something” can be represented as a function, and functions are
-just values, we can pass our action as a function value:
-
-[source,javascript]
-----
-function forEach(array, action) {
- for (var i = 0; i < array.length; i++)
- action(array[i]);
-}
-
-forEach(["Wampeter", "Foma", "Granfalloon"], console.log);
-// → Wampeter
-// → Foma
-// → Granfalloon
-----
-
-Often, you don't pass a pre-defined function to `forEach`, but create
-a function value on the spot instead.
-
-[source,javascript]
-----
-var numbers = [1, 2, 3, 4, 5], sum = 0;
-forEach(numbers, function(number) {
- sum += number;
-});
-console.log(sum);
-// → 15
-----
-
-(((loop body)))(((curly braces)))This looks quite a lot like the
-classical `for` loop, with its body written as a block below it.
-Except that now the body is inside of the function value, as well as
-inside of the ((parentheses)) of the call to `forEach`. This is why it
-has to be closed with the closing brace _and_ closing parenthesis.
-
-(((local variable)))(((parameter)))Using this pattern, we can
-specify a variable name for the current element (`number`), rather
-than having to pick it out of the array manually.
-
-(((array,methods)))(((function,higher-order)))(((forEach
-method)))(((array)))In fact, we don't need to write `forEach`
-ourselves. It is available as a standard method on arrays. Since the
-array is already provided as the thing the method acts on, `forEach`
-only takes one required argument: the function to be executed for each
-element.
-
-To illustrate how helpful this is, let's look back at this function
-from link:04_data.html#analysis[the previous chapter]. It contains two
-array-traversing ((loop))s:
-
-[source,javascript]
-----
-function gatherCorrelations(journal) {
- var phis = {};
- for (var entry = 0; entry < journal.length; entry++) {
- var events = journal[entry].events;
- for (var i = 0; i < events.length; i++) {
- var event = events[i];
- if (!(event in phis))
- phis[event] = phi(tableFor(event, journal));
- }
- }
- return phis;
-}
-----
-
-(((forEach method)))Working with `forEach` makes it slightly shorter
-and quite a bit cleaner:
-
-[source,javascript]
-----
-function gatherCorrelations(journal) {
- var phis = {};
- journal.forEach(function(entry) {
- entry.events.forEach(function(event) {
- if (!(event in phis))
- phis[event] = phi(tableFor(event, journal));
- });
- });
- return phis;
-}
-----
-
-== Higher-order functions ==
-
-(((function,higher-order)))(((function,as value)))Functions that
-operate on other functions, either by taking them as arguments or
-returning them, are called _higher-order functions_. If you have
-already accepted the fact that functions are regular values, there is
-nothing particularly remarkable about the fact that such functions
-exist. The term comes from ((mathematics)), where the distinction
-between functions and other values is taken more seriously.
-
-(((abstraction)))Higher-order functions allow us to abstract over
-_actions_, not just values. They come in several forms. For example,
-you can have functions that create new functions:
-
-[source,javascript]
-----
-function greaterThan(n) {
- return function(m) { return m > n; };
-}
-var greaterThan10 = greaterThan(10);
-console.log(greaterThan10(11));
-// → true
-----
-
-Or functions that change other functions:
-
-[source,javascript]
-----
-function noisy(f) {
- return function(arg) {
- console.log("calling with", arg);
- var val = f(arg);
- console.log("called with", arg, "- got", val);
- return val;
- };
-}
-noisy(Boolean)(0);
-// → calling with 0
-// → called with 0 - got false
-----
-
-Or functions that build new types of ((control flow)):
-
-[source,javascript]
-----
-function unless(test, then) {
- if (!test) then();
-}
-function repeat(times, body) {
- for (var i = 0; i < times; i++) body(i);
-}
-
-repeat(3, function(n) {
- unless(n % 2, function() {
- console.log(n, "is even");
- });
-});
-// → 0 is even
-// → 2 is even
-----
-
-(((inner function)))(((nesting,of functions)))((({}
-(block))))(((local variable)))(((closure)))The ((lexical scoping))
-rules that we discussed in link:03_functions.html#scoping[Chapter 3]
-work to our advantage when using functions in this way. In the example
-above, the `n` variable is a ((parameter)) to the outer function.
-Because the inner function lives inside the environment of the outer
-one, it can use `n`. The bodies of such inner functions can access the
-variables around them. They can play a role similar to the `{}` blocks
-used in regular loops and conditional statements. An important
-difference is that variables declared inside inner functions do not
-end up in the environment of the outer function. And that is usually a
-good thing.
-
-== Passing along arguments ==
-
-(((function,wrapping)))(((arguments object)))The `noisy` function
-above, which wraps its argument in another function, has a rather
-serious deficit.
-
-[source,javascript]
-----
-function noisy(f) {
- return function(arg) {
- console.log("calling with", arg);
- var val = f(arg);
- console.log("called with", arg, "- got", val);
- return val;
- };
-}
-----
-
-If `f` takes more than one ((parameter)), it only gets the first one.
-We could add a bunch of arguments to the inner function (`arg1`,
-`arg2`, and so on) and pass them all to `f`, it is not clear how many
-would be enough. This solution would also deprive `f` of the
-information in `arguments.length`. Since we'd always pass the same
-number of arguments, it wouldn't know how many argument were
-originally given.
-
-(((apply method)))(((array-like object)))(((function,application)))For
-these kinds of situations, JavaScript functions have an `apply`
-method. You pass it an array (or array-like object) of arguments, and
-it will call the function with those arguments.
-
-[source,javascript]
-----
-function transparentWrapping(f) {
- return function() {
- return f.apply(null, arguments);
- };
-}
-----
-
-(((null)))That's a useless function, but it shows the pattern we are
-interested in—the function it returns passes all of the given
-arguments, and only those arguments, to `f`. It does this by passing
-its own `arguments` object to `apply`. The first argument to `apply`,
-for which we are passing `null` here, can be used to simulate a
-((method)) call. More on that in the
-link:06_object.html#call_method[next chapter].
-
-== JSON ==
-
-(((array)))(((function,higher-order)))(((forEach method)))(((data
-set)))Higher-order functions that somehow apply a function to the
-elements of an array are widely used in JavaScript. The `forEach`
-method is the most primitive such function. There are a number of
-other variants available as methods on arrays. In order to familiarize
-ourselves with them, let's play around with another data set.
-
-(((ancestry example)))A few years ago, someone crawled through a lot
-of archives and put together a book on the history of my family name
-(“Haverbeke”—literally “Oatbrook”). I opened it hoping to find
-knights, pirates, and alchemists ... but the book turns out to be
-mostly full of Flemish ((farmer))s. For my amusement, I extracted the
-information on my direct ancestors, and put it into a
-computer-readable format.
-
-(((data format)))(((JSON)))The file I created looks something like
-this:
-
-[source,application/json]
-----
-[
- {"name": "Emma de Milliano", "sex": "f",
- "born": 1876, "died": 1956,
- "father": "Petrus de Milliano",
- "mother": "Sophia van Damme"},
- {"name": "Carolus Haverbeke", "sex": "m",
- "born": 1832, "died": 1905,
- "father": "Carel Haverbeke",
- "mother": "Maria van Brussel"},
- … and so on
-]
-----
-
-indexsee:[JavaScript Object Notation,JSON]
-
-(((World Wide Web)))This format is called JSON (pronounced “Jason”),
-which stands for JavaScript Object Notation. It is widely used as a
-data storage and communication format on the Web.
-
-(((array)))(((object)))(((quoting,in JSON)))JSON is similar to
-JavaScript's way of writing arrays and objects, with a few
-restrictions. All property names have to be surrounded by quotes, and
-only simple data expressions are allowed—no function calls, or
-variables, or anything that involves actual computation.
-
-(((JSON.stringify function)))(((JSON.parse
-function)))(((serialization)))(((deserialization)))(((parsing)))JavaScript
-gives us functions, `JSON.stringify` and `JSON.parse`, that convert
-data from and to this format. The first takes a JavaScript value, and
-returns a JSON-encoded string. The second takes such a string, and
-converts it to the value it encodes.
-
-[source,javascript]
-----
-var string = JSON.stringify({name: "X", born: 1980});
-console.log(string);
-// → {"name":"X","born":1980}
-console.log(JSON.parse(string).born);
-// → 1980
-----
-
-(((ANCESTRY_FILE data set)))The variable `ANCESTRY_FILE`, available in
-the ((sandbox)) for this chapter as well as in
-http://eloquentjavascript.net/code/ancestry.js[a downloadable file] on
-the website(!tex (_eloquentjavascript.net/code_)!), contains the
-content of my ((JSON)) file as a string. Let's decode it and see how
-many people it contains:
-
-// include_code strip_log
-
-[source,javascript]
-----
-var ancestry = JSON.parse(ANCESTRY_FILE);
-console.log(ancestry.length);
-// → 39
-----
-
-== Filtering an array ==
-
-(((array,methods)))(((array,filtering)))(((filter
-method)))(((function,higher-order)))(((predicate function)))To find
-the people in the ancestry data set that were young in 1924, the
-following function might be helpful. It filters out the elements in an
-array that don't pass a test.
-
-[source,javascript]
-----
-function filter(array, test) {
- var passed = [];
- for (var i = 0; i < array.length; i++) {
- if (test(array[i]))
- passed.push(array[i]);
- }
- return passed;
-}
-
-console.log(filter(ancestry, function(person) {
- return person.born > 1900 && person.born < 1925;
-}));
-// → [{name: "Philibert Haverbeke", …}, …]
-----
-
-(((function,as value)))(((function,application)))This uses the
-argument named `test`, a function value, to fill in a “gap” in the
-computation. The test function is called for each element, and its
-return value determines whether an element is included in the returned
-array or not.
-
-(((ancestry example)))Three people in the file were alive and young in
-1924: my grandfather, grandmother, and great-aunt.
-
-(((filter method)))(((pure function)))(((side effect)))Note how the
-`filter` function, rather than delete elements from the existing
-array, builds up a new array with only the elements that pass the
-test. This function is _pure_, it does not modify the array it is
-given.
-
-Like `forEach`, `filter` is also a ((standard)) method on arrays. The
-example defined the function only in order to show what it does
-internally. From now on, we'll use it like this instead:
-
-[source,javascript]
-----
-console.log(ancestry.filter(function(person) {
- return person.father == "Carel Haverbeke";
-}));
-// → [{name: "Carolus Haverbeke", …}]
-----
-
-== Transforming with map ==
-
-(((array,methods)))(((map method)))(((ancestry example)))Say we
-have an array of objects representing people, produced by filtering
-the `ancestry` array somehow. But we want an array of names, which is
-easier to read through.
-
-(((function,higher-order)))The `map` method transforms an array by
-applying a function to all of its elements, and building a new array
-from the returned values. The new array will have the same length as
-the input array, but its content will have been “mapped” to a new form
-by the function.
-
-// test: join
-
-[source,javascript]
-----
-function map(array, transform) {
- var mapped = [];
- for (var i = 0; i < array.length; i++)
- mapped.push(transform(array[i]));
- return mapped;
-}
-
-var overNinety = ancestry.filter(function(person) {
- return person.died - person.born > 90;
-});
-console.log(map(overNinety, function(person) {
- return person.name;
-}));
-// → ["Clara Aernoudts", "Emile Haverbeke",
-// "Maria Haverbeke"]
-----
-
-Interestingly, the people that lived to over 90 years of age are the
-same three people that we saw before—the people who were young in the
-1920s, which happens to be the most recent generation in my data set.
-I guess ((medicine)) has come a long way.
-
-Like `forEach` and `filter`, `map` is also a standard method on
-arrays.
-
-== Summarizing with reduce ==
-
-(((array,methods)))(((summing example)))(((reduce method)))(((ancestry
-example)))Another common pattern of computation on arrays is computing
-a single value from them. Our recurring example, summing a collection
-of numbers, is an instance of this. Another example would be finding
-the person with the earliest year of birth in the data set.
-
-(((function,higher-order)))(((fold function)))The higher-order
-operation that represents this pattern is called _reduce_ (or
-sometimes _fold_). You can think of it as folding up the array, one
-element at a time. When summing numbers, you'd start with the number
-zero, and for each element, combine it with the current sum by adding
-the two.
-
-The parameters to the `reduce` function are, apart from the array, a
-combining function and a start value. This function is a little less
-straightforward than `filter` and `map`, so pay careful attention.
-
-[source,javascript]
-----
-function reduce(array, combine, start) {
- var current = start;
- for (var i = 0; i < array.length; i++)
- current = combine(current, array[i]);
- return current;
-}
-
-console.log(reduce([1, 2, 3, 4], function(a, b) {
- return a + b;
-}, 0));
-// → 10
-----
-
-(((reduce method)))The standard array method `reduce`, which of course
-corresponds to this function, has an added convenience. If your array
-contains at least one element, you are allowed to leave off the
-`start` argument. The method will take the first element of the array
-as its start value, and start reducing at the second element.
-
-(((ancestry example)))(((minimum)))To use `reduce` to find my most
-ancient known ancestor, we can write something like this:
-
-// test: no
-
-[source,javascript]
-----
-console.log(ancestry.reduce(function(min, cur) {
- if (cur.born < min.born) return cur;
- else return min;
-}));
-// → {name: "Pauwels van Haverbeke", born: 1535, …}
-----
-
-== Composability ==
-
-(((loop)))(((minimum)))(((ancestry example)))Consider how we would
-have written the previous example (finding the person with the
-earliest year of birth) without higher-order functions. The code is
-not that much worse:
-
-// test: no
-
-[source,javascript]
-----
-var min = ancestry[0];
-for (var i = 1; i < ancestry.length; i++) {
- var cur = ancestry[i];
- if (cur.born < min.born)
- min = cur;
-}
-console.log(min);
-// → {name: "Pauwels van Haverbeke", born: 1535, …}
-----
-
-There are a few more ((variable))s, and the program is two lines
-longer, but still quite easy to understand.
-
-[[average_function]]
-(((average
-function)))(((composability)))(((function,higher-order)))Higher-order
-functions start to shine when you need to _compose_ functions. As an
-example, let us write code that finds the average age for men and for
-women in the data set.
-
-// test: clip
-
-[source,javascript]
-----
-function average(array) {
- function plus(a, b) { return a + b; }
- return array.reduce(plus) / array.length;
-}
-function age(p) { return p.died - p.born; }
-function male(p) { return p.sex == "m"; }
-function female(p) { return p.sex == "f"; }
-
-console.log(average(ancestry.filter(male).map(age)));
-// → 61.67
-console.log(average(ancestry.filter(female).map(age)));
-// → 54.56
-----
-
-(((plus function)))(((+ operator)))(((function,as value)))(It's a bit
-silly that we have to define `plus` as a function, but operators in
-JavaScript, unlike functions, are not values, so you can't pass them
-as arguments.)
-
-(((abstraction)))(((vocabulary)))Instead of tangling the logic into a
-big ((loop)), it is neatly composed into the concepts we are
-interested in—determining sex, computing age, averaging numbers. We
-can apply these one by one to get the result we were looking for.
-
-This is _fabulous_ for writing clear code. Unfortunately, this clarity
-comes at a cost.
-
-== The cost ==
-
-(((efficiency)))(((optimization)))In the happy land of elegant code
-and pretty rainbows, there lives a spoil-sport monster called
-“__inefficiency__”.
-
-(((elegance)))(((array,creation)))(((pure
-function)))(((composability)))A program that processes an array is most
-elegantly expressed as a sequence of cleanly separated steps that each
-do something with the array and produce a new array. But building up
-all those intermediate arrays is somewhat expensive.
-
-(((readability)))(((function,application)))(((forEach
-method)))(((function,as value)))Likewise, passing a function to
-`forEach` and letting that method handle the array iteration for us is
-convenient and easy to read. But function calls in JavaScript are
-costly compared to simple loop bodies.
-
-(((abstraction)))And so it goes with a lot of techniques that help
-improve the clarity of a program. Abstractions add layers between the
-raw things the computer is doing and the concepts we are working with,
-and thus cause the machine to perform more work. This is not an iron
-law—there are programming languages that have better support for
-building abstractions without adding inefficiencies, and even in
-JavaScript, an experienced programmer can find ways to write abstract
-code that is still fast. But it is a problem that comes up a lot.
-
-(((profiling)))Fortunately, most computers are insanely fast. If you
-are processing a modest set of data, or doing something that only has
-to happen on a human time scale (say, every time the user clicks a
-button), then it _does not matter_ whether you wrote a pretty solution
-that takes half a millisecond or a super-optimized solution that takes
-a tenth of a millisecond.
-
-(((nesting,of loops)))(((inner loop)))(((complexity)))It is helpful to
-roughly keep track of how often a piece of your program is going to
-run. If you have a ((loop)) inside a loop (either directly, or through
-the outer loop calling a function that ends up performing the inner
-loop), the code inside the inner loop will end up running __N__×__M__
-times, where _N_ is the number of times the outer loop repeats, and
-_M_ the number of times the inner loop repeats within each iteration
-of the outer loop. If that inner loop contains another loop that makes
-_P_ rounds, its body will run __M__×__N__×__P__ times, and so on. This
-can add up to large numbers, and when a program is slow, the problem
-can often be traced to only a small part of the code, which sits in
-such an inner loop.
-
-== Great-great-great-great-... ==
-
-(((ancestry example)))My ((grandfather)), Philibert Haverbeke, is
-included in the data file. By starting with him, I can trace my
-lineage to find out whether the most ancient person in the data,
-Pauwels van Haverbeke, is my direct ancestor. And if he is, I would
-like to know how much ((DNA)) I theoretically share with him.
-
-(((byName object)))(((map)))(((data structure)))(((object,as
-map)))To be able to go from a parent's name to the actual object that
-represents this person, we first build up an object that associates
-names with people.
-
-// include_code strip_log
-
-[source,javascript]
-----
-var byName = {};
-ancestry.forEach(function(person) {
- byName[person.name] = person;
-});
-
-console.log(byName["Philibert Haverbeke"]);
-// → {name: "Philibert Haverbeke", …}
-----
-
-Now, the problem is not entirely as simple as following the `father`
-properties and counting how many we need to reach Pauwels. There are
-several cases in the family ((tree)) where people married their second
-cousins (tiny villages and all that). This causes the branches of the
-family tree to re-join in a few places, which means I share more than
-1/2^_G_^ of my genes with this person, where _G_ for the number of
-generations between Pauwels and me. This formula comes from the idea
-that each generation splits the gene pool in two.
-
-(((reduce method)))(((data structure)))A reasonable way to think about
-this problem is to look at it as being analogous to `reduce`, which
-condenses an array down to a single value by repeatedly combining
-values, left to right. In this case, we also want to condense our data
-structure down to a single value, but in a way that follows family
-lines. The _shape_ of the data is that of a family tree, rather than a
-flat list.
-
-The way we want to reduce this shape is by computing a value for a
-given person by combining values from their ancestors. This can be
-done recursively: if we are interested in person _A_, we have to
-compute the values for __A__’s parents, which in turn requires us to
-compute the value for __A__’s grandparents, and so on. In principle,
-that'd require us to look at an infinite number of people, but since
-our data set is finite, we have to stop somewhere. We'll allow a
-((default value)) to be given to our reduction function, which will be
-used for people that are not in the data. In our case, that value is
-simply zero, on the assumption that people not in the list don't share
-DNA with the ancestor we are looking at.
-
-(((recursion)))(((reduceAncestors function)))Given a person, a
-function to combine values from the two parents of a given person, and
-a default value, `reduceAncestors` condenses a value from a family
-tree.
-
-// include_code
-
-[source,javascript]
-----
-function reduceAncestors(person, f, defaultValue) {
- function valueFor(person) {
- if (person == null)
- return defaultValue;
- else
- return f(person, valueFor(byName[person.mother]),
- valueFor(byName[person.father]));
- }
- return valueFor(person);
-}
-----
-
-(((function,higher-order)))The inner function (`valueFor`) handles a
-single person. Through the ((magic)) of recursion, it can simply call
-itself to handle the father and the mother of this person. The
-results, along with the person object itself, are passed to `f`, which
-returns the actual value for this person.
-
-We can then use this to compute the amount of ((DNA)) my
-((grandfather)) shared with Pauwels van Haverbeke, and divide that by
-four.
-
-// start_code bottom_lines: 2
-// test: clip
-// include_code top_lines: 6
-
-[source,javascript]
-----
-function sharedDNA(person, fromMother, fromFather) {
- if (person.name == "Pauwels van Haverbeke")
- return 1;
- else
- return (fromMother + fromFather) / 2;
-}
-var ph = byName["Philibert Haverbeke"];
-console.log(reduceAncestors(ph, sharedDNA, 0) / 4);
-// → 0.00049
-----
-
-The person with the name Pauwels van Haverbeke obviously shared 100%
-of his DNA with Pauwels van Haverbeke (there are no people who share
-names in the data set), so the function returns 1 for him. All other
-people share the average of the amounts that their parents share.
-
-So, statistically speaking, I share about 0.05% of my ((DNA)) with
-this 16th-century person. It should be noted that this is only a
-statistical approximation, not an exact amount. It is a rather small
-number, but given how much genetic material we carry (about 3 billion
-base pairs), there's still probably some aspect in the biological
-machine that is me that originates with Pauwels.
-
-(((ancestry example)))(((reduceAncestors
-function)))(((abstraction)))We could also have computed this number
-without relying on `reduceAncestors`. But separating the general
-approach (condensing a family tree) from the specific case (computing
-shared DNA) can improve the clarity of the code, and allows us to
-reuse the abstract part of the program for other cases. For example,
-the following code finds the percentage of known ancestors, for a
-given person, that lived past 70:
-
-// test: clip
-
-[source,javascript]
-----
-function countAncestors(person, test) {
- function combine(person, fromMother, fromFather) {
- var thisOneCounts = test(person);
- return fromMother + fromFather + (thisOneCounts ? 1 : 0);
- }
- return reduceAncestors(person, combine, 0);
-}
-function longLivingPercentage(person) {
- var all = countAncestors(person, function(person) {
- return true;
- });
- var longLiving = countAncestors(person, function(person) {
- return (person.died - person.born) >= 70;
- });
- return longLiving / all;
-}
-console.log(longLivingPercentage(byName["Emile Haverbeke"]));
-// → 0.145
-----
-
-Such numbers are not to be taken too seriously, given the fact that
-our data set contains a rather arbitrary collection of people. But the
-code illustrates the fact that `reduceAncestors` gives us a useful
-piece of ((vocabulary)) for working with the family tree data
-structure.
-
-== Binding ==
-
-(((bind method)))(((partial
-application)))(((function,application)))The `bind` method, which all
-functions have, creates a new function that will call the original
-function, but with some of the arguments already fixed.
-
-(((filter method)))(((function,as value)))The code below shows an
-example of `bind` in use. First it defines a function `isInSet` that
-tells us whether a person is in a given set of strings. To call
-`filter` in order to collect those person objects whose names are in a
-specific set, we can either write a function expression that makes a
-call to `isInSet` with our set as its first argument, or _partially
-apply_ the `isInSet` function.
-
-[source,javascript]
-----
-var theSet = ["Carel Haverbeke", "Maria van Brussel",
- "Donald Duck"];
-function isInSet(set, person) {
- return set.indexOf(person.name) > -1;
-}
-
-console.log(ancestry.filter(function(person) {
- return isInSet(theSet, person);
-}));
-// → [{name: "Maria van Brussel", …},
-// {name: "Carel Haverbeke", …}]
-console.log(ancestry.filter(isInSet.bind(null, theSet)));
-// → … same result
-----
-
-The above `bind` call returns a function that will call `isInSet` with
-`theSet` as first argument, followed by any remaining arguments given
-to the bound function.
-
-(((null)))The first argument, where the example passes `null`, is used
-for ((method call))s, similar to the first argument to `apply`. We'll
-describe this in more detail in the
-link:06_object.html#call_method[next chapter].
-
-== Summary ==
-
-Being able to pass function values to other functions is not just a
-gimmick, but a deeply useful aspect of JavaScript. It allows us to
-write computations with “gaps” in them as functions, and have the code
-that calls these functions fill in those gaps by providing function
-values that describe the missing computations.
-
-Arrays provide a number of very useful higher-order methods—`forEach`
-to do something with each element in an array, `filter` to build a new
-array with some elements filtered out, `map` to build a new array
-where each element has been put through a function, and `reduce` to
-combine all an array's elements into a single value.
-
-Functions have an `apply` method that can be used to call them with an
-array specifying their arguments. They also have a `bind` method,
-which is used to create a partially applied version of the function.
-
-== Exercises ==
-
-=== Flattening ===
-
-(((flattening (exercise))))(((reduce method)))(((concat
-method)))(((array)))Use the `reduce` method in combination with
-the `concat` method to “flatten” an array of arrays into a single
-array that has all the elements of the input arrays.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-var arrays = [[1, 2, 3], [4, 5], [6]];
-// Your code here.
-// → [1, 2, 3, 4, 5, 6]
-----
-endif::html_target[]
-
-=== Mother-child age difference ===
-
-(((ancestry example)))(((age difference (exercise))))(((average
-function)))Using the example data set from this chapter, compute the
-average age difference between mothers and children (the age of the
-mother when the child is born). You can use the `average` function
-defined link:05_higher_order.html#average_function[earlier] in this
-chapter.
-
-(((byName object)))Note that not all the mothers mentioned in the data
-are themselves present in the array. The `byName` object, which makes
-it easy to find a person's object from their name, might be useful
-here.
-
-ifdef::html_target[]
-
-// test: no
-// include_code
-
-[source,javascript]
-----
-function average(array) {
- function plus(a, b) { return a + b; }
- return array.reduce(plus) / array.length;
-}
-
-var byName = {};
-ancestry.forEach(function(person) {
- byName[person.name] = person;
-});
-
-// Your code here.
-
-// → 31.2
-----
-endif::html_target[]
-
-!!solution!!
-
-(((age difference (exercise))))(((filter method)))(((map
-method)))(((null)))(((average function)))Because not all elements in
-the `ancestry` array produce useful data (we can't compute the age
-difference unless we know the birth date of the mother), we will have
-to apply `filter` in some manner before calling `average`. You could
-do it as a first pass, by defining a `hasKnownMother` function and
-filtering on that first. Alternatively, you could start by calling
-`map`, and in your mapping function return either the age difference,
-or `null` if no mother is known. Then, you can call `filter` to remove
-the `null` elements before passing the array to `average`.
-
-!!solution!!
-
-=== Historical life expectancy ===
-
-(((life expectancy (exercise))))When we looked up all the people in
-our data set that lived more than ninety years, only the very latest
-generation in the data came out. Let's take a closer look at that
-phenomenon.
-
-(((average function)))Compute and output the average age of the people
-in the ancestry data set per century. A person is assigned to a
-((century)) by taking their year of death, dividing it by a hundred,
-and rounding it up, as in `Math.ceil(person.died / 100)`.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-function average(array) {
- function plus(a, b) { return a + b; }
- return array.reduce(plus) / array.length;
-}
-
-// Your code here.
-
-// → 16: 43.5
-// 17: 51.2
-// 18: 52.8
-// 19: 54.8
-// 20: 84.7
-// 21: 94
-----
-endif::html_target[]
-
-!!solution!!
-
-(((life expectancy (exercise))))The essence of this example lies in
-((grouping)) the elements of a collection by some aspect of
-theirs—splitting the array of ancestors into smaller arrays with the
-ancestors for each century.
-
-(((array)))(((map)))(((object,as map)))During the grouping
-process, keep an object that associates ((century)) names (numbers)
-with arrays of either person objects or ages. Since we do not know in
-advance what categories we will find, we'll have to create them on the
-fly. For each person, after computing their century, we test whether
-that century was already known. If not, add an array for it. Then add
-the person (or age) to the array for the proper century.
-
-(((for/in loop)))(((average function)))Finally, a `for`/`in` loop can
-be used to print the average ages for the individual centuries.
-
-!!solution!!
-
-(((grouping)))(((map)))(((object,as map)))(((groupBy function)))For
-bonus points, write a function `groupBy` that abstracts the grouping
-operation. It should accept as arguments an array and a function that
-computes the group for an element in the array, and returns the object
-containing the groups.
-
-=== Every and then some ===
-
-(((predicate function)))(((every and some (exercise))))(((every
-method)))(((some method)))(((array,methods)))(((&& operator)))(((||
-operator)))Arrays also come with the standard methods `every` and
-`some`. Both take a predicate function that, when called with an array
-element as argument, returns true or false. Just like `&&` only
-returns a true value when the expressions on both sides are true,
-`every` only returns true when the predicate returned true for _all_
-elements of the array. Similarly, `some` returns true as soon as the
-predicate returned true for _any_ of the elements. They do not process
-more elements than necessary—for example, if `some` finds that the
-predicate holds for the first element of the array, it will not look
-at the values after that.
-
-Write two functions, `every` and `some`, that behave like these
-methods, except that they take the array as their first argument,
-rather than being a method.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-console.log(every([NaN, NaN, NaN], isNaN));
-// → true
-console.log(every([NaN, NaN, 4], isNaN));
-// → false
-console.log(some([NaN, 3, 4], isNaN));
-// → true
-console.log(some([2, 3, 4], isNaN));
-// → false
-----
-endif::html_target[]
-
-!!solution!!
-
-(((every and some (exercise))))(((short-circuit evaluation)))(((return
-keyword)))The functions can follow a similar pattern to the
-link:05_higher_order.html#forEach[definition] of `forEach` at the
-start of the chapter, except that they must return immediately (with
-the right value) when the predicate function returns false—or true.
-Don't forget to put another `return` statement after the loop, so that
-the function also returns the correct value when it reaches the end of
-the array.
-
-!!solution!!
diff --git a/06_object.md b/06_object.md
new file mode 100644
index 000000000..86b92bd50
--- /dev/null
+++ b/06_object.md
@@ -0,0 +1,914 @@
+{{meta {load_files: ["code/chapter/06_object.js"], zip: "node/html"}}}
+
+# The Secret Life of Objects
+
+{{quote {author: "Barbara Liskov", title: "Programming with Abstract Data Types", chapter: true}
+
+An abstract data type is realized by writing a special kind of program […] which defines the type in terms of the operations which can be performed on it.
+
+quote}}
+
+{{index "Liskov, Barbara", "abstract data type"}}
+
+{{figure {url: "img/chapter_picture_6.jpg", alt: "Illustration of a rabbit next to its prototype, a schematic representation of a rabbit", chapter: framed}}}
+
+[Chapter ?](data) introduced JavaScript's objects as containers that hold other data. In programming culture, _((object-oriented programming))_ is a set of techniques that use objects as the central principle of program organization. Though no one really agrees on its precise definition, object-oriented programming has shaped the design of many programming languages, including JavaScript. This chapter describes the way these ideas can be applied in JavaScript.
+
+## Abstract Data Types
+
+{{index "abstract data type", type, "mixer example"}}
+
+The main idea in object-oriented programming is to use objects, or rather _types_ of objects, as the unit of program organization. Setting up a program as a number of strictly separated object types provides a way to think about its structure and thus to enforce some kind of discipline, preventing everything from becoming entangled.
+
+The way to do this is to think of objects somewhat like you'd think of an electric mixer or other consumer ((appliance)). The people who design and assemble a mixer have to do specialized work requiring material science and understanding of electricity. They cover all that up in a smooth plastic shell so that the people who only want to mix pancake batter don't have to worry about all that—they have to understand only the few knobs that the mixer can be operated with.
+
+{{index "class"}}
+
+Similarly, an _abstract data type_, or _object class_, is a subprogram that may contain arbitrarily complicated code but exposes a limited set of methods and properties that people working with it are supposed to use. This allows large programs to be built up out of a number of appliance types, limiting the degree to which these different parts are entangled by requiring them to only interact with each other in specific ways.
+
+{{index encapsulation, isolation, modularity}}
+
+If a problem is found in one such object class, it can often be repaired or even completely rewritten without impacting the rest of the program. Even better, it may be possible to use object classes in multiple different programs, avoiding the need to recreate their functionality from scratch. You can think of JavaScript's built-in data structures, such as arrays and strings, as such reusable abstract data types.
+
+{{id interface}}
+{{index [interface, object]}}
+
+Each abstract data type has an _interface_, the collection of operations that external code can perform on it. Any details beyond that interface are _encapsulated_, treated as internal to the type and of no concern to the rest of the program.
+
+Even basic things like numbers can be thought of as an abstract data type whose interface allows us to add them, multiply them, compare them, and so on. In fact, the fixation on single _objects_ as the main unit of organization in classical object-oriented programming is somewhat unfortunate since useful pieces of functionality often involve a group of different object classes working closely together.
+
+{{id obj_methods}}
+
+## Methods
+
+{{index "rabbit example", method, [property, access]}}
+
+In JavaScript, methods are nothing more than properties that hold function values. This is a simple method:
+
+```{includeCode: "top_lines:6"}
+function speak(line) {
+ console.log(`The ${this.type} rabbit says '${line}'`);
+}
+let whiteRabbit = {type: "white", speak};
+let hungryRabbit = {type: "hungry", speak};
+
+whiteRabbit.speak("Oh my fur and whiskers");
+// → The white rabbit says 'Oh my fur and whiskers'
+hungryRabbit.speak("Got any carrots?");
+// → The hungry rabbit says 'Got any carrots?'
+```
+
+{{index "this binding", "method call"}}
+
+Typically a method needs to do something with the object on which it was called. When a function is called as a method—looked up as a property and immediately called, as in `object.method()`—the binding called `this` in its body automatically points at the object on which it was called.
+
+{{id call_method}}
+
+{{index "call method"}}
+
+You can think of `this` as an extra ((parameter)) that is passed to the function in a different way than regular parameters. If you want to provide it explicitly, you can use a function's `call` method, which takes the `this` value as its first argument and treats further arguments as normal parameters.
+
+```
+speak.call(whiteRabbit, "Hurry");
+// → The white rabbit says 'Hurry'
+```
+
+Since each function has its own `this` binding whose value depends on the way it is called, you cannot refer to the `this` of the wrapping scope in a regular function defined with the `function` keyword.
+
+{{index "this binding", "arrow function"}}
+
+Arrow functions are different—they do not bind their own `this` but can see the `this` binding of the scope around them. Thus, you can do something like the following code, which references `this` from inside a local function:
+
+```
+let finder = {
+ find(array) {
+ return array.some(v => v == this.value);
+ },
+ value: 5
+};
+console.log(finder.find([4, 5]));
+// → true
+```
+
+A property like `find(array)` in an object expression is a shorthand way of defining a method. It creates a property called `find` and gives it a function as its value.
+
+If I had written the argument to `some` using the `function` keyword, this code wouldn't work.
+
+{{id prototypes}}
+
+## Prototypes
+
+One way to create a rabbit object type with a `speak` method would be to create a helper function that has a rabbit type as its parameter and returns an object holding that as its `type` property and our speak function in its `speak` property.
+
+All rabbits share that same method. Especially for types with many methods, it would be nice if there were a way to keep a type's methods in a single place, rather than adding them to each object individually.
+
+{{index [property, inheritance], [object, property], "Object prototype"}}
+
+In JavaScript, _((prototype))s_ are the way to do that. Objects can be linked to other objects, to magically get all the properties that other object has. Plain old objects created with `{}` notation are linked to an object called `Object.prototype`.
+
+{{index "toString method"}}
+
+```
+let empty = {};
+console.log(empty.toString);
+// → function toString(){…}
+console.log(empty.toString());
+// → [object Object]
+```
+
+It looks like we just pulled a property out of an empty object. But in fact, `toString` is a method stored in `Object.prototype`, meaning it is available in most objects.
+
+When an object gets a request for a property that it doesn't have, its prototype will be searched for the property. If that doesn't have it, the _prototype's_ prototype is searched, and so on until an object without prototype is reached (`Object.prototype` is such an object).
+
+```
+console.log(Object.getPrototypeOf({}) == Object.prototype);
+// → true
+console.log(Object.getPrototypeOf(Object.prototype));
+// → null
+```
+
+{{index "getPrototypeOf function"}}
+
+As you'd guess, `Object.getPrototypeOf` returns the prototype of an object.
+
+{{index inheritance, "Function prototype", "Array prototype", "Object prototype"}}
+
+Many objects don't directly have `Object.prototype` as their ((prototype)) but instead have another object that provides a different set of default properties. Functions derive from `Function.prototype` and arrays derive from `Array.prototype`.
+
+```
+console.log(Object.getPrototypeOf(Math.max) ==
+ Function.prototype);
+// → true
+console.log(Object.getPrototypeOf([]) == Array.prototype);
+// → true
+```
+
+{{index "Object prototype"}}
+
+Such a prototype object will itself have a prototype, often `Object.prototype`, so that it still indirectly provides methods like `toString`.
+
+{{index "rabbit example", "Object.create function"}}
+
+You can use `Object.create` to create an object with a specific ((prototype)).
+
+```{includeCode: "top_lines: 7"}
+let protoRabbit = {
+ speak(line) {
+ console.log(`The ${this.type} rabbit says '${line}'`);
+ }
+};
+let blackRabbit = Object.create(protoRabbit);
+blackRabbit.type = "black";
+blackRabbit.speak("I am fear and darkness");
+// → The black rabbit says 'I am fear and darkness'
+```
+
+{{index "shared property"}}
+
+The "proto" rabbit acts as a container for the properties shared by all rabbits. An individual rabbit object, like the black rabbit, contains properties that apply only to itself—in this case its type—and derives shared properties from its prototype.
+
+{{id classes}}
+
+## Classes
+
+{{index "object-oriented programming", "abstract data type"}}
+
+JavaScript's ((prototype)) system can be interpreted as a somewhat free-form take on abstract data types or ((class))es. A _class_ defines the shape of a type of object—what methods and properties it has. Such an object is called an _((instance))_ of the class.
+
+{{index [property, inheritance]}}
+
+Prototypes are useful for defining properties for which all instances of a class share the same value. Properties that differ per instance, such as our rabbits' `type` property, need to be stored directly in the objects themselves.
+
+{{id constructors}}
+
+To create an instance of a given class, you have to make an object that derives from the proper prototype, but you _also_ have to make sure it itself has the properties that instances of this class are supposed to have. This is what a _((constructor))_ function does.
+
+```
+function makeRabbit(type) {
+ let rabbit = Object.create(protoRabbit);
+ rabbit.type = type;
+ return rabbit;
+}
+```
+
+JavaScript's ((class)) notation makes it easier to define this type of function, along with a ((prototype)) object.
+
+{{index "rabbit example", constructor}}
+
+```{includeCode: true}
+class Rabbit {
+ constructor(type) {
+ this.type = type;
+ }
+ speak(line) {
+ console.log(`The ${this.type} rabbit says '${line}'`);
+ }
+}
+```
+
+{{index "prototype property", [braces, class]}}
+
+The `class` keyword starts a ((class declaration)), which allows us to define a constructor and a set of methods together. Any number of methods may be written inside the declaration's braces. This code has the effect of defining a binding called `Rabbit`, which holds a function that runs the code in `constructor` and has a `prototype` property that holds the `speak` method.
+
+{{index "new operator", "this binding", [object, creation]}}
+
+This function cannot be called like a normal function. Constructors, in JavaScript, are called by putting the keyword `new` in front of them. Doing so creates a fresh instance object whose prototype is the object from the function's `prototype` property, then runs the function with `this` bound to the new object, and finally returns the object.
+
+```{includeCode: true}
+let killerRabbit = new Rabbit("killer");
+```
+
+In fact, `class` was only introduced in the 2015 edition of JavaScript. Any function can be used as a constructor, and before 2015, the way to define a class was to write a regular function and then manipulate its `prototype` property.
+
+```
+function ArchaicRabbit(type) {
+ this.type = type;
+}
+ArchaicRabbit.prototype.speak = function(line) {
+ console.log(`The ${this.type} rabbit says '${line}'`);
+};
+let oldSchoolRabbit = new ArchaicRabbit("old school");
+```
+
+For this reason, all non-arrow functions start with a `prototype` property holding an empty object.
+
+{{index capitalization}}
+
+By convention, the names of constructors are capitalized so that they can easily be distinguished from other functions.
+
+{{index "prototype property", "getPrototypeOf function"}}
+
+It is important to understand the distinction between the way a prototype is associated with a constructor (through its `prototype` property) and the way objects _have_ a prototype (which can be found with `Object.getPrototypeOf`). The actual prototype of a constructor is `Function.prototype` since constructors are functions. The constructor function's `prototype` _property_ holds the prototype used for instances created through it.
+
+```
+console.log(Object.getPrototypeOf(Rabbit) ==
+ Function.prototype);
+// → true
+console.log(Object.getPrototypeOf(killerRabbit) ==
+ Rabbit.prototype);
+// → true
+```
+
+{{index constructor}}
+
+Constructors will typically add some per-instance properties to `this`. It is also possible to declare properties directly in the ((class declaration)). Unlike methods, such properties are added to ((instance)) objects and not the prototype.
+
+```
+class Particle {
+ speed = 0;
+ constructor(position) {
+ this.position = position;
+ }
+}
+```
+
+Like `function`, `class` can be used both in statements and in expressions. When used as an expression, it doesn't define a binding but just produces the constructor as a value. You are allowed to omit the class name in a class expression.
+
+```
+let object = new class { getWord() { return "hello"; } };
+console.log(object.getWord());
+// → hello
+```
+
+
+## Private Properties
+
+{{index [property, private], [property, public], "class declaration"}}
+
+It is common for classes to define some properties and ((method))s for internal use that are not part of their ((interface)). These are called _private_ properties, as opposed to _public_ ones, which are part of the object's external interface.
+
+{{index [method, private]}}
+
+To declare a private method, put a `#` sign in front of its name. Such methods can be called only from inside the `class` declaration that defines them.
+
+```
+class SecretiveObject {
+ #getSecret() {
+ return "I ate all the plums";
+ }
+ interrogate() {
+ let shallISayIt = this.#getSecret();
+ return "never";
+ }
+}
+```
+
+When a class does not declare a constructor, it will automatically get an empty one.
+
+If you try to call `#getSecret` from outside the class, you get an error. Its existence is entirely hidden inside the class declaration.
+
+To use private instance properties, you must declare them. Regular properties can be created by just assigning to them, but private properties _must_ be declared in the class declaration to be available at all.
+
+This class implements an appliance for getting a random whole number below a given maximum number. It has only one ((public)) property: `getNumber`.
+
+```
+class RandomSource {
+ #max;
+ constructor(max) {
+ this.#max = max;
+ }
+ getNumber() {
+ return Math.floor(Math.random() * this.#max);
+ }
+}
+```
+
+## Overriding derived properties
+
+{{index "shared property", overriding, [property, inheritance]}}
+
+When you add a property to an object, whether it is present in the prototype or not, the property is added to the object _itself_. If there was already a property with the same name in the prototype, this property will no longer affect the object, as it is now hidden behind the object's own property.
+
+```
+Rabbit.prototype.teeth = "small";
+console.log(killerRabbit.teeth);
+// → small
+killerRabbit.teeth = "long, sharp, and bloody";
+console.log(killerRabbit.teeth);
+// → long, sharp, and bloody
+console.log((new Rabbit("basic")).teeth);
+// → small
+console.log(Rabbit.prototype.teeth);
+// → small
+```
+
+{{index [prototype, diagram]}}
+
+The following diagram sketches the situation after this code has run. The `Rabbit` and `Object` ((prototype))s lie behind `killerRabbit` as a kind of backdrop, where properties that are not found in the object itself can be looked up.
+
+{{figure {url: "img/rabbits.svg", alt: "A diagram showing the object structure of rabbits and their prototypes. There is a box for the 'killerRabbit' instance (holding instance properties like 'type'), with its two prototypes, 'Rabbit.prototype' (holding the 'speak' method) and 'Object.prototype' (holding methods like 'toString') stacked behind it.",width: "8cm"}}}
+
+{{index "shared property"}}
+
+Overriding properties that exist in a prototype can be a useful thing to do. As the rabbit teeth example shows, overriding can be used to express exceptional properties in instances of a more generic class of objects while letting the nonexceptional objects take a standard value from their prototype.
+
+{{index "toString method", "Array prototype", "Function prototype"}}
+
+Overriding is also used to give the standard function and array prototypes a different `toString` method than the basic object prototype.
+
+```
+console.log(Array.prototype.toString ==
+ Object.prototype.toString);
+// → false
+console.log([1, 2].toString());
+// → 1,2
+```
+
+{{index "toString method", "join method", "call method"}}
+
+Calling `toString` on an array gives a result similar to calling `.join(",")` on it—it puts commas between the values in the array. Directly calling `Object.prototype.toString` with an array produces a different string. That function doesn't know about arrays, so it simply puts the word _object_ and the name of the type between square brackets.
+
+```
+console.log(Object.prototype.toString.call([1, 2]));
+// → [object Array]
+```
+
+## Maps
+
+{{index "map method"}}
+
+We saw the word _map_ used in the [previous chapter](higher_order#map) for an operation that transforms a data structure by applying a function to its elements. Confusing as it is, in programming the same word is used for a related but rather different thing.
+
+{{index "map (data structure)", "ages example", ["data structure", map]}}
+
+A _map_ (noun) is a data structure that associates values (the keys) with other values. For example, you might want to map names to ages. It is possible to use objects for this.
+
+```
+let ages = {
+ Boris: 39,
+ Liang: 22,
+ Júlia: 62
+};
+
+console.log(`Júlia is ${ages["Júlia"]}`);
+// → Júlia is 62
+console.log("Is Jack's age known?", "Jack" in ages);
+// → Is Jack's age known? false
+console.log("Is toString's age known?", "toString" in ages);
+// → Is toString's age known? true
+```
+
+{{index "Object.prototype", "toString method"}}
+
+Here, the object's property names are the people's names and the property values are their ages. But we certainly didn't list anybody named toString in our map. Yet because plain objects derive from `Object.prototype`, it looks like the property is there.
+
+{{index "Object.create function", prototype}}
+
+For this reason, using plain objects as maps is dangerous. There are several possible ways to avoid this problem. First, you can create objects with _no_ prototype. If you pass `null` to `Object.create`, the resulting object will not derive from `Object.prototype` and can be safely used as a map.
+
+```
+console.log("toString" in Object.create(null));
+// → false
+```
+
+{{index [property, naming]}}
+
+Object property names must be strings. If you need a map whose keys can't easily be converted to strings—such as objects—you cannot use an object as your map.
+
+{{index "Map class"}}
+
+Fortunately, JavaScript comes with a class called `Map` that is written for this exact purpose. It stores a mapping and allows any type of keys.
+
+```
+let ages = new Map();
+ages.set("Boris", 39);
+ages.set("Liang", 22);
+ages.set("Júlia", 62);
+
+console.log(`Júlia is ${ages.get("Júlia")}`);
+// → Júlia is 62
+console.log("Is Jack's age known?", ages.has("Jack"));
+// → Is Jack's age known? false
+console.log(ages.has("toString"));
+// → false
+```
+
+{{index [interface, object], "set method", "get method", "has method", encapsulation}}
+
+The methods `set`, `get`, and `has` are part of the interface of the `Map` object. Writing a data structure that can quickly update and search a large set of values isn't easy, but we don't have to worry about that. Someone else did it for us, and we can go through this simple interface to use their work.
+
+{{index "hasOwn function", "in operator"}}
+
+If you do have a plain object that you need to treat as a map for some reason, it is useful to know that `Object.keys` returns only an object's _own_ keys, not those in the prototype. As an alternative to the `in` operator, you can use the `Object.hasOwn` function, which ignores the object's prototype.
+
+```
+console.log(Object.hasOwn({x: 1}, "x"));
+// → true
+console.log(Object.hasOwn({x: 1}, "toString"));
+// → false
+```
+
+## Polymorphism
+
+{{index "toString method", "String function", polymorphism, overriding, "object-oriented programming"}}
+
+When you call the `String` function (which converts a value to a string) on an object, it will call the `toString` method on that object to try to create a meaningful string from it. I mentioned that some of the standard prototypes define their own version of `toString` so they can create a string that contains more useful information than `"[object Object]"`. You can also do that yourself.
+
+```{includeCode: "top_lines: 3"}
+Rabbit.prototype.toString = function() {
+ return `a ${this.type} rabbit`;
+};
+
+console.log(String(killerRabbit));
+// → a killer rabbit
+```
+
+{{index "object-oriented programming", [interface, object]}}
+
+This is a simple instance of a powerful idea. When a piece of code is written to work with objects that have a certain interface—in this case, a `toString` method—any kind of object that happens to support this interface can be plugged into the code and will be able to work with it.
+
+This technique is called _polymorphism_. Polymorphic code can work with values of different shapes, as long as they support the interface it expects.
+
+{{index "forEach method"}}
+
+An example of a widely used interface is that of ((array-like object))s that have a `length` property holding a number and numbered properties for each of their elements. Both arrays and strings support this interface, as do various other objects, some of which we'll see later in the chapters about the browser. Our implementation of `forEach` from [Chapter ?](higher_order) works on anything that provides this interface. In fact, so does `Array.prototype.forEach`.
+
+```
+Array.prototype.forEach.call({
+ length: 2,
+ 0: "A",
+ 1: "B"
+}, elt => console.log(elt));
+// → A
+// → B
+```
+
+## Getters, setters, and statics
+
+{{index [interface, object], [property, definition], "Map class"}}
+
+Interfaces often contain plain properties, not just methods. For example, `Map` objects have a `size` property that tells you how many keys are stored in them.
+
+It is not necessary for such an object to compute and store such a property directly in the instance. Even properties that are accessed directly may hide a method call. Such methods are called _((getter))s_ and are defined by writing `get` in front of the method name in an object expression or class declaration.
+
+```{test: no}
+let varyingSize = {
+ get size() {
+ return Math.floor(Math.random() * 100);
+ }
+};
+
+console.log(varyingSize.size);
+// → 73
+console.log(varyingSize.size);
+// → 49
+```
+
+{{index "temperature example"}}
+
+Whenever someone reads from this object's `size` property, the associated method is called. You can do a similar thing when a property is written to, using a _((setter))_.
+
+```{startCode: true, includeCode: "top_lines: 16"}
+class Temperature {
+ constructor(celsius) {
+ this.celsius = celsius;
+ }
+ get fahrenheit() {
+ return this.celsius * 1.8 + 32;
+ }
+ set fahrenheit(value) {
+ this.celsius = (value - 32) / 1.8;
+ }
+
+ static fromFahrenheit(value) {
+ return new Temperature((value - 32) / 1.8);
+ }
+}
+
+let temp = new Temperature(22);
+console.log(temp.fahrenheit);
+// → 71.6
+temp.fahrenheit = 86;
+console.log(temp.celsius);
+// → 30
+```
+
+The `Temperature` class allows you to read and write the temperature in either degrees ((Celsius)) or degrees ((Fahrenheit)), but internally it stores only Celsius and automatically converts to and from Celsius in the `fahrenheit` getter and setter.
+
+{{index "static method", "static property"}}
+
+Sometimes you want to attach some properties directly to your constructor function rather than to the prototype. Such methods won't have access to a class instance but can, for example, be used to provide additional ways to create instances.
+
+Inside a class declaration, methods or properties that have `static` written before their name are stored on the constructor. For example, the `Temperature` class allows you to write `Temperature.fromFahrenheit(100)` to create a temperature using degrees Fahrenheit.
+
+```
+let boil = Temperature.fromFahrenheit(212);
+console.log(boil.celsius);
+// → 100
+```
+
+## Symbols
+
+{{index "for/of loop", "iterator interface"}}
+
+I mentioned in [Chapter ?](data#for_of_loop) that a `for`/`of` loop can loop over several kinds of data structures. This is another case of polymorphism—such loops expect the data structure to expose a specific interface, which arrays and strings do. And we can also add this interface to our own objects! But before we can do that, we need to briefly take a look at the symbol type.
+
+It is possible for multiple interfaces to use the same property name for different things. For example, on array-like objects, `length` refers to the number of elements in the collection. But an object interface describing a hiking route could use `length` to provide the length of the route in meters. It would not be possible for an object to conform to both these interfaces.
+
+An object trying to be a route and array-like (maybe to enumerate its waypoints) is somewhat far-fetched, and this kind of problem isn't that common in practice. For things like the iteration protocol, though, the language designers needed a type of property that _really_ doesn't conflict with any others. So in 2015, _((symbol))s_ were added to the language.
+
+{{index "Symbol function", [property, naming]}}
+
+Most properties, including all those we have seen so far, are named with strings. But it is also possible to use symbols as property names. Symbols are values created with the `Symbol` function. Unlike strings, newly created symbols are unique—you cannot create the same symbol twice.
+
+```
+let sym = Symbol("name");
+console.log(sym == Symbol("name"));
+// → false
+Rabbit.prototype[sym] = 55;
+console.log(killerRabbit[sym]);
+// → 55
+```
+
+The string you pass to `Symbol` is included when you convert it to a string and can make it easier to recognize a symbol when, for example, showing it in the console. But it has no meaning beyond that—multiple symbols may have the same name.
+
+Being both unique and usable as property names makes symbols suitable for defining interfaces that can peacefully live alongside other properties, no matter what their names are.
+
+```{includeCode: "top_lines: 1"}
+const length = Symbol("length");
+Array.prototype[length] = 0;
+
+console.log([1, 2].length);
+// → 2
+console.log([1, 2][length]);
+// → 0
+```
+
+{{index [property, naming]}}
+
+It is possible to include symbol properties in object expressions and classes by using ((square bracket))s around the property name. That causes the expression between the brackets to be evaluated to produce the property name, analogous to the square bracket property access notation.
+
+```
+let myTrip = {
+ length: 2,
+ 0: "Lankwitz",
+ 1: "Babelsberg",
+ [length]: 21500
+};
+console.log(myTrip[length], myTrip.length);
+// → 21500 2
+```
+
+## The iterator interface
+
+{{index "iterable interface", "Symbol.iterator symbol", "for/of loop"}}
+
+The object given to a `for`/`of` loop is expected to be _iterable_. This means it has a method named with the `Symbol.iterator` symbol (a symbol value defined by the language, stored as a property of the `Symbol` function).
+
+{{index "iterator interface", "next method"}}
+
+When called, that method should return an object that provides a second interface, _iterator_. This is the actual thing that iterates. It has a `next` method that returns the next result. That result should be an object with a `value` property that provides the next value, if there is one, and a `done` property, which should be true when there are no more results and false otherwise.
+
+Note that the `next`, `value`, and `done` property names are plain strings, not symbols. Only `Symbol.iterator`, which is likely to be added to a _lot_ of different objects, is an actual symbol.
+
+We can directly use this interface ourselves.
+
+```
+let okIterator = "OK"[Symbol.iterator]();
+console.log(okIterator.next());
+// → {value: "O", done: false}
+console.log(okIterator.next());
+// → {value: "K", done: false}
+console.log(okIterator.next());
+// → {value: undefined, done: true}
+```
+
+{{index ["data structure", list], "linked list", collection}}
+
+Let's implement an iterable data structure similar to the linked list from the exercise in [Chapter ?](data). We'll write the list as a class this time.
+
+```{includeCode: true}
+class List {
+ constructor(value, rest) {
+ this.value = value;
+ this.rest = rest;
+ }
+
+ get length() {
+ return 1 + (this.rest ? this.rest.length : 0);
+ }
+
+ static fromArray(array) {
+ let result = null;
+ for (let i = array.length - 1; i >= 0; i--) {
+ result = new this(array[i], result);
+ }
+ return result;
+ }
+}
+```
+
+Note that `this`, in a static method, points at the constructor of the class, not an instance—there is no instance around when a static method is called.
+
+Iterating over a list should return all the list's elements from start to end. We'll write a separate class for the iterator.
+
+{{index "ListIterator class"}}
+
+```{includeCode: true}
+class ListIterator {
+ constructor(list) {
+ this.list = list;
+ }
+
+ next() {
+ if (this.list == null) {
+ return {done: true};
+ }
+ let value = this.list.value;
+ this.list = this.list.rest;
+ return {value, done: false};
+ }
+}
+```
+
+The class tracks the progress of iterating through the list by updating its `list` property to move to the next list object whenever a value is returned and reports that it is done when that list is empty (null).
+
+Let's set up the `List` class to be iterable. Throughout this book, I'll occasionally use after-the-fact prototype manipulation to add methods to classes so that the individual pieces of code remain small and self contained. In a regular program, where there is no need to split the code into small pieces, you'd declare these methods directly in the class instead.
+
+```{includeCode: true}
+List.prototype[Symbol.iterator] = function() {
+ return new ListIterator(this);
+};
+```
+
+{{index "for/of loop"}}
+
+We can now loop over a list with `for`/`of`.
+
+```
+let list = List.fromArray([1, 2, 3]);
+for (let element of list) {
+ console.log(element);
+}
+// → 1
+// → 2
+// → 3
+```
+
+{{index spread}}
+
+The `...` syntax in array notation and function calls similarly works with any iterable object. For example, you can use `[...value]` to create an array containing the elements in an arbitrary iterable object.
+
+```
+console.log([..."PCI"]);
+// → ["P", "C", "I"]
+```
+
+## Inheritance
+
+{{index inheritance, "linked list", "object-oriented programming", "LengthList class"}}
+
+Imagine we need a list type much like the `List` class we saw before, but because we will be asking for its length all the time, we don't want it to have to scan through its `rest` every time. Instead, we want to store the length in every instance for efficient access.
+
+{{index overriding, prototype}}
+
+JavaScript's prototype system makes it possible to create a _new_ class, much like the old class, but with new definitions for some of its properties. The prototype for the new class derives from the old prototype but adds a new definition for, say, the `length` getter.
+
+In object-oriented programming terms, this is called _((inheritance))_. The new class inherits properties and behavior from the old class.
+
+```{includeCode: "top_lines: 12"}
+class LengthList extends List {
+ #length;
+
+ constructor(value, rest) {
+ super(value, rest);
+ this.#length = super.length;
+ }
+
+ get length() {
+ return this.#length;
+ }
+}
+
+console.log(LengthList.fromArray([1, 2, 3]).length);
+// → 3
+```
+
+The use of the word `extends` indicates that this class shouldn't be directly based on the default `Object` prototype but on some other class. This is called the _((superclass))_. The derived class is the _((subclass))_.
+
+To initialize a `LengthList` instance, the constructor calls the constructor of its superclass through the `super` keyword. This is necessary because if this new object is to behave (roughly) like a `List`, it is going to need the instance properties that lists have.
+
+The constructor then stores the list's length in a private property. If we had written `this.length` there, the class's own getter would have been called, which doesn't work yet since `#length` hasn't been filled in yet. We can use `super.something` to call methods and getters on the superclass's prototype, which is often useful.
+
+Inheritance allows us to build slightly different data types from existing data types with relatively little work. It is a fundamental part of the object-oriented tradition, alongside encapsulation and polymorphism. But while the latter two are now generally regarded as wonderful ideas, inheritance is more controversial.
+
+{{index complexity, reuse, "class hierarchy"}}
+
+Whereas ((encapsulation)) and polymorphism can be used to _separate_ pieces of code from one another, reducing the tangledness of the overall program, ((inheritance)) fundamentally ties classes together, creating _more_ tangle. When inheriting from a class, you usually have to know more about how it works than when simply using it. Inheritance can be a useful tool to make some types of programs more succinct, but it shouldn't be the first tool you reach for, and you probably shouldn't actively go looking for opportunities to construct class hierarchies (family trees of classes).
+
+## The instanceof operator
+
+{{index type, "instanceof operator", constructor, object}}
+
+It is occasionally useful to know whether an object was derived from a specific class. For this, JavaScript provides a binary operator called `instanceof`.
+
+```
+console.log(
+ new LengthList(1, null) instanceof LengthList);
+// → true
+console.log(new LengthList(2, null) instanceof List);
+// → true
+console.log(new List(3, null) instanceof LengthList);
+// → false
+console.log([1] instanceof Array);
+// → true
+```
+
+{{index inheritance}}
+
+The operator will see through inherited types, so a `LengthList` is an instance of `List`. The operator can also be applied to standard constructors like `Array`. Almost every object is an instance of `Object`.
+
+## Summary
+
+Objects do more than just hold their own properties. They have prototypes, which are other objects. They'll act as if they have properties they don't have as long as their prototype has that property. Simple objects have `Object.prototype` as their prototype.
+
+Constructors, which are functions whose names usually start with a capital letter, can be used with the `new` operator to create new objects. The new object's prototype will be the object found in the `prototype` property of the constructor. You can make good use of this by putting the properties that all values of a given type share into their prototype. There's a `class` notation that provides a clear way to define a constructor and its prototype.
+
+You can define getters and setters to secretly call methods every time an object's property is accessed. Static methods are methods stored in a class's constructor rather than its prototype.
+
+The `instanceof` operator can, given an object and a constructor, tell you whether that object is an instance of that constructor.
+
+One useful thing to do with objects is to specify an interface for them and tell everybody that they are supposed to talk to your object only through that interface. The rest of the details that make up your object are now _encapsulated_, hidden behind the interface. You can use private properties to hide a part of your object from the outside world.
+
+More than one type may implement the same interface. Code written to use an interface automatically knows how to work with any number of different objects that provide the interface. This is called _polymorphism_.
+
+When implementing multiple classes that differ in only some details, it can be helpful to write the new classes as _subclasses_ of an existing class, _inheriting_ part of its behavior.
+
+## Exercises
+
+{{id exercise_vector}}
+
+### A vector type
+
+{{index dimensions, "Vec class", coordinates, "vector (exercise)"}}
+
+Write a ((class)) `Vec` that represents a vector in two-dimensional space. It takes `x` and `y` parameters (numbers), that it saves to properties of the same name.
+
+{{index addition, subtraction}}
+
+Give the `Vec` prototype two methods, `plus` and `minus`, that take another vector as a parameter and return a new vector that has the sum or difference of the two vectors' (`this` and the parameter) _x_ and _y_ values.
+
+Add a ((getter)) property `length` to the prototype that computes the length of the vector—that is, the distance of the point (_x_, _y_) from the origin (0, 0).
+
+{{if interactive
+
+```{test: no}
+// Your code here.
+
+console.log(new Vec(1, 2).plus(new Vec(2, 3)));
+// → Vec{x: 3, y: 5}
+console.log(new Vec(1, 2).minus(new Vec(2, 3)));
+// → Vec{x: -1, y: -1}
+console.log(new Vec(3, 4).length);
+// → 5
+```
+if}}
+
+{{hint
+
+{{index "vector (exercise)"}}
+
+Look back to the `Rabbit` class example if you're unsure how `class` declarations look.
+
+{{index Pythagoras, "defineProperty function", "square root", "Math.sqrt function"}}
+
+Adding a getter property to the constructor can be done by putting the word `get` before the method name. To compute the distance from (0, 0) to (x, y), you can use the Pythagorean theorem, which says that the square of the distance we are looking for is equal to the square of the x-coordinate plus the square of the y-coordinate. Thus, [√(x^2^ + y^2^)]{if html}[[$\sqrt{x^2 + y^2}$]{latex}]{if tex} is the number you want. `Math.sqrt` is the way you compute a square root in JavaScript and `x ** 2` can be used to square a number.
+
+hint}}
+
+### Groups
+
+{{index "groups (exercise)", "Set class", "Group class", "set (data structure)"}}
+
+{{id groups}}
+
+The standard JavaScript environment provides another data structure called `Set`. Like an instance of `Map`, a set holds a collection of values. Unlike `Map`, it does not associate other values with those—it just tracks which values are part of the set. A value can be part of a set only once—adding it again doesn't have any effect.
+
+{{index "add method", "delete method", "has method"}}
+
+Write a class called `Group` (since `Set` is already taken). Like `Set`, it has `add`, `delete`, and `has` methods. Its constructor creates an empty group, `add` adds a value to the group (but only if it isn't already a member), `delete` removes its argument from the group (if it was a member), and `has` returns a Boolean value indicating whether its argument is a member of the group.
+
+{{index "=== operator", "indexOf method"}}
+
+Use the `===` operator, or something equivalent such as `indexOf`, to determine whether two values are the same.
+
+{{index "static method"}}
+
+Give the class a static `from` method that takes an iterable object as its argument and creates a group that contains all the values produced by iterating over it.
+
+{{if interactive
+
+```{test: no}
+class Group {
+ // Your code here.
+}
+
+let group = Group.from([10, 20]);
+console.log(group.has(10));
+// → true
+console.log(group.has(30));
+// → false
+group.add(10);
+group.delete(10);
+console.log(group.has(10));
+// → false
+```
+
+if}}
+
+{{hint
+
+{{index "groups (exercise)", "Group class", "indexOf method", "includes method"}}
+
+The easiest way to do this is to store an array of group members in an instance property. The `includes` or `indexOf` methods can be used to check whether a given value is in the array.
+
+{{index "push method"}}
+
+Your class's ((constructor)) can set the member collection to an empty array. When `add` is called, it must check whether the given value is in the array or add it otherwise, possibly using `push`.
+
+{{index "filter method"}}
+
+Deleting an element from an array, in `delete`, is less straightforward, but you can use `filter` to create a new array without the value. Don't forget to overwrite the property holding the members with the newly filtered version of the array.
+
+{{index "for/of loop", "iterable interface"}}
+
+The `from` method can use a `for`/`of` loop to get the values out of the iterable object and call `add` to put them into a newly created group.
+
+hint}}
+
+### Iterable groups
+
+{{index "groups (exercise)", [interface, object], "iterator interface", "Group class"}}
+
+{{id group_iterator}}
+
+Make the `Group` class from the previous exercise iterable. Refer to the section about the iterator interface earlier in the chapter if you aren't clear on the exact form of the interface anymore.
+
+If you used an array to represent the group's members, don't just return the iterator created by calling the `Symbol.iterator` method on the array. That would work, but it defeats the purpose of this exercise.
+
+It is okay if your iterator behaves strangely when the group is modified during iteration.
+
+{{if interactive
+
+```{test: no}
+// Your code here (and the code from the previous exercise)
+
+for (let value of Group.from(["a", "b", "c"])) {
+ console.log(value);
+}
+// → a
+// → b
+// → c
+```
+
+if}}
+
+{{hint
+
+{{index "groups (exercise)", "Group class", "next method"}}
+
+It is probably worthwhile to define a new class `GroupIterator`. Iterator instances should have a property that tracks the current position in the group. Every time `next` is called, it checks whether it is done and, if not, moves past the current value and returns it.
+
+The `Group` class itself gets a method named by `Symbol.iterator` that, when called, returns a new instance of the iterator class for that group.
+
+hint}}
diff --git a/06_object.txt b/06_object.txt
deleted file mode 100644
index 3dd75cee2..000000000
--- a/06_object.txt
+++ /dev/null
@@ -1,1225 +0,0 @@
-:chap_num: 6
-:prev_link: 05_higher_order
-:next_link: 07_elife
-:load_files: ["code/mountains.js", "code/chapter/06_object.js"]
-
-= The Secret Life of Objects =
-
-[chapterquote="true"]
-[quote, Joe Armstrong, interviewed in Coders at Work]
-____
-The problem with object-oriented languages
-is they’ve got all this implicit environment that they carry around
-with them. You wanted a banana but what you got was a gorilla holding
-the banana and the entire jungle.
-____
-
-(((Armstrong+++,+++ Joe)))(((object)))(((holy war)))When a programmer
-says “object”, this is a loaded term. In my profession, objects are a
-way of life, the subject of holy wars, and a beloved buzzword that
-still hasn't quite lost its power.
-
-To an outsider, this is probably a little confusing. Let's start with
-a brief ((history)) of objects as a programming construct.
-
-== History ==
-
-(((isolation)))(((history)))(((object-oriented programming)))(((object)))This story, like most programming stories, starts with the
-problem of ((complexity)). One philosophy is that complexity can be
-made manageable by separating it into small compartments that are
-isolated from each other. These compartments have ended up with the
-name “objects”.
-
-[[interface]]
-(((complexity)))(((encapsulation)))(((method)))(((interface)))An
-object is a hard shell that hides the gooey complexity inside of it,
-and instead offers us a few knobs and connectors (like ((method))s)
-that present an _interface_ through which the object is to be used.
-The idea is that the interface is relatively simple, and all the
-complex things going on _inside_ of the object can be ignored when
-working with it.
-
-image::img/object.jpg[alt="A simple interface can hide a lot of complexity",width="6cm"]
-
-As an example, you can imagine an object that provides an interface to
-an area on your screen. It provides a way to draw shapes or text onto
-this area, but hides all the details of how these shapes are converted
-to the actual pixels that make up the screen. You'd have a set of
-methods, for example `drawCircle`, and those are the only things you
-need to know in order to use such an object.
-
-(((object-oriented programming)))These ideas were initially worked out
-in the 1970s and 80s, and, in the 90s, were carried up by a huge wave
-of ((hype))—the object-oriented programming revolution. Suddenly,
-there was a large tribe of people declaring that objects were the
-_right_ way to program, and that anything that did not involve objects
-was outdated nonsense.
-
-That kind of zealotry always produces a lot of impractical silliness,
-and there has been a sort of counter-revolution since then. In some
-circles, objects have a rather bad reputation nowadays.
-
-I prefer to look at the issue from a practical, rather than
-ideological angle. There are several very useful concepts, most
-importantly that of _((encapsulation))_ (distinguishing between
-internal complexity and external interface), that the object-oriented
-culture has popularized. These are worth studying.
-
-This chapter describes JavaScript's rather eccentric take on objects,
-and the way they relate to some classical object-oriented techniques.
-
-[[obj_methods]]
-== Methods ==
-
-(((rabbit example)))(((method)))(((property)))Methods are simply
-properties that hold function values. This is a very simple method:
-
-[source,javascript]
-----
-var rabbit = {};
-rabbit.speak = function(line) {
- console.log("The rabbit says '" + line + "'");
-};
-
-rabbit.speak("I'm alive.");
-// → The rabbit says 'I'm alive.'
-----
-
-(((this)))(((method call)))Usually a method needs to do something with
-the object it was called on. When a function is called as a
-method—looked up as a property and immediately called, as in
-++object.method()++—the special variable `this` in its body will point
-to the object that it was called on.
-
-// test: join
-// include_code top_lines:6
-
-[source,javascript]
-----
-function speak(line) {
- console.log("The " + this.type + " rabbit says '" +
- line + "'");
-}
-var whiteRabbit = {type: "white", speak: speak};
-var fatRabbit = {type: "fat", speak: speak};
-
-whiteRabbit.speak("Oh my ears and whiskers, " +
- "how late it's getting!");
-// → The white rabbit says 'Oh my ears and whiskers, how
-// late it's getting!'
-fatRabbit.speak("I could sure use a carrot right now.");
-// → The fat rabbit says 'I could sure use a carrot
-// right now.'
-----
-
-(((rabbit example)))The code uses the `this` keyword to output the
-type of rabbit that is speaking.
-
-(((apply method)))(((bind method)))(((this)))Recall that the `apply`
-and `bind` methods both take a first argument that can be used to
-simulate method calls. This first argument is in fact used to give a
-value to `this`.
-
-[[call_method]]
-(((call method)))There is a method similar to `apply`, called `call`.
-It also calls the function it is a method of, but takes its arguments
-normally, rather than as an array. Like `apply` and `bind`, `call` can
-be passed a specific `this` value.
-
-[source,javascript]
-----
-speak.apply(fatRabbit, ["Burp!"]);
-// → The fat rabbit says 'Burp!'
-speak.call({type: "old"}, "Oh my.");
-// → The old rabbit says 'Oh my.'
-----
-
-[[prototypes]]
-== Prototypes ==
-
-(((toString method)))Watch closely.
-
-[source,javascript]
-----
-var empty = {};
-console.log(empty.toString);
-// → function toString(){…}
-console.log(empty.toString());
-// → [object Object]
-----
-
-(((magic)))I just pulled a property out of an empty object. Magic!
-
-(((property)))(((object)))Well, not really. I have simply been
-withholding information about the way JavaScript objects work. In
-addition to their set of properties, almost all objects also have a
-_prototype_. A ((prototype)) is another object that is used as a
-fallback source of properties. When an object gets a request for a
-property that it does not have, its prototype will be searched for the
-property, and then the prototype's prototype, and so on.
-
-(((Object prototype)))So who is the ((prototype)) of that empty
-object? It is the great ancestral prototype, the entity behind almost
-all objects, `Object.prototype`.
-
-[source,javascript]
-----
-console.log(Object.getPrototypeOf({}) ==
- Object.prototype);
-// → true
-console.log(Object.getPrototypeOf(Object.prototype));
-// → null
-----
-
-(((getPrototypeOf function)))As you might expect, the
-`Object.getPrototypeOf` function returns the prototype of an object.
-
-(((toString method)))The prototype relations of JavaScript objects
-form a ((tree))-shaped structure, and at the root of this structure
-sits `Object.prototype`. It provides a few ((method))s that show up in
-all objects, such as `toString`, which converts an object to a string
-representation.
-
-(((inheritance)))(((Function prototype)))(((Array
-prototype)))(((Object prototype)))Many objects don't directly have
-`Object.prototype` as their ((prototype)), but instead have another
-object, which provides its own default properties. Functions derive
-from `Function.prototype`, arrays from `Array.prototype`.
-
-[source,javascript]
-----
-console.log(Object.getPrototypeOf(isNaN) ==
- Function.prototype);
-// → true
-console.log(Object.getPrototypeOf([]) ==
- Array.prototype);
-// → true
-----
-
-(((Object prototype)))Such a prototype object will itself have a
-prototype, often `Object.prototype` so that it still indirectly
-provides methods like `toString`.
-
-(((getPrototypeOf function)))(((rabbit example)))(((Object.create
-function)))The `Object.getPrototypeOf` function obviously returns the
-prototype of an object. You can use `Object.create` to create an
-object with a specific ((prototype)).
-
-[source,javascript]
-----
-var protoRabbit = {
- speak: function(line) {
- console.log("The " + this.type + " rabbit says '" +
- line + "'");
- }
-};
-var killerRabbit = Object.create(protoRabbit);
-killerRabbit.type = "killer";
-killerRabbit.speak("SKREEEE!");
-// → The killer rabbit says 'SKREEEE!'
-----
-
-(((shared property)))The “proto” rabbit acts as a container for the
-properties that are shared by all rabbits. An individual rabbit
-object, like the killer rabbit, contains properties that apply only to
-itself—in this case its type—and derives shared properties from its
-prototype.
-
-[[constructors]]
-== Constructors ==
-
-(((new operator)))(((this variable)))(((return keyword)))(((object,creation)))A more convenient way to create objects that derive
-from some shared prototype is to use a _((constructor))_. In
-JavaScript, calling a function with the `new` keyword in front of it
-causes it to be treated as a constructor. The constructor will have
-its `this` variable bound to a fresh object, and, unless it explicitly
-returns another object value, this new object will be returned from
-the call.
-
-An object created with `new` is said to be an _((instance))_ of its
-constructor.
-
-(((rabbit example)))(((capitalization)))Here is a simple constructor
-for rabbits. It is a convention to capitalize the names of
-constructors, so that they are easily distinguished from other
-functions.
-
-// include_code top_lines:6
-
-[source,javascript]
-----
-function Rabbit(type) {
- this.type = type;
-}
-
-var killerRabbit = new Rabbit("killer");
-var blackRabbit = new Rabbit("black");
-console.log(blackRabbit.type);
-// → black
-----
-
-(((prototype property)))(((constructor)))Constructors (in fact, all
-functions) automatically get a property named `prototype`, which by
-default holds a plain, empty object that derives from
-`Object.prototype`. Every instance created with this constructor will
-have this object as its ((prototype)). So to add a `speak` method to
-rabbits created with the `Rabbit` constructor, we can simply do this:
-
-// include_code top_lines:4
-
-[source,javascript]
-----
-Rabbit.prototype.speak = function(line) {
- console.log("The " + this.type + " rabbit says '" +
- line + "'");
-};
-blackRabbit.speak("Doom...");
-// → The black rabbit says 'Doom...'
-----
-
-(((prototype property)))(((getPrototypeOf function)))It is important
-to note the distinction between the way a prototype is associated with
-a constuctor (through its `prototype` property), and the way objects
-_have_ a prototype (which can be retrieved with
-`Object.getPrototypeOf`). The actual prototype of a constructor is
-`Function.prototype`, since constructors are functions. Its
-`prototype` _property_ will be the prototype of instances created
-though it, but is not its _own_ prototype.
-
-== Overriding derived properties ==
-
-(((shared property)))(((overriding)))When you add a ((property)) to an
-object, whether it is present in the prototype or not, the property is
-added to the object _itself_, which will henceforth have it as its own
-property. If there _is_ a property by the same name in the prototype,
-this property will no longer affect the object. The prototype itself
-is not changed.
-
-[source,javascript]
-----
-Rabbit.prototype.teeth = "small";
-console.log(killerRabbit.teeth);
-// → small
-killerRabbit.teeth = "long, sharp, and bloody";
-console.log(killerRabbit.teeth);
-// → long, sharp, and bloody
-console.log(blackRabbit.teeth);
-// → small
-console.log(Rabbit.prototype.teeth);
-// → small
-----
-
-(((prototype,diagram)))The following diagram sketches the situation
-after this code has run. The `Rabbit` and `Object` ((prototype))s lie
-behind `killerRabbit` as a kind of backdrop, where properties that are
-not found in the object itself can be looked up.
-
-image::img/rabbits.svg[alt="Rabbit object prototype schema",width="8cm"]
-
-(((shared property)))Overriding properties that exist in a prototype
-is often a useful thing to do. As the rabbit teeth example shows, it
-can be used to express exceptional properties in instances of a more
-generic class of objects, while letting the non-exceptional objects
-simply take a standard value from their prototype.
-
-(((toString method)))(((Array prototype)))(((Function prototype)))It
-is also used to give the standard function and array prototypes a
-different `toString` method than the basic object prototype.
-
-[source,javascript]
-----
-console.log(Array.prototype.toString ==
- Object.prototype.toString);
-// → false
-console.log([1, 2].toString());
-// → 1,2
-console.log(Object.prototype.toString.call([1, 2]));
-// → [object Array]
-----
-
-(((toString method)))(((join method)))(((call method)))Calling
-`toString` on an array gives a result similar to calling `.join(",")`
-on it—it puts commas between the values in the array. Directly calling
-`Object.prototype.toString` with an array produces a different string.
-That function doesn't know about arrays, so it simply puts the word
-“object” and the name of the type between square brackets.
-
-[source,javascript]
-----
-console.log(Object.prototype.toString.call([1, 2]));
-// → [object Array]
-----
-
-== Prototype interference ==
-
-(((prototype,interference)))(((rabbit example)))(((mutability)))A
-((prototype)) can be used at any time to add new properties and
-methods to all objects based on it. For example, it might become
-necessary for our rabbits to dance.
-
-[source,javascript]
-----
-Rabbit.prototype.dance = function() {
- console.log("The " + this.type + " rabbit dances a jig.");
-};
-killerRabbit.dance();
-// → The killer rabbit dances a jig.
-----
-
-(((map)))(((object,as map)))That's very convenient. But there are
-situations where it causes problems. In previous chapters, we used an
-object as a way to associate values with names by creating properties
-for the names and giving them the corresponding value as their value.
-Here's an example from link:04_data.html#object_map[Chapter 4]:
-
-// include_code
-
-[source,javascript]
-----
-var map = {};
-function storePhi(event, phi) {
- map[event] = phi;
-}
-
-storePhi("pizza", 0.069);
-storePhi("touched tree", -0.081);
-----
-
-(((for/in loop)))(((in operator)))We can iterate over all phi values
-in the object using a `for`/`in` loop, and test whether a name is in
-there using the regular `in` operator. But unfortunately, the object's
-prototype gets in the way.
-
-[source,javascript]
-----
-Object.prototype.nonsense = "hi";
-for (var name in map)
- console.log(name);
-// → pizza
-// → touched tree
-// → nonsense
-console.log("nonsense" in map);
-// → true
-console.log("toString" in map);
-// → true
-
-// Delete the problematic property again
-delete Object.prototype.nonsense;
-----
-
-(((prototype,pollution)))(((toString method)))That's all wrong. There
-is no event called “nonsense” in our data set. And there _definitely_
-is no event called “toString”.
-
-(((enumerability)))(((for/in loop)))(((property)))Oddly, `toString`
-did not show up in the `for`/`in` loop, but the `in` operator did
-return true for it. This is because JavaScript distinguishes between
-_enumerable_ and _non-enumerable_ properties.
-
-(((Object prototype)))All properties that we create by simply
-assigning to them are enumerable. The standard properties in
-`Object.prototype` are all non-enumerable, which is why they do not
-show up in such a `for`/`in` loop.
-
-(((defineProperty function)))It is possible to define our own
-non-enumerable properties by using the `Object.defineProperty`
-function, which allows us to control the type of property we are
-creating.
-
-[source,javascript]
-----
-Object.defineProperty(Object.prototype, "hiddenNonsense",
- {enumerable: false, value: "hi"});
-for (var name in map)
- console.log(name);
-// → pizza
-// → touched tree
-console.log(map.hiddenNonsense);
-// → hi
-----
-
-(((in operator)))(((map)))(((object,as map)))(((hasOwnProperty
-method)))So now the property is there, but it won't show up in a loop.
-That's good. But we still have the problem with the regular `in`
-operator claiming that the `Object.prototype` properties exist in our
-object. For that, we can use the object's `hasOwnProperty` method.
-
-[source,javascript]
-----
-console.log(map.hasOwnProperty("toString"));
-// → false
-----
-
-(((property,own)))This method tells us whether the object _itself_ has
-the property, without looking at its prototypes. This is often a more
-useful piece of information than what the `in` operator gives us.
-
-(((prototype,pollution)))(((for/in loop)))When you are worried that
-someone (some other code you loaded into your program) might have
-messed with the base object prototype, I recommend to write your
-`for`/`in` loops like this:
-
-[source,javascript]
-----
-for (var name in map) {
- if (map.hasOwnProperty(name)) {
- // ... this is an own property
- }
-}
-----
-
-== Prototype-less objects ==
-
-(((map)))(((object,as map)))(((hasOwnProperty method)))But the
-rabbit hole doesn't end there. What if someone registered the name
-`hasOwnProperty` in our `map` object and set it to the value 42? Now
-the call to `map.hasOwnProperty` will try to call the local property,
-which holds a number, not a function.
-
-(((Object.create function)))(((prototype,avoidance)))In such a case,
-prototypes just get in the way, and we would actually prefer to have
-objects without prototype. We saw the `Object.create` function, which
-allows us to create an object with a specific prototype. You are
-allowed to pass `null` as the prototype to create a fresh object with
-no prototype. For objects like `map`, where the properties could be
-anything, this is exactly what we want.
-
-[source,javascript]
-----
-var map = Object.create(null);
-map["pizza"] = 0.069;
-console.log("toString" in map);
-// → false
-console.log("pizza" in map);
-// → true
-----
-
-(((in operator)))(((for/in loop)))(((Object prototype)))Much
-better! We no longer need the `hasOwnProperty` kludge, because all the
-properties the object has are its own properties. Now we can safely
-use `for`/`in` loops, no matter what people have been doing to
-`Object.prototype`.
-
-== Polymorphism ==
-
-(((toString method)))(((String
-function)))(((polymorphism)))(((overriding)))When you call the
-`String` function, which converts a value to a string, on an object,
-it will call the `toString` method on that object to try and create a
-meaningful string to return. I mentioned that some of the standard
-prototypes define their own version of `toString`, in order to be able
-to create a string that contains more useful information than
-`"[object Object]"`.
-
-(((object-oriented programming)))This is a simple instance of a very
-powerful idea. When a piece of code is written to work with objects
-that have a certain ((interface))—in this case, a `toString`
-method—any kind of object that happens to support this interface can
-be plugged into the code, and it will just work.
-
-This technique is called __polymorphism__—though no actual
-shape-shifting is involved. Polymorphic code can work with values of
-different shapes, as long as they support the interface it expects.
-
-[[tables]]
-== Laying out a table ==
-
-(((MOUNTAINS data set)))(((table example)))I am going to work through
-a slightly more involved example in an attempt to give you a better
-idea what ((polymorphism)), as well as ((object-oriented programming))
-in general, looks like. The project is this: we will write a program
-that, given an array of arrays of ((table)) cells, builds up a string
-that contains a nicely laid out table—meaning that the columns are
-straight and the rows are aligned. Something like this:
-
-[source,text/plain]
-----
-name height country
------------- ------ -------------
-Kilimanjaro 5895 Tanzania
-Everest 8848 Nepal
-Mount Fuji 3776 Japan
-Mont Blanc 4808 Italy/France
-Vaalserberg 323 Netherlands
-Denali 6168 United States
-Popocatepetl 5465 Mexico
-----
-
-The way our table-building system will work is that the builder
-function will ask each cell how wide and high it wants to be, and then
-use this information to determine the width of the columns and the
-height of the rows. The builder function will then ask the cells to
-draw themselves at the correct size, and assemble the results into a
-single string.
-
-[[table_interface]]
-(((table example)))The layout program will communicate with the cell
-objects through a well-defined ((interface)). That way, the types of
-cells that the program supports is not fixed in advance. We can add
-new cell styles later— for example, underlined cells for table
-headers—and if they support our interface, they will just work,
-without requiring changes to the layout program.
-
-This is the interface:
-
-* `minHeight()` returns a number indicating the minimum height this
- cell requires (in lines).
-
-* `minWidth()` returns a number indicating this cell's minimum width (in
- characters).
-
-* `draw(width, height)` returns an array of length
- `height`, which contains a series of strings that are each `width` characters wide.
- This represents the content of the cell.
-
-(((function,higher-order)))I'm going to make heavy use of higher-order
-array methods in this example, since it lends itself well to that
-approach.
-
-(((rowHeights function)))(((colWidths function)))(((maximum)))(((map
-method)))(((reduce method)))The first part of the program computes
-arrays of minimum column widths and row heights for a grid of cells.
-The `rows` variable will hold an array of arrays, each inner array
-representing a row of cells.
-
-// include_code
-
-[source,javascript]
-----
-function rowHeights(rows) {
- return rows.map(function(row) {
- return row.reduce(function(max, cell) {
- return Math.max(max, cell.minHeight());
- }, 0);
- });
-}
-
-function colWidths(rows) {
- return rows[0].map(function(_, i) {
- return rows.reduce(function(max, row) {
- return Math.max(max, row[i].minWidth());
- }, 0);
- });
-}
-----
-
-(((underscore character)))(((programming style)))Using a variable name
-starting with an underscore (“_”), or consisting entirely of a single
-underscore, is a way to indicate (to human readers) that this argument
-is not going to be used.
-
-The `rowHeights` function shouldn't be too hard to follow. It uses
-`reduce` to compute the maximum height of an array of cells, and wraps
-that in `map` in order to do it for all rows in the `rows` array.
-
-(((map method)))(((filter method)))(((forEach
-method)))(((array,indexing)))(((reduce method)))Things are slightly
-harder for the `colWidths` function, because the outer array is an
-array of rows, not of columns. I have failed to mention so far that
-`map` (as well as `forEach`, `filter`, and similar array methods)
-passes a second argument to the function it is given: the ((index)) of
-the current element. By mapping over the elements of the first row and
-only using the mapping function's second argument, `colWidths` builds
-up an array with one element for every column index. The call to
-`reduce` runs over the outer `rows` array for each index, and picks
-out the width of the widest cell at that index.
-
-(((table example)))(((drawTable function)))Here's the code to draw a
-table:
-
-// include_code
-
-[source,javascript]
-----
-function drawTable(rows) {
- var heights = rowHeights(rows);
- var widths = colWidths(rows);
-
- function drawLine(blocks, lineNo) {
- return blocks.map(function(block) {
- return block[lineNo];
- }).join(" ");
- }
-
- function drawRow(row, rowNum) {
- var blocks = row.map(function(cell, colNum) {
- return cell.draw(widths[colNum], heights[rowNum]);
- });
- return blocks[0].map(function(_, lineNo) {
- return drawLine(blocks, lineNo);
- }).join("\n");
- }
-
- return rows.map(drawRow).join("\n");
-}
-----
-
-(((inner function)))(((nesting,of functions)))The `drawTable` function
-uses the internal helper function `drawRow` to draw all rows, then
-joins them together with newline characters.
-
-(((table example)))The `drawRow` function itself first converts the
-cell objects in the row to _blocks_, which are arrays of strings
-representing the content of the cells, split by line. A single cell
-containing simply the number 3776 might be represented by a
-single-element array like `["3776"]`, whereas an underlined cell might
-take up two lines, and be represented by the array `["name", "----"]`.
-
-(((map method)))(((join method)))The blocks for a row, which all have
-the same height, should appear next to each other in the final output.
-The second call to `map` in `drawRow` builds up this output line by
-line, by mapping over the lines in the leftmost block, and for each of
-those, collecting a line that spans the full width of the table. These
-lines are then joined with newline characters to provide the whole row
-as `drawRow`’s return value.
-
-The function `drawLine` extracts lines that should appear next
-to each other from an array of blocks, and joins them with a space
-character, to create a one-character gap between the table's columns.
-
-[[split]]
-(((split method)))(((string,methods)))(((table example)))Now
-let's write a constructor for cells that contain text, which
-implements the ((interface)) for table cells. The constructor splits a
-string into an array of lines, using the string method `split`, which
-cuts up a string at every occurrence of its argument, and returns an
-array of the pieces. The `minWidth` method finds the maximum line
-width in this array.
-
-// include_code
-
-[source,javascript]
-----
-function repeat(string, times) {
- var result = "";
- for (var i = 0; i < times; i++)
- result += string;
- return result;
-}
-
-function TextCell(text) {
- this.text = text.split("\n");
-}
-TextCell.prototype.minWidth = function() {
- return this.text.reduce(function(width, line) {
- return Math.max(width, line.length);
- }, 0);
-};
-TextCell.prototype.minHeight = function() {
- return this.text.length;
-};
-TextCell.prototype.draw = function(width, height) {
- var result = [];
- for (var i = 0; i < height; i++) {
- var line = this.text[i] || "";
- result.push(line + repeat(" ", width - line.length));
- }
- return result;
-};
-----
-
-(((TextCell type)))The code uses a helper function called `repeat`,
-which builds a string whose value is the `string` argument repeated
-`times` number of times. The `draw` method uses it to add “padding” to
-lines, so that they all have the required length.
-
-Let's try out everything we've written so far by building up a 5 × 5
-checkerboard.
-
-[source,javascript]
-----
-var rows = [];
-for (var i = 0; i < 5; i++) {
- var row = [];
- for (var j = 0; j < 5; j++) {
- if ((j + i) % 2 == 0)
- row.push(new TextCell("##"));
- else
- row.push(new TextCell(" "));
- }
- rows.push(row);
-}
-console.log(drawTable(rows));
-// → ## ## ##
-// ## ##
-// ## ## ##
-// ## ##
-// ## ## ##
-----
-
-It works! But since all cells have the same size, the table-layout
-code doesn't really do anything interesting.
-
-[[mountains]]
-(((data set)))(((MOUNTAINS data set)))The source data for the table of
-mountains that we are trying to build is available in the `MOUNTAINS`
-variable in the ((sandbox)), and also
-http://eloquentjavascript.net/code/mountains.js[downloadable] from the
-list of data sets on the website(!tex (http://eloquentjavascript.net/code[_eloquentjavascript.net/code_])!).
-
-(((table example)))We will want to highlight the top row, which
-contains the column names, by underlining the cells with a series of
-dash characters. No problem—we simply write a cell type that handles
-underlining.
-
-// include_code
-
-[source,javascript]
-----
-function UnderlinedCell(inner) {
- this.inner = inner;
-};
-UnderlinedCell.prototype.minWidth = function() {
- return this.inner.minWidth();
-};
-UnderlinedCell.prototype.minHeight = function() {
- return this.inner.minHeight() + 1;
-};
-UnderlinedCell.prototype.draw = function(width, height) {
- return this.inner.draw(width, height - 1)
- .concat([repeat("-", width)]);
-};
-----
-
-(((UnterlinedCell type)))An underlined cell _contains_ another cell.
-It reports its minimum size as being the same as that of its inner
-cell (by calling through to that cell's `minWidth` and `minHeight`
-methods), but adds one to the height, to account for the space taken
-up by the underline.
-
-(((concat method)))(((concatenation)))Drawing such a cell is quite
-simple—we take the content of the inner cell, and concatenate a single
-line full of dashes to it.
-
-(((dataTable function)))Having an underlining mechanism, we can now
-write a function that builds up a grid of cells from our data set.
-
-// test: wrap, trailing
-
-[source,javascript]
-----
-function dataTable(data) {
- var keys = Object.keys(data[0]);
- var headers = keys.map(function(name) {
- return new UnderlinedCell(new TextCell(name));
- });
- var body = data.map(function(row) {
- return keys.map(function(name) {
- return new TextCell(String(row[name]));
- });
- });
- return [headers].concat(body);
-}
-
-console.log(drawTable(dataTable(MOUNTAINS)));
-// → name height country
-// ------------ ------ -------------
-// Kilimanjaro 5895 Tanzania
-// … etcetera
-----
-
-[[keys]]
-(((Object.keys function)))(((property)))(((for/in loop)))The standard
-`Object.keys` function returns an array of property names in an
-object. The top row of the table must contain underlined cells that
-give the names of the columns. Below that, the values of all the
-objects in the data set appear as normal cells—we extract them by
-mapping over the `keys` array, so that we are sure that the order of
-the cells is the same in every row.
-
-(((right-aligning)))The resulting table resembles the example shown
-before, except that it does not right-align the numbers in the
-`height` colomn. We will get to that in a moment.
-
-== Getters and setters ==
-
-(((getter)))(((setter)))(((property)))When specifying an interface, it
-is possible to include properties that are not methods. We could have
-defined `minHeight` and `minWidth` to simply hold numbers. But that'd
-have required us to compute them in the ((constructor)), which adds
-code there that isn't strictly relevant to _constructing_ the object.
-It would cause problems if, for example, the inner cell of an
-underlined cell was changed, at which point the size of the underlined
-cell should also change.
-
-(((programming style)))This has led some people to adopt a principle
-of never including non-method properties in interfaces. Rather than
-directly access a simple value property, they'd use `getSomething` and
-`setSomething` methods to read and write the property. This approach
-has the downside that you will end up writing—and reading—a lot of
-additional methods.
-
-Fortunately, JavaScript provides a technique that gets us the best of
-both worlds. We can specify properties that, from the outside, look
-like normal properties, but secretly have ((method))s associated with
-them.
-
-[source,javascript]
-----
-var pile = {
- elements: ["eggshell", "orange peel", "worm"],
- get height() {
- return this.elements.length;
- },
- set height(value) {
- console.log("Ignoring attempt to set height to", value);
- }
-};
-
-console.log(pile.height);
-// → 3
-pile.height = 100;
-// → Ignoring attempt to set height to 100
-----
-
-(((defineProperty function)))((({}
-(object))))(((getter)))(((setter)))In object literal, the `get` or
-`set` notation for properties allows you to specify a function to be
-run when the property is read or written. You can also add such a
-property to an existing object, for example a prototype, using the
-`Object.defineProperty` function (which we previously used to create
-non-enumerable properties).
-
-[source,javascript]
-----
-Object.defineProperty(TextCell.prototype, "heightProp", {
- get: function() { return this.text.length; }
-});
-
-var cell = new TextCell("no\nway");
-console.log(cell.heightProp);
-// → 2
-cell.heightProp = 100;
-console.log(cell.heightProp);
-// → 2
-----
-
-You can use a similar `set` property, in the object passed to
-`defineProperty`, to specify a setter method. When a getter but no
-setter is defined, writing to the property is simply ignored.
-
-== Inheritance ==
-
-(((inheritance)))(((table example)))(((aligmnent)))(((TextCell
-type)))We are not quite done yet with our table layout exercise. It
-helps readability to right-align columns of numbers. We should create
-another cell type that is like `TextCell`, but rather than padding the
-lines on the right side, pads them on the left side, so that they
-align to the right.
-
-(((RTextCell type)))We could simply write a whole new ((constructor)),
-with all three methods in its prototype. But prototypes may themselves
-have prototypes, and this allows us to do something clever:
-
-// include_code
-
-[source,javascript]
-----
-function RTextCell(text) {
- TextCell.call(this, text);
-}
-RTextCell.prototype = Object.create(TextCell.prototype);
-RTextCell.prototype.draw = function(width, height) {
- var result = [];
- for (var i = 0; i < height; i++) {
- var line = this.text[i] || "";
- result.push(repeat(" ", width - line.length) + line);
- }
- return result;
-};
-----
-
-(((shared property)))(((overriding)))(((interface)))We reuse the
-constructor and the `minHeight` and `minWidth` methods from the
-regular `TextCell`. An `RTextCell` is now basically equivalent to a
-`TextCell`, except that its `draw` method contains a different
-function.
-
-(((call method)))This pattern is called _((inheritance))_. It allows
-us to build slightly different data types from existing datatypes with
-relatively little work. Typically, the new constructor will call the
-old ((constructor)) (using the `call` method in order to be able to
-give it the new object as its `this` value). Once this constructor has
-been called, we can assume that all the fields that the old object
-type is supposed to contain have been added. We arrange for the
-constructor's ((prototype)) to derive from the old prototype, so that
-instances of this type will also have access to the properties in that
-prototype. Finally, we can override some of these properties by adding
-them to our new prototype.
-
-(((dataTable function)))Now if we slightly adjust the `dataTable`
-function to use `RTextCell`'s for cells whose value is a number, we
-get the table we were aiming for:
-
-// start_code bottom_lines: 1
-// include_code strip_log
-
-[source,javascript]
-----
-function dataTable(data) {
- var keys = Object.keys(data[0]);
- var headers = keys.map(function(name) {
- return new UnderlinedCell(new TextCell(name));
- });
- var body = data.map(function(row) {
- return keys.map(function(name) {
- var value = row[name];
- // This was changed:
- if (typeof value == "number")
- return new RTextCell(String(value));
- else
- return new TextCell(String(value));
- });
- });
- return [headers].concat(body);
-}
-
-console.log(drawTable(dataTable(MOUNTAINS)));
-// → … beautifully aligned table
-----
-
-(((object-oriented programming)))Inheritance is a fundamental part of
-the object-oriented tradition, alongside encapsulation and
-polymorphism. But while the latter two are now generally regarded as
-wonderful ideas, inheritance is somewhat controversial.
-
-(((complexity)))The main reason for this is that it is often confused
-with ((polymorphism)), sold as a more powerful tool than it really
-is—and subsequently overused in all kinds of ugly ways. Whereas
-((encapsulation)) and polymorphism can be used to _separate_ pieces of
-code from each other, reducing the tangledness of the overall program,
-((inheritance)) fundamentally ties types together, creating _more_
-tangle.
-
-(((code structure)))(((programming style)))You can have
-polymorphism without inheritance, as we saw. I am not going to tell
-you to avoid inheritance entirely—I use it regularly in my own
-programs. But see it as a slightly dodgy trick that can help you
-define new types with little code, not as a grand principle of code
-organization. A preferable way to extend types is through
-((composition)), such as how `UnderlinedCell` builds on another cell
-object by simply storing it in a property and forwarding method calls
-to it in its own ((method))s.
-
-== The instanceof operator ==
-
-(((type)))(((instanceof operator)))(((constructor)))(((object)))It is occasionally useful to know whether an object was derived
-from a specific constructor. For this, JavaScript provides a binary
-operator called `instanceof`:
-
-[source,javascript]
-----
-console.log(new RTextCell("A") instanceof RTextCell);
-// → true
-console.log(new RTextCell("A") instanceof TextCell);
-// → true
-console.log(new TextCell("A") instanceof RTextCell);
-// → false
-console.log([1] instanceof Array);
-// → true
-----
-
-(((inheritance)))The operator will also see through inherited types.
-An `RTextCell` is an instance of `TextCell`, because
-`RTextCell.prototype` derives from `TextCell.prototype`. The operator
-can be applied to standard constructors like `Array`. Almost every
-object is an instance of `Object`.
-
-== Summary ==
-
-So objects are more complicated than I initially portrayed them. They
-have prototypes, which are other objects, and will act as if they have
-properties they don't have as long as the prototype has that property.
-Simple objects have `Object.prototype` as their prototype.
-
-Constructors, which are functions whose name usually starts with a
-capital letter, can be used with the `new` operator to create new
-objects. The new object's prototype will be the object found in the
-`prototype` property of the constructor function. You can make good
-use of this by putting the properties that all values of a given type
-share into their prototype. The `instanceof` operator can, given an
-object and a constructor, tell you whether that object is an instance
-of that constructor.
-
-One useful thing to do with objects is to specify an interface for
-them, and tell everybody that they are only supposed to talk to your
-object through that interface. The rest of the details that make up
-your object are now _encapsulated_, hidden behind the interface.
-
-Once you are talking in terms of interfaces, who says that only one
-kind of object may implement this interface? Having different objects
-expose the same interface, and then writing code that works on any
-object with the interface, is called _polymorphism_. It is very
-useful.
-
-When implementing multiple types that only differ in some details, it
-can be helpful to simply make the prototype of your new type derive
-from the prototype of your old type, and have your new constructor
-call the old one. This gives you an object type very similar to the
-old type, but for which you can add and override properties as you see
-fit.
-
-== Exercises ==
-
-[[exercise_vector]]
-=== A vector type ===
-
-(((dimensions)))(((Vector type)))(((coordinates)))(((vector (exercise))))Write a
-((constructor)) `Vector` that represents a vector in two-dimensional
-space. It takes `x` and `y` parameters (numbers), which it should save
-to properties of the same name.
-
-(((addition)))(((subtraction)))Give the `Vector` prototype two
-methods, `plus` and `minus`, which take another vector as a parameter,
-and return a new vector that has the sum or difference of the two
-vectors’ (the one in `this` and the parameter) x and y values.
-
-Add a ((getter)) property `length` to the prototype that computes the
-length of the vector—that is, the distance of the point (x, y) from
-the origin (0, 0).
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-console.log(new Vector(1, 2).plus(new Vector(2, 3)));
-// → Vector{x: 3, y: 5}
-console.log(new Vector(1, 2).minus(new Vector(2, 3)));
-// → Vector{x: -1, y: -1}
-console.log(new Vector(3, 4).length);
-// → 5
-----
-endif::html_target[]
-
-!!solution!!
-
-(((vector (exercise))))Your solution can follow the pattern of the
-`Rabbit` constructor from this chapter quite closely.
-
-(((Pythagoras)))(((defineProperty function)))(((square
-root)))(((Math.sqrt function)))Adding a getter property to the
-constructor can be done with the `Object.defineProperty` function. To
-compute the distance from (0, 0) to (x, y), you can use the
-Pythagorean theorem, which says that the square of the distance we are
-looking for is equal to the square of the x coordinate plus the square
-of the y coordinate. Thus (!html √(x^2^ + y^2^)!)(!tex pass:[$\sqrt{x^2 + y^2}$]!)
-is the number you want, and `Math.sqrt` is the way you compute a square
-root in JavaScript.
-
-!!solution!!
-
-=== Another cell ===
-
-(((StretchCell (exercise))))(((interface)))Implement a cell type named
-`StretchCell(inner, width, height)` that conforms to the
-link:06_object.html#table_interface[table cell interface] described
-earlier in the chapter. It should wrap another cell (like
-`UnderlinedCell` does), and ensure that the resulting cell has at
-least the given `width` and `height`, even if the inner cell would
-naturally be smaller.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-var sc = new StretchCell(new TextCell("abc"), 1, 2);
-console.log(sc.minWidth());
-// → 3
-console.log(sc.minHeight());
-// → 2
-console.log(sc.draw(3, 2));
-// → ["abc", " "]
-----
-
-endif::html_target[]
-
-!!solution!!
-
-(((StretchCell (exercise))))You'll have to store all three constructor
-arguments in the instance object. The `minWidth` and `minHeight`
-methods should call through to the corresponding methods in the
-`inner` cell, but ensure that no number less than the given size is
-returned (possibly using `Math.max`).
-
-Don't forget to add a `draw` method that simply forwards the call to
-the inner cell.
-
-!!solution!!
-
-=== Sequence interface ===
-
-(((sequence (exercise))))Design an _((interface))_ that abstracts
-((iteration)) over a ((collection)) of values. An object that provides
-this interface represents a sequence, and the interface must somehow
-make it possible for code that uses such an object to iterate over the
-sequence, looking at the element values it is made up of, and having
-some way to find out when the end of the sequence is reached.
-
-When you have specified your interface, try to write a function
-`logFive` that takes a sequence object and calls `console.log` on its
-first five elements—or less, if the sequence has less than five
-elements.
-
-Then implement an object type `ArraySeq` that wraps an array and
-allows iteration over the array using the interface you designed.
-Implement another object type `RangeSeq` that iterates over a range of
-integers (taking `from` and `to` arguments to its constructor)
-instead.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-logFive(new ArraySeq([1, 2]));
-// → 1
-// → 2
-logFive(new RangeSeq(100, 1000));
-// → 100
-// → 101
-// → 102
-// → 103
-// → 104
-----
-
-endif::html_target[]
-
-!!solution!!
-
-(((sequence (exercise))))(((collection)))One way to solve this is to
-give the sequence objects _((state))_, meaning their properties are
-changed in the process of using them. You could store a counter that
-indicates how far the sequence object has advanced.
-
-Your ((interface)) will need to expose at least a way to get the next
-element, and to find out whether the iteration has reached the end of
-the sequence yet. It is tempting to roll these into one method,
-`next`, which returns `null` or `undefined` when the sequence is at
-its end. But now you have a problem when a sequence actually contains
-`null`. So a separate method (or getter property) to find out whether
-the end has been reached is probably preferable.
-
-(((mutation)))(((pure function)))(((efficiency)))Another solution is
-to avoid changing state in the object. You can expose a method for
-getting the current element (without advancing any counter), and
-another for getting a new sequence that represents the remaining
-elements after the current one (or a special value if the end of the
-sequence is reached). This is quite elegant—a sequence value will
-“stay itself” even after it is used, and can thus be shared with other
-code without worrying about what might happen to it. It is,
-unfortunately, also somewhat inefficient in a language like
-JavaScript, because it involves creating a lot of objects during
-iteration.
-
-!!solution!!
diff --git a/07_elife.txt b/07_elife.txt
deleted file mode 100644
index dd299d4f2..000000000
--- a/07_elife.txt
+++ /dev/null
@@ -1,1238 +0,0 @@
-:chap_num: 7
-:prev_link: 06_object
-:next_link: 08_error
-:load_files: ["code/chapter/07_elife.js", "js/animateworld.js"]
-
-= Project: Electronic Life =
-
-[chapterquote="true"]
-[quote, Edsger Dijkstra, The Threats to Computing Science]
-____
-[...] the question of whether Machines Can Think [...] is about as
-relevant as the question of whether Submarines Can Swim.
-____
-
-(((artificial intelligence)))(((Dijkstra+++,+++ Edsger)))(((project
-chapter)))(((reading code)))(((writing code)))In “project” chapters,
-I'll stop pummeling you with new theory for a brief moment, and
-instead work through a program with you. Theory is indispensable when
-learning to program, but it should be accompanied by reading and
-understanding non-trivial programs.
-
-(((artificial life)))(((electronic life)))(((ecosystem)))Our
-project in this chapter is to build a virtual ecosystem, a little
-world populated with ((critter))s that move around and struggle for
-survival.
-
-== Definition ==
-
-(((dimensions)))(((electronic life)))In order to make this
-task manageable, we will radically simplify the concept of a
-_((world))_. Namely, a world will be a two-dimensional ((grid)) where
-each entity takes up one full square of the grid. On every _((turn))_,
-the critters all get a chance to take some action.
-
-(((discretization)))(((simulation)))Thus, we chop both time and space
-into units with a fixed size: squares for space and turns for time. Of
-course, this is a somewhat crude and inaccurate ((approximation)). But
-our simulation is intended to be amusing, not accurate, so we can
-freely cut such corners.
-
-[[plan]]
-(((array)))We can define a world with a “plan”, an array of
-strings that lays out the world's grid using one character per square.
-
-// include_code
-
-[source,javascript]
-----
-var plan =
- ["############################",
- "# # # o ##",
- "# #",
- "# ##### #",
- "## # # ## #",
- "### ## # #",
- "# ### # #",
- "# #### #",
- "# ## o #",
- "# o # o ### #",
- "# # #",
- "############################"];
-----
-
-The “#” characters in this plan represent ((wall))s and rocks, and the
-“o” characters represent critters. The spaces, as you might have
-guessed, are empty space.
-
-(((object)))(((toString method)))(((turn)))A plan array can be
-used to create a ((world)) object. Such an object keeps track of the
-size and content of the world. It has a `toString` method, which
-converts the world back to a printable string (similar to the plan it
-was based on), so that we can see what's going on inside. The world
-object also has a `turn` method, which allows all the critters in it
-take one turn, and updates the world to reflect their actions.
-
-[[grid]]
-== Representing space ==
-
-(((array,as grid)))(((Vector type)))(((coordinates)))The ((grid))
-that models the world has a fixed width and height. Squares are
-identified by their x- and y-coordinates. We use a very simple type,
-`Vector` (as seen in the exercises for the
-link:06_object.html#exercise_vector[previous chapter]) to represent
-these coordinate pairs.
-
-// include_code
-
-[source,javascript]
-----
-function Vector(x, y) {
- this.x = x;
- this.y = y;
-}
-Vector.prototype.plus = function(other) {
- return new Vector(this.x + other.x, this.y + other.y);
-};
-----
-
-(((object)))(((encapsulation)))Next, we need an object type that
-models the grid itself. A grid is part of a world, but we are making
-it a separate object (which will be a property of a ((world)) object)
-to keep the world object itself simple. The world should concern
-itself with world-related things, the grid with grid-related things.
-
-(((array)))(((data structure)))To store a grid of values, we have
-several options. We can use an array of row arrays and use two
-property accesses to get to a specific square, like this:
-
-[source,javascript]
-----
-var grid = [["top left", "top middle", "top right"],
- ["bottom left", "bottom middle", "bottom right"]];
-console.log(grid[1][2]);
-// → bottom right
-----
-
-(((array,indexing)))(((coordinates)))(((grid)))Or, we can use a
-single array, with size width×height, and decide that the element at
-(x,y) is found at position x + (y × width) in the array.
-
-[source,javascript]
-----
-var grid = ["top left", "top middle", "top right",
- "bottom left", "bottom middle", "bottom right"];
-console.log(grid[2 + (1 * 3)]);
-// → bottom right
-----
-
-(((encapsulation)))(((abstraction)))(((Array constructor)))(((array,creation)))(((array,length
-of)))Since the actual access to this array will be wrapped in methods
-on the grid object type, it doesn't matter to outside code which
-approach we take. I chose the second representation, because it makes
-it much easier to create the array. When calling the `Array`
-constructor with a single number as argument, it creates a new empty
-array of the given length.
-
-(((Grid type)))This code defines the `Grid` object, with some basic
-methods:
-
-// include_code
-
-[source,javascript]
-----
-function Grid(width, height) {
- this.space = new Array(width * height);
- this.width = width;
- this.height = height;
-}
-Grid.prototype.isInside = function(vector) {
- return vector.x >= 0 && vector.x < this.width &&
- vector.y >= 0 && vector.y < this.height;
-};
-Grid.prototype.get = function(vector) {
- return this.space[vector.x + this.width * vector.y];
-};
-Grid.prototype.set = function(vector, value) {
- this.space[vector.x + this.width * vector.y] = value;
-};
-----
-
-And a trivial test:
-
-[source,javascript]
-----
-var grid = new Grid(5, 5);
-console.log(grid.get(new Vector(1, 1)));
-// → undefined
-grid.set(new Vector(1, 1), "X");
-console.log(grid.get(new Vector(1, 1)));
-// → X
-----
-
-== A critter's programming interface ==
-
-(((record)))(((electronic life)))(((interface)))Before we can
-start on the `World` ((constructor)), we must get more specific about
-the ((critter)) objects that will be living inside it. I mentioned
-that the world will ask the critters what actions they want to take.
-This works as follows: each critter object has an `act` ((method))
-that, when called, returns an _action_. An action is an object with a
-`type` property, which names the type of action the critter wants to
-take, for example `"move"`. The action may also contain extra
-information, such as the direction the critter wants to move in.
-
-[[directions]]
-(((Vector type)))(((View type)))(((directions object)))(((object,as map)))Critters are terribly myopic, and can only see the
-squares directly around them on the grid. But even this limited vision
-can be useful when deciding which action to take. When the `act`
-method is called, it is given a _view_ object that allows the critter
-to inspect its surroundings. We name the eight surrounding squares by
-their ((compass direction))s: `"n"` for north, `"ne"` for northeast,
-and so on. Here's the object we will use to map from direction names
-to coordinate offsets:
-
-// include_code
-
-[source,javascript]
-----
-var directions = {
- "n": new Vector( 0, -1),
- "ne": new Vector( 1, -1),
- "e": new Vector( 1, 0),
- "se": new Vector( 1, 1),
- "s": new Vector( 0, 1),
- "sw": new Vector(-1, 1),
- "w": new Vector(-1, 0),
- "nw": new Vector(-1, -1)
-};
-----
-
-(((View type)))The view object has a method `look`, which takes a
-direction and returns a character, for example `"#"` when there is a
-wall in that direction, or `" "` (space) when there is nothing there.
-The object also provides the convenient methods `find` and `findAll`.
-Both take a map character as argument. The first returns a direction
-in which the character can be found next to the critter, or `null` if
-no such direction exists. The second returns an array containing all
-directions with that character. For example, a creature sitting left
-(west) of a wall will get `["ne", "e", "se"]` when calling `findAll`
-on its view object with the `"#"` character as argument.
-
-(((bouncing)))(((behavior)))(((BouncingCritter type)))Here is a
-simple, stupid critter that just follows its nose until it hits an
-obstacle, then bounces off in a random open direction:
-
-// include_code
-
-[source,javascript]
-----
-function randomElement(array) {
- return array[Math.floor(Math.random() * array.length)];
-}
-
-function BouncingCritter() {
- this.direction = randomElement(Object.keys(directions));
-};
-
-BouncingCritter.prototype.act = function(view) {
- if (view.look(this.direction) != " ")
- this.direction = view.find(" ") || "s";
- return {type: "move", direction: this.direction};
-};
-----
-
-(((random number)))(((Math.random function)))(((randomElement
-function)))(((array,indexing)))The `randomElement` helper
-function simply picks a random element from an array, using
-`Math.random` plus some arithmetic to get a random index. We'll use
-this again later, as randomness can be useful in ((simulation))s.
-
-(((Object.keys function)))The `BouncingCritter` constructor calls
-`Object.keys`. We saw this function in the
-link:06_object.html#keys[previous chapter]: it returns an array
-containing all the property names in the given object. Here, it gets
-all the direction names from the `directions` object we defined
-link:07_elife.html#directions[earlier].
-
-(((|| operator)))(((null)))The “++|| "s"++” in the `act` method is
-there to prevent `this.direction` from getting the value `null` if the
-critter is somehow trapped with no empty space around it (for example
-when crowded into a corner by other critters).
-
-== The world object ==
-
-(((World type)))(((electronic life)))Now we can start on the
-`World` object type. The ((constructor)) takes a plan (the array of
-strings representing the world's grid, described
-link:07_elife.html#grid[earlier]) and a _((legend))_ as arguments. A
-legend is an object that tells us what each character in the map
-means. It contains a constructor for every character— except for the
-space character, which always refers to `null`, the value we'll use to
-represent empty space.
-
-// include_code
-
-[source,javascript]
-----
-function elementFromChar(legend, ch) {
- if (ch == " ")
- return null;
- var element = new legend[ch]();
- element.originChar = ch;
- return element;
-}
-
-function World(map, legend) {
- var grid = new Grid(map[0].length, map.length);
- this.grid = grid;
- this.legend = legend;
-
- map.forEach(function(line, y) {
- for (var x = 0; x < line.length; x++)
- grid.set(new Vector(x, y),
- elementFromChar(legend, line[x]));
- });
-}
-----
-
-(((elementFromChar function)))(((object,as map)))In `elementFromChar`,
-first we create an instance of the right type by looking up the
-character's constructor and applying `new` to it. Then we add an
-`originChar` ((property)) to it to make it easy to find out what
-character the element was originally created from.
-
-(((toString method)))(((nesting,of loops)))(((for
-loop)))(((coordinates)))We need this `originChar` property when
-implementing the world's `toString` method. This method builds up a
-map-like string from the world's current state, by performing a
-two-dimensional loop over the squares on the grid.
-
-// include_code
-
-[source,javascript]
-----
-function charFromElement(element) {
- if (element == null)
- return " ";
- else
- return element.originChar;
-}
-
-World.prototype.toString = function() {
- var output = "";
- for (var y = 0; y < this.grid.height; y++) {
- for (var x = 0; x < this.grid.width; x++) {
- var element = this.grid.get(new Vector(x, y));
- output += charFromElement(element);
- }
- output += "\n";
- }
- return output;
-};
-----
-
-(((electronic life)))(((constructor)))(((Wall type)))A ((wall)) is
-a very simple object—it is only used for taking up space, and has no
-`act` method.
-
-// include_code
-
-[source,javascript]
-----
-function Wall() {}
-----
-
-(((World type)))When we try out the `World` object by creating an
-instance based on the plan from link:07_elife.html#plan[earlier in the
-chapter], and then calling `toString` on it, we get a string very
-similar to the plan we put in.
-
-// include_code strip_log
-// test: trim
-
-[source,javascript]
-----
-var world = new World(plan,
- {"#": Wall,
- "o": BouncingCritter});
-console.log(world.toString());
-// → ############################
-// # # # o ##
-// # #
-// # ##### #
-// ## # # ## #
-// ### ## # #
-// # ### # #
-// # #### #
-// # ## o #
-// # o # o ### #
-// # # #
-// ############################
-----
-
-== this and its scope ==
-
-(((forEach
-method)))(((function,scope)))(((this)))(((scope)))(((self
-variable)))(((global object)))The `World` ((constructor)) contains a
-call to `forEach`. One interesting thing to note is that inside the
-function passed to `forEach`, we are no longer directly in the
-function scope of the constructor. Each function call gets its own
-`this` binding, so the `this` inside of the inner function does _not_
-refer to the newly constructed object that the outer `this` refers to.
-In fact, when a function isn't called as a method, `this` will refer
-to the global object.
-
-This means that we can't write `this.grid` to access the grid from
-inside the ((loop)). Instead, the outer function creates a normal
-local variable, `grid`, through which the inner function gets access
-to the grid.
-
-(((future)))(((ECMAScript 6)))(((arrow function)))(((self
-variable)))This is a bit of a design blunder in JavaScript.
-Fortunately, the next version of the language provides a solution for
-this problem. Meanwhile, there are workarounds. A common pattern is to
-say `var self = this` and from then on refer to `self`, which is a
-normal variable and thus visible to inner functions.
-
-(((bind method)))(((this)))Another solution is to use the `bind`
-method, which allows us to provide an explicit `this` object to bind
-to.
-
-[source,javascript]
-----
-var test = {
- prop: 10,
- addPropTo: function(array) {
- return array.map(function(elt) {
- return this.prop + elt;
- }.bind(this));
- }
-};
-console.log(test.addPropTo([5]));
-// → [15]
-----
-
-(((map method)))The function passed to `map` is the result of the
-`bind` call, and thus has its `this` bound to the first argument given
-to ++bind++—the outer function's `this` value (which holds the `test`
-object).
-
-(((context parameter)))(((function,higher-order)))Most ((standard))
-higher-order methods on arrays, such as `forEach` and `map`, take an
-optional second argument that can also be used to provide a `this` for
-the calls to the iteration function. So you could express the example
-above in a slightly simpler way:
-
-[source,javascript]
-----
-var test = {
- prop: 10,
- addPropTo: function(array) {
- return array.map(function(elt) {
- return this.prop + elt;
- }, this); // ← no bind
- }
-};
-console.log(test.addPropTo([5]));
-// → [15]
-----
-
-This only works for higher-order functions that
-support such a _context_ parameter. When they don't, you'll need to
-use one of the other approaches.
-
-(((context parameter)))(((function,higher-order)))(((call method)))In
-our own higher-order functions, we can support such a context
-parameter by using the `call` method to call the function given as
-argument. For example, here is a `forEach` method for our `Grid` type,
-which calls a given function for each element in the grid that isn't
-null or undefined:
-
-// include_code
-
-[source,javascript]
-----
-Grid.prototype.forEach = function(f, context) {
- for (var y = 0; y < this.height; y++) {
- for (var x = 0; x < this.width; x++) {
- var value = this.space[x + y * this.width];
- if (value != null)
- f.call(context, value, new Vector(x, y));
- }
- }
-};
-----
-
-== Animating life ==
-
-(((simulation)))(((electronic life)))(((World type)))The next
-step is to write a `turn` method for the world object that gives the
-((critter))s a chance to act. It will go over the grid using the
-`forEach` method we just defined, looking for objects with an `act`
-method. When it finds one, `turn` calls that method to get an action
-object, and carries out the action when it is valid. For now, only
-`"move"` actions are understood.
-
-(((grid)))There is one potential problem with this approach. Can you
-spot it? If we let critters move as we come across them, they may move
-to a square that we haven't looked at yet, and we'll allow them to
-move _again_ when we reach that square. Thus, we have to keep an array
-of critters that have already had their turn, and ignore those when we
-see them again.
-
-// include_code
-
-[source,javascript]
-----
-World.prototype.turn = function() {
- var acted = [];
- this.grid.forEach(function(critter, vector) {
- if (critter.act && acted.indexOf(critter) == -1) {
- acted.push(critter);
- this.letAct(critter, vector);
- }
- }, this);
-};
-----
-
-(((this)))We use the second parameter to the grid's `forEach` method
-to be able to access the correct `this` inside of the inner function.
-The `letAct` method contains the actual logic that allows the critters
-to move.
-
-// include_code
-
-[[checkDestination]]
-[source,javascript]
-----
-World.prototype.letAct = function(critter, vector) {
- var action = critter.act(new View(this, vector));
- if (action && action.type == "move") {
- var dest = this.checkDestination(action, vector);
- if (dest && this.grid.get(dest) == null) {
- this.grid.set(vector, null);
- this.grid.set(dest, critter);
- }
- }
-};
-
-World.prototype.checkDestination = function(action, vector) {
- if (directions.hasOwnProperty(action.direction)) {
- var dest = vector.plus(directions[action.direction]);
- if (this.grid.isInside(dest))
- return dest;
- }
-};
-----
-
-(((View type)))(((electronic life)))First, we simply ask the
-critter to act, passing it a view object that knows about the world
-and the critter's current position in that world (we'll define `View`
-in a link:07_elife.html#view[moment]). The `act` method returns an
-action of some kind.
-
-If the action's `type` is not `"move"`, it is ignored. If it _is_
-`"move"`, and it has a `direction` property that refers to a valid
-direction, _and_ the square in that direction is empty (null), we set
-the square where the critter used to be to hold null, and store the
-critter in the destination square.
-
-(((error tolerance)))(((defensive programming)))(((sloppy
-programming)))(((validation)))Note that `letAct` takes care to ignore
-nonsense ((input))—it doesn't assume that the action's `direction`
-property is valid, or that the `type` property makes sense. This kind
-of _defensive_ programming makes sense in some situations. The main
-reason for doing it is to validate inputs coming from sources you
-don't control (such as user or file input), but it can also be useful
-to isolate subsystems from each other. In this case, the intention is
-that the critters themselves can be programmed sloppily—they don't
-have to verify if their intended actions make sense. They can just
-request an action, and the world will figure out whether to allow it.
-
-(((interface)))(((private property)))(((access
-control)))(((property,naming)))(((underscore character)))(((World
-type)))These two methods are not part of the external interface of a
-`World` object. They are an internal detail. Some languages provide
-ways to explicitly declare certain methods and properties _private_
-and signal an error when you try to use them from outside the object.
-JavaScript does not, so you will have to rely on some other form of
-communication to describe what is part of an object's interface.
-Sometimes it can help to use a naming scheme to distinguish between
-external and internal properties, for example by prefixing all
-internal ones with an underscore character (“_”). This will make
-accidental uses of properties that are not part of an object's
-interface easier to spot.
-
-[[view]]
-(((View type)))The one missing part, the `View` type, looks like this:
-
-// include_code
-
-[source,javascript]
-----
-function View(world, vector) {
- this.world = world;
- this.vector = vector;
-}
-View.prototype.look = function(dir) {
- var target = this.vector.plus(directions[dir]);
- if (this.world.grid.isInside(target))
- return charFromElement(this.world.grid.get(target));
- else
- return "#";
-};
-View.prototype.findAll = function(ch) {
- var found = [];
- for (var dir in directions)
- if (this.look(dir) == ch)
- found.push(dir);
- return found;
-};
-View.prototype.find = function(ch) {
- var found = this.findAll(ch);
- if (found.length == 0) return null;
- return randomElement(found);
-};
-----
-
-(((defensive programming)))The `look` method figures out the
-coordinates that we are trying to look at, and, if they are inside the
-((grid)), finds the character corresponding to the element that sits
-there. For coordinates outside the grid, `look` simply pretends that
-there is a wall there, so that if you define a world that isn't walled
-in, the critters still won't be tempted to try and walk off the edges.
-
-== It moves ==
-
-(((electronic life)))(((simulation)))We instantiated a world
-object before. Now that we've added all the necessary methods, it
-should be possible to actually make it move.
-
-[source,javascript]
-----
-for (var i = 0; i < 5; i++) {
- world.turn();
- console.log(world.toString());
-}
-// → … five turns of moving critters
-----
-
-ifdef::tex_target[]
-
-The first two maps that are displayed will look something like this
-(depending on the random direction the critters picked):
-
-----
-############################ ############################
-# # # ## # # # ##
-# o # # #
-# ##### # # ##### o #
-## # # ## # ## # # ## #
-### ## # # ### ## # #
-# ### # # # ### # #
-# #### # # #### #
-# ## # # ## #
-# # o ### # #o # ### #
-#o # o # # # o o #
-############################ ############################
-----
-
-(((animation)))They move! To get a more interactive view of these
-critters crawling around and bouncing off the walls, open this chapter
-in the online version of the book at
-http://eloquentjavascript.net[_eloquentjavascript.net_].
-
-endif::tex_target[]
-
-ifdef::html_target[]
-
-Simply printing out many copies of the map is a rather unpleasant
-way to observe a world, though. That's why the sandbox provides an
-`animateWorld` function that will run a world as an on-screen
-animation, moving three turns per second, until you hit the stop
-button.
-
-// test: no
-
-[source,javascript]
-----
-animateWorld(world);
-// → … life!
-----
-
-The implementation of `animateWorld` will remain a mystery for now,
-but after you've read the link:13_dom.html#dom[later chapters] of this
-book, which discuss JavaScript integration in web browsers, it won't
-look so magical anymore.
-
-endif::html_target[]
-
-== More lifeforms ==
-
-The dramatic highlight of our world, if you watch for a bit, is when
-two critters bounce off each other. Can you think of another
-interesting form of ((behavior))?
-
-(((wall following)))The one I came up with is a ((critter)) that moves
-along walls. Conceptually, the critter keeps its left hand (paw,
-tentacle, whatever) to the wall and follows along. This turns out to
-be not entirely trivial to implement.
-
-(((WallFollower type)))(((directions object)))First, we need to be
-able to “compute” with ((compass direction))s. Since directions are
-modeled by a set of strings, we need to define our own operation
-(`dirPlus`) to calculate relative directions. So `dirPlus("n", 1)`
-means one 45-degree turn clockwise from North, giving `"ne"`.
-Similarly, `dirPlus("s", -2)` means 90 degrees counterclockwise from
-South, which is East.
-
-// include_code
-
-[source,javascript]
-----
-var directionNames = Object.keys(directions);
-function dirPlus(dir, n) {
- var index = directionNames.indexOf(dir);
- return directionNames[(index + n + 8) % 8];
-}
-
-function WallFollower() {
- this.dir = "s";
-}
-
-WallFollower.prototype.act = function(view) {
- var start = this.dir;
- if (view.look(dirPlus(this.dir, -3)) != " ")
- start = this.dir = dirPlus(this.dir, -2);
- while (view.look(this.dir) != " ") {
- this.dir = dirPlus(this.dir, 1);
- if (this.dir == start) break;
- }
- return {type: "move", direction: this.dir};
-};
-----
-
-(((artificial intelligence)))(((pathfinding)))(((View type)))The `act`
-method only has to “scan” the critter's surroundings, starting from
-its left-hand side and going clockwise until it finds an empty square.
-It then moves in the direction of that empty square.
-
-What complicates things is that a critter may end up in the middle of
-empty space, either as its start position or as a result of walking
-around another critter. If we apply the approach I just described in
-empty space, the poor critter will just keep on turning left at every
-step, running in circles.
-
-So there is an extra check (the `if` statement) to start scanning to
-the left only if it looks like the critter has just passed some kind
-of ((obstacle))—that is, if the space behind-and-to-the-left of the
-critter is not empty. Otherwise, the critter starts scanning directly
-ahead, so that it'll walk straight when in empty space.
-
-(((infinite loop)))And finally, there's a test comparing `this.dir` to
-`start` after every pass through the loop, to make sure that the loop
-won't run forever when the critter is walled in or crowded in by other
-critters and can't find empty space.
-
-ifdef::html_target[]
-
-This small world demonstrates the wall-following creatures.
-
-// test: no
-
-[source,javascript]
-----
-animateWorld(new World(
- ["############",
- "# # #",
- "# ~ ~ #",
- "# ## #",
- "# ## o####",
- "# #",
- "############"],
- {"#": Wall,
- "~": WallFollower,
- "o": BouncingCritter}
-));
-----
-
-endif::html_target[]
-
-== A more lifelike simulation ==
-
-(((simulation)))(((electronic life)))To make life in our world
-more interesting, we will add the concepts of ((food)) and
-((reproduction)). Each living thing in the world gets a new property,
-`energy`, which is reduced by performing actions and increased by
-eating things. When the critter has enough ((energy)), it can
-reproduce, generating a new critter of the same kind. To keep things
-simple, the critters in our world reproduce asexually, all by
-themselves.
-
-(((energy)))(((entropy)))If critters only move around and eat one
-another, the world will soon succumb to the law of increasing entropy,
-run out of energy, and become a lifeless wasteland. To prevent this
-from happening (too quickly, at least), we add ((plant))s to the
-world. Plants do not move. They just use ((photosynthesis)) to grow
-(that is, increase their energy) and reproduce.
-
-(((World type)))To make this work, we'll need a world with a different
-`letAct` method. We could just replace the method of the `World`
-prototype, but I've become very attached to our simulation with the
-wall-following critters, and would hate to break that old world.
-
-(((actionTypes object)))(((LifeLikeWorld type)))One solution is to use
-((inheritance)). We create a new ((constructor)), `LifelikeWorld`,
-whose prototype is based on the `World` prototype, but which overrides
-the `letAct` method. The new `letAct` method delegates the work of
-actually performing an action to various functions stored in the
-`actionTypes` object.
-
-// include_code
-
-[source,javascript]
-----
-function LifelikeWorld(map, legend) {
- World.call(this, map, legend);
-}
-LifelikeWorld.prototype = Object.create(World.prototype);
-
-var actionTypes = Object.create(null);
-
-LifelikeWorld.prototype.letAct = function(critter, vector) {
- var action = critter.act(new View(this, vector));
- var handled = action &&
- action.type in actionTypes &&
- actionTypes[action.type].call(this, critter,
- vector, action);
- if (!handled) {
- critter.energy -= 0.2;
- if (critter.energy <= 0)
- this.grid.set(vector, null);
- }
-};
-----
-
-(((electronic life)))(((function,as value)))(((call
-method)))(((this)))The new `letAct` method first checks whether an
-action was returned at all, then whether a handler function for this
-type of action exists, and finally, whether that handler returned
-true, indicating that it successfully handled the action. Note the use
-of `call` to give the handler access to the world, though its `this`
-binding.
-
-If the action didn't work for whatever reason, the default action is
-for the creature to simply wait. It loses one fifth point of ((energy)),
-and if its energy level drops to zero or below, the creature dies, and
-is removed from the grid.
-
-== Action handlers ==
-
-(((photosynthesis)))The simplest action a creature can perform is
-`"grow"`, used by ((plant))s. When an action object like `{type:
-"grow"}` is returned, the following handler method will be called:
-
-// include_code
-
-[source,javascript]
-----
-actionTypes.grow = function(critter) {
- critter.energy += 0.5;
- return true;
-};
-----
-
-Growing always succeeds, and adds half a point to the plant's
-((energy)) level.
-
-Moving is more involved:
-
-// include_code
-
-[source,javascript]
-----
-actionTypes.move = function(critter, vector, action) {
- var dest = this.checkDestination(action, vector);
- if (dest == null ||
- critter.energy <= 1 ||
- this.grid.get(dest) != null)
- return false;
- critter.energy -= 1;
- this.grid.set(vector, null);
- this.grid.set(dest, critter);
- return true;
-};
-----
-
-(((validation)))This action first checks, using the `checkDestination`
-method defined link:07_elife.html#checkDestination[earlier], whether
-the action provides a valid destination. If not, or when the
-destination isn't empty, or if the critter lacks the required
-((energy)), `move` returns false to indicate no action was taken.
-Otherwise, it moves the critter, and subtracts the energy cost.
-
-(((food)))In addition to moving, critters can also eat:
-
-// include_code
-
-[source,javascript]
-----
-actionTypes.eat = function(critter, vector, action) {
- var dest = this.checkDestination(action, vector);
- var atDest = dest != null && this.grid.get(dest);
- if (atDest == null || atDest.energy == null)
- return false;
- critter.energy += atDest.energy;
- this.grid.set(dest, null);
- return true;
-};
-----
-
-(((validation)))Eating another ((critter)) also involves providing a
-valid destination square. This time, the destination must not be
-empty, and must contain something with ((energy)), like a critter (but
-not a wall—walls are not edible). If so, the energy from the eaten is
-transferred to the eater, and the victim is removed from the grid.
-
-(((reproduction)))And finally, we allow our critters to reproduce:
-
-// include_code
-
-[source,javascript]
-----
-actionTypes.reproduce = function(critter, vector, action) {
- var baby = elementFromChar(this.legend,
- critter.originChar);
- var dest = this.checkDestination(action, vector);
- if (dest == null ||
- critter.energy <= 2 * baby.energy ||
- this.grid.get(dest) != null)
- return false;
- critter.energy -= 2 * baby.energy;
- this.grid.set(dest, baby);
- return true;
-};
-----
-
-(((electronic life)))Reproducing costs twice the ((energy))
-level of the newborn critter. So we first create a (hypothetical) baby
-using `elementFromChar` on the critter's own origin character. Once we
-have a baby, we can find its energy level and test whether the parent
-has enough energy to successfully bring it into the world. We also
-require a valid (and empty) destination.
-
-(((reproduction)))If everything is okay, the baby is put onto the grid
-(it is now no longer hypothetical) and the energy is spent.
-
-== Populating the new world ==
-
-(((Plant type)))(((electronic life)))We now have a
-((framework)) to simulate these more lifelike creatures. We could put
-the critters from the old world into it, but they would just die,
-since they don't have an ((energy)) property. So let us make new ones.
-First we'll write a ((plant)), which is a rather simple lifeform.
-
-// include_code
-
-[source,javascript]
-----
-function Plant() {
- this.energy = 3 + Math.random() * 4;
-}
-Plant.prototype.act = function(context) {
- if (this.energy > 15) {
- var space = context.find(" ");
- if (space)
- return {type: "reproduce", direction: space};
- }
- if (this.energy < 20)
- return {type: "grow"};
-};
-----
-
-(((reproduction)))(((photosynthesis)))(((random
-number)))(((Math.random function)))Plants start with an energy level
-between 3 and 7, randomized so that they don't all reproduce in the
-same turn. When a plant reaches 15 energy points and there is empty
-space nearby, it reproduces into that empty space. If a plant can't
-reproduce, it simply grows until it reaches energy level 20.
-
-(((critter)))(((PlantEater type)))(((herbivore)))(((food chain)))Next,
-a plant eater.
-
-// include_code
-
-[source,javascript]
-----
-function PlantEater() {
- this.energy = 20;
-}
-PlantEater.prototype.act = function(context) {
- var space = context.find(" ");
- if (this.energy > 60 && space)
- return {type: "reproduce", direction: space};
- var plant = context.find("*");
- if (plant)
- return {type: "eat", direction: plant};
- if (space)
- return {type: "move", direction: space};
-};
-----
-
-We'll use the `*` character for ((plant))s, so that's what this
-creature will look for when it searches for ((food)).
-
-== Bringing it to life ==
-
-(((electronic life)))And that gives us enough elements to try
-our new world. Imagine the map below as a grassy valley with a herd of
-((herbivore))s in it, some boulders, and lush ((plant)) life
-everywhere.
-
-// include_code
-
-[source,javascript]
-----
-var valley = new LifelikeWorld(
- ["############################",
- "##### ######",
- "## *** **##",
- "# *##** ** O *##",
- "# *** O ##** *#",
- "# O ##*** #",
- "# ##** #",
- "# O #* #",
- "#* #** O #",
- "#*** ##** O **#",
- "##**** ###*** *###",
- "############################"],
- {"#": Wall,
- "O": PlantEater,
- "*": Plant}
-);
-----
-
-(((animation)))(((simulation)))Let's see what happens if we run this.
-(!tex These snapshots illustrate a typical run of this world.!)
-
-ifdef::html_target[]
-
-// start_code
-// test: no
-
-[source,javascript]
-----
-animateWorld(valley);
-----
-
-endif::html_target[]
-
-ifdef::tex_target[]
-
-----
-############################ ############################
-##### ###### ##### ** ######
-## *** O *## ## ** * O ##
-# *##* ** *## # **## ##
-# ** ##* *# # ** O ##O #
-# ##* # # *O * * ## #
-# ## O # # *** ## O #
-# #* O # #** #*** #
-#* #** O # #** O #**** #
-#* O O ##* **# #*** ##*** O #
-##* ###* ### ##** ###** O ###
-############################ ############################
-
-############################ ############################
-#####O O ###### ##### O ######
-## ## ## ##
-# ##O ## # ## O ##
-# O O *## # # ## #
-# O O O **## O # # ## #
-# **## O # # O ## * #
-# # *** * # # # O #
-# # O***** O # # O # O #
-# ##****** # # ## O O #
-## ###****** ### ## ### O ###
-############################ ############################
-
-############################ ############################
-##### ###### ##### ######
-## ## ## ** * ##
-# ## ## # ## ***** ##
-# ## # # ##**** #
-# ##* * # # ##***** #
-# O ## * # # ##****** #
-# # # # # ** ** #
-# # # # # #
-# ## # # ## #
-## ### ### ## ### ###
-############################ ############################
-----
-
-endif::tex_target[]
-
-(((stability)))(((reproduction)))(((extinction)))(((starvation)))Most
-of the time, the plants multiply and expand quite quickly, but then
-the abundance of ((food)) causes a population explosion of the
-((herbivore))s, who proceed to wipe out all or nearly all of the
-((plant))s, resulting in a mass starvation of the critters. Sometimes,
-the ((ecosystem)) recovers and another cycle starts. At other times,
-one of the species dies out completely. If it's the herbivores, the
-whole space will fill with plants. If it's the plants, the remaining
-critters starve, and the valley becomes a desolate wasteland. Ah, the
-cruelty of nature.
-
-== Exercises ==
-
-=== Artificial stupidity ===
-
-(((artificial stupidity (exercise))))(((artificial
-intelligence)))(((extinction)))Having the inhabitants of our world go
-extinct after a few minutes is kind of depressing. To deal with this,
-we could try to create a smarter plant eater.
-
-(((pathfinding)))(((reproduction)))(((food)))There are several obvious
-problems with our herbivores. First, they are terribly greedy,
-stuffing themselves with every plant they see until they have wiped
-out the local plant life. Second, their randomized movement (recall
-that the `view.find` method returns a random direction when multiple
-directions match) causes them to stumble around ineffectively, and
-starve if there don't happen to be any plants nearby. And finally,
-they breed very fast, which makes the cycles between abundance and
-famine quite intense.
-
-Write a new critter type that tries to address one or more of these
-points, and substitute it for the old `PlantEater` type in the valley
-world. See how it fares. Tweak it some more if necessary.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here
-function SmartPlantEater() {}
-
-animateWorld(new LifelikeWorld(
- ["############################",
- "##### ######",
- "## *** **##",
- "# *##** ** O *##",
- "# *** O ##** *#",
- "# O ##*** #",
- "# ##** #",
- "# O #* #",
- "#* #** O #",
- "#*** ##** O **#",
- "##**** ###*** *###",
- "############################"],
- {"#": Wall,
- "O": SmartPlantEater,
- "*": Plant}
-));
-----
-
-endif::html_target[]
-
-!!solution!!
-
-(((artificial stupidity (exercise))))(((artificial
-intelligence)))(((behavior)))(((state)))The greediness problem can be
-attacked in several ways. The critters could stop eating when they
-reach a certain ((energy)) level. Or only eat every N turns (by
-keeping a counter of the turns since their last meal in a property on
-the creature object). Or, to make sure plants never go entirely
-extinct, the animals could refuse to eat a ((plant)) unless they see
-at least one other plant nearby (using the `findAll` method on the
-view). A combination of these, or some entirely different strategy,
-might also work.
-
-(((pathfinding)))(((wall following)))Making the critters move more
-effectively could be done by stealing one of the movement strategies
-from the critters in our old, energyless world. Both the bouncing
-behavior and the wall-following behavior showed a much wider range of
-movement than completely random staggering.
-
-(((reproduction)))(((stability)))Making creatures breed more slowly is
-trivial. Just increase the minimum energy level at which they
-reproduce. Of course, making the ecosystem more stable also makes it
-more boring. If you have a handful of fat, immobile critters forever
-munching on a sea of plants, and never reproducing, that makes for a
-very stable ecosystem. But no one wants to watch that.
-
-!!solution!!
-
-=== Predators ===
-
-(((predators (exercise))))(((carnivore)))(((food chain)))Any serious
-((ecosystem)) has a food chain longer than a single link. Write
-another ((critter)) that survives by eating the ((herbivore)) critter.
-You'll notice that ((stability)) is even harder to achieve now that there
-are cycles at multiple levels. Try to find a strategy to make the
-ecosystem run smoothly for at least a little while.
-
-(((Tiger type)))One thing that will help is to make the world bigger.
-This way, local population booms or busts are less likely to wipe out
-a species entirely, and there is space for the relatively large prey
-population needed to sustain a small predator population.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here
-function Tiger() {}
-
-animateWorld(new LifelikeWorld(
- ["####################################################",
- "# #### **** ###",
- "# * @ ## ######## OO ##",
- "# * ## O O **** *#",
- "# ##* ########## *#",
- "# ##*** * **** **#",
- "#* ** # * *** ######### **#",
- "#* ** # * # * **#",
- "# ## # O # *** ######",
- "#* @ # # * O # #",
- "#* # ###### ** #",
- "### **** *** ** #",
- "# O @ O #",
- "# * ## ## ## ## ### * #",
- "# ** # * ##### O #",
- "## ** O O # # *** *** ### ** #",
- "### # ***** ****#",
- "####################################################"],
- {"#": Wall,
- "@": Tiger,
- "O": SmartPlantEater, // from previous exercise
- "*": Plant}
-));
-----
-
-endif::html_target[]
-
-!!solution!!
-
-(((predators (exercise))))(((reproduction)))(((starvation)))Many of
-the same tricks that worked for the previous exercise also apply here.
-Making the predators big (lots of energy), and having them reproduce
-slowly is recommended. That'll make them less vulnerable to periods of
-starvation when the herbivores are scarce.
-
-Beyond staying alive, keeping its ((food)) stock alive is a
-predator's main objective. Find some way to make predators hunt
-more aggressively when there are a lot of ((herbivore))s, and more
-slowly (or not at all) when prey is rare. Since plant eaters move
-around, the simple trick of only eating one when others are nearby is
-unlikely to work—that'll happen so rarely that your predator will
-starve. But you could keep track of observations in previous turns, in
-some ((data structure)) kept on the predator objects, and have it base
-its ((behavior)) on what it has seen recently.
-
-!!solution!!
diff --git a/07_robot.md b/07_robot.md
new file mode 100644
index 000000000..3c942cfb3
--- /dev/null
+++ b/07_robot.md
@@ -0,0 +1,491 @@
+{{meta {load_files: ["code/chapter/07_robot.js", "code/animatevillage.js"], zip: html}}}
+
+# Project: A Robot
+
+{{quote {author: "Edsger Dijkstra", title: "The Threats to Computing Science", chapter: true}
+
+The question of whether Machines Can Think [...] is about as relevant as the question of whether Submarines Can Swim.
+
+quote}}
+
+{{index "artificial intelligence", "Dijkstra, Edsger"}}
+
+{{figure {url: "img/chapter_picture_7.jpg", alt: "Illustration of a robot holding a stack of packages", chapter: framed}}}
+
+{{index "project chapter", "reading code", "writing code"}}
+
+In "project" chapters, I'll stop pummeling you with new theory for a brief moment, and instead we'll work through a program together. Theory is necessary to learn to program, but reading and understanding actual programs is just as important.
+
+Our project in this chapter is to build an ((automaton)), a little program that performs a task in a ((virtual world)). Our automaton will be a mail-delivery ((robot)) picking up and dropping off parcels.
+
+## Meadowfield
+
+{{index "roads array"}}
+
+The village of ((Meadowfield)) isn't very big. It consists of 11 places with 14 roads between them. It can be described with this array of roads:
+
+```{includeCode: true}
+const roads = [
+ "Alice's House-Bob's House", "Alice's House-Cabin",
+ "Alice's House-Post Office", "Bob's House-Town Hall",
+ "Daria's House-Ernie's House", "Daria's House-Town Hall",
+ "Ernie's House-Grete's House", "Grete's House-Farm",
+ "Grete's House-Shop", "Marketplace-Farm",
+ "Marketplace-Post Office", "Marketplace-Shop",
+ "Marketplace-Town Hall", "Shop-Town Hall"
+];
+```
+
+{{figure {url: "img/village2x.png", alt: "Pixel art illustration of a small village with 11 locations, labeled with letters, and roads going being them"}}}
+
+The network of roads in the village forms a _((graph))_. A graph is a collection of points (places in the village) with lines between them (roads). This graph will be the world that our robot moves through.
+
+{{index "roadGraph object"}}
+
+The array of strings isn't very easy to work with. What we're interested in is the destinations that we can reach from a given place. Let's convert the list of roads to a data structure that, for each place, tells us what can be reached from there.
+
+```{includeCode: true}
+function buildGraph(edges) {
+ let graph = Object.create(null);
+ function addEdge(from, to) {
+ if (from in graph) {
+ graph[from].push(to);
+ } else {
+ graph[from] = [to];
+ }
+ }
+ for (let [from, to] of edges.map(r => r.split("-"))) {
+ addEdge(from, to);
+ addEdge(to, from);
+ }
+ return graph;
+}
+
+const roadGraph = buildGraph(roads);
+```
+
+{{index "split method"}}
+
+Given an array of edges, `buildGraph` creates a map object that, for each node, stores an array of connected nodes. It uses the `split` method to go from the road strings—which have the form `"Start-End"`)—to two-element arrays containing the start and end as separate strings.
+
+## The task
+
+Our ((robot)) will be moving around the village. There are parcels in various places, each addressed to some other place. The robot picks up parcels when it comes across them and delivers them when it arrives at their destinations.
+
+The automaton must decide, at each point, where to go next. It has finished its task when all parcels have been delivered.
+
+{{index simulation, "virtual world"}}
+
+To be able to simulate this process, we must define a virtual world that can describe it. This model tells us where the robot is and where the parcels are. When the robot has decided to move somewhere, we need to update the model to reflect the new situation.
+
+{{index [state, in objects]}}
+
+If you're thinking in terms of ((object-oriented programming)), your first impulse might be to start defining objects for the various elements in the world: a ((class)) for the robot, one for a parcel, maybe one for places. These could then hold properties that describe their current ((state)), such as the pile of parcels at a location, which we could change when updating the world.
+
+This is wrong. At least, it usually is. The fact that something sounds like an object does not automatically mean that it should be an object in your program. Reflexively writing classes for every concept in your application tends to leave you with a collection of interconnected objects that each have their own internal, changing state. Such programs are often hard to understand and thus easy to break.
+
+{{index [state, in objects]}}
+
+Instead, let's condense the village's state down to the minimal set of values that define it. There's the robot's current location and the collection of undelivered parcels, each of which has a current location and a destination address. That's it.
+
+{{index "VillageState class", "persistent data structure"}}
+
+While we're at it, let's make it so that we don't _change_ this state when the robot moves but rather compute a _new_ state for the situation after the move.
+
+```{includeCode: true}
+class VillageState {
+ constructor(place, parcels) {
+ this.place = place;
+ this.parcels = parcels;
+ }
+
+ move(destination) {
+ if (!roadGraph[this.place].includes(destination)) {
+ return this;
+ } else {
+ let parcels = this.parcels.map(p => {
+ if (p.place != this.place) return p;
+ return {place: destination, address: p.address};
+ }).filter(p => p.place != p.address);
+ return new VillageState(destination, parcels);
+ }
+ }
+}
+```
+
+The `move` method is where the action happens. It first checks whether there is a road going from the current place to the destination, and if not, it returns the old state, since this is not a valid move.
+
+{{index "map method", "filter method"}}
+
+Next, the method creates a new state with the destination as the robot's new place. It also needs to create a new set of parcels—parcels that the robot is carrying (that are at the robot's current place) need to be moved along to the new place. And parcels that are addressed to the new place need to be delivered—that is, they need to be removed from the set of undelivered parcels. The call to `map` takes care of the moving, and the call to `filter` does the delivering.
+
+Parcel objects aren't changed when they are moved but re-created. The `move` method gives us a new village state but leaves the old one entirely intact.
+
+```
+let first = new VillageState(
+ "Post Office",
+ [{place: "Post Office", address: "Alice's House"}]
+);
+let next = first.move("Alice's House");
+
+console.log(next.place);
+// → Alice's House
+console.log(next.parcels);
+// → []
+console.log(first.place);
+// → Post Office
+```
+
+The move causes the parcel to be delivered, which is reflected in the next state. But the initial state still describes the situation where the robot is at the post office and the parcel is undelivered.
+
+## Persistent data
+
+{{index "persistent data structure", mutability, ["data structure", immutable]}}
+
+Data structures that don't change are called _((immutable))_ or _persistent_. They behave a lot like strings and numbers in that they are who they are and stay that way, rather than containing different things at different times.
+
+In JavaScript, just about everything _can_ be changed, so working with values that are supposed to be persistent requires some restraint. There is a function called `Object.freeze` that changes an object so that writing to its properties is ignored. You could use that to make sure your objects aren't changed, if you want to be careful. Freezing does require the computer to do some extra work, and having updates ignored is just about as likely to confuse someone as having them do the wrong thing. I usually prefer to just tell people that a given object shouldn't be messed with and hope they remember it.
+
+```
+let object = Object.freeze({value: 5});
+object.value = 10;
+console.log(object.value);
+// → 5
+```
+
+Why am I going out of my way to not change objects when the language is obviously expecting me to? Because it helps me understand my programs. This is about complexity management again. When the objects in my system are fixed, stable things, I can consider operations on them in isolation—moving to Alice's house from a given start state always produces the same new state. When objects change over time, that adds a whole new dimension of complexity to this kind of reasoning.
+
+For a small system like the one we are building in this chapter, we could handle that bit of extra complexity. But the most important limit on what kind of systems we can build is how much we can understand. Anything that makes your code easier to understand makes it possible to build a more ambitious system.
+
+Unfortunately, although understanding a system built on persistent data structures is easier, _designing_ one, especially when your programming language isn't helping, can be a little harder. We'll look for opportunities to use persistent data structures in this book, but we'll also be using changeable ones.
+
+## Simulation
+
+{{index simulation, "virtual world"}}
+
+A delivery ((robot)) looks at the world and decides in which direction it wants to move. So we could say that a robot is a function that takes a `VillageState` object and returns the name of a nearby place.
+
+{{index "runRobot function"}}
+
+Because we want robots to be able to remember things so they can make and execute plans, we also pass them their memory and allow them to return a new memory. Thus, the thing a robot returns is an object containing both the direction it wants to move in and a memory value that will be given back to it the next time it is called.
+
+```{includeCode: true}
+function runRobot(state, robot, memory) {
+ for (let turn = 0;; turn++) {
+ if (state.parcels.length == 0) {
+ console.log(`Done in ${turn} turns`);
+ break;
+ }
+ let action = robot(state, memory);
+ state = state.move(action.direction);
+ memory = action.memory;
+ console.log(`Moved to ${action.direction}`);
+ }
+}
+```
+
+Consider what a robot has to do to "solve" a given state. It must pick up all parcels by visiting every location that has a parcel and deliver them by visiting every location to which a parcel is addressed, but only after picking up the parcel.
+
+What is the dumbest strategy that could possibly work? The robot could just walk in a random direction every turn. That means, with great likelihood, it will eventually run into all parcels and then also at some point reach the place where they should be delivered.
+
+{{index "randomPick function", "randomRobot function"}}
+
+Here's what that could look like:
+
+```{includeCode: true}
+function randomPick(array) {
+ let choice = Math.floor(Math.random() * array.length);
+ return array[choice];
+}
+
+function randomRobot(state) {
+ return {direction: randomPick(roadGraph[state.place])};
+}
+```
+
+{{index "Math.random function", "Math.floor function", [array, "random element"]}}
+
+Remember that `Math.random()` returns a number between 0 and 1—but always below 1. Multiplying such a number by the length of an array and then applying `Math.floor` to it gives us a random index for the array.
+
+Since this robot does not need to remember anything, it ignores its second argument (remember that JavaScript functions can be called with extra arguments without ill effects) and omits the `memory` property in its returned object.
+
+To put this sophisticated robot to work, we'll first need a way to create a new state with some parcels. A static method (written here by directly adding a property to the constructor) is a good place to put that functionality.
+
+```{includeCode: true}
+VillageState.random = function(parcelCount = 5) {
+ let parcels = [];
+ for (let i = 0; i < parcelCount; i++) {
+ let address = randomPick(Object.keys(roadGraph));
+ let place;
+ do {
+ place = randomPick(Object.keys(roadGraph));
+ } while (place == address);
+ parcels.push({place, address});
+ }
+ return new VillageState("Post Office", parcels);
+};
+```
+
+{{index "do loop"}}
+
+We don't want any parcels to be sent from the same place that they are addressed to. For this reason, the `do` loop keeps picking new places when it gets one that's equal to the address.
+
+Let's start up a virtual world.
+
+```{test: no}
+runRobot(VillageState.random(), randomRobot);
+// → Moved to Marketplace
+// → Moved to Town Hall
+// → …
+// → Done in 63 turns
+```
+
+It takes the robot a lot of turns to deliver the parcels because it isn't planning ahead very well. We'll address that soon.
+
+{{if interactive
+
+For a more pleasant perspective on the simulation, you can use the `runRobotAnimation` function that's available in [this chapter's programming environment](https://eloquentjavascript.net/code/#7). This runs the simulation, but instead of outputting text, it shows you the robot moving around the village map.
+
+```{test: no}
+runRobotAnimation(VillageState.random(), randomRobot);
+```
+
+The way `runRobotAnimation` is implemented will remain a mystery for now, but after you've read the [later chapters](dom) of this book, which discuss JavaScript integration in web browsers, you'll be able to guess how it works.
+
+if}}
+
+## The mail truck's route
+
+{{index "mailRoute array"}}
+
+We should be able to do a lot better than the random ((robot)). An easy improvement would be to take a hint from the way real-world mail delivery works. If we find a route that passes all places in the village, the robot could run that route twice, at which point it is guaranteed to be done. Here is one such route (starting from the post office):
+
+```{includeCode: true}
+const mailRoute = [
+ "Alice's House", "Cabin", "Alice's House", "Bob's House",
+ "Town Hall", "Daria's House", "Ernie's House",
+ "Grete's House", "Shop", "Grete's House", "Farm",
+ "Marketplace", "Post Office"
+];
+```
+
+{{index "routeRobot function"}}
+
+To implement the route-following robot, we'll need to make use of robot memory. The robot keeps the rest of its route in its memory and drops the first element every turn.
+
+```{includeCode: true}
+function routeRobot(state, memory) {
+ if (memory.length == 0) {
+ memory = mailRoute;
+ }
+ return {direction: memory[0], memory: memory.slice(1)};
+}
+```
+
+This robot is a lot faster already. It'll take a maximum of 26 turns (twice the 13-step route) but usually less.
+
+{{if interactive
+
+```{test: no}
+runRobotAnimation(VillageState.random(), routeRobot, []);
+```
+
+if}}
+
+## Pathfinding
+
+Still, I wouldn't really call blindly following a fixed route intelligent behavior. The ((robot)) could work more efficiently if it adjusted its behavior to the actual work that needs to be done.
+
+{{index pathfinding}}
+
+To do that, it has to be able to deliberately move toward a given parcel or toward the location where a parcel has to be delivered. Doing that, even when the goal is more than one move away, will require some kind of route-finding function.
+
+The problem of finding a route through a ((graph)) is a typical _((search problem))_. We can tell whether a given solution (a route) is valid, but we can't directly compute the solution the way we could for 2 + 2. Instead, we have to keep creating potential solutions until we find one that works.
+
+The number of possible routes through a graph is infinite. But when searching for a route from _A_ to _B_, we are interested only in the ones that start at _A_. We also don't care about routes that visit the same place twice—those are definitely not the most efficient route anywhere. So that cuts down on the number of routes that the route finder has to consider.
+
+In fact, since we are mostly interested in the _shortest_ route, we want to make sure we look at short routes before we look at longer ones. A good approach would be to "grow" routes from the starting point, exploring every reachable place that hasn't been visited yet until a route reaches the goal. That way, we'll explore only routes that are potentially interesting, and we know that the first route we find is the shortest route (or one of the shortest routes, if there are more than one).
+
+{{index "findRoute function"}}
+
+{{id findRoute}}
+
+Here is a function that does this:
+
+```{includeCode: true}
+function findRoute(graph, from, to) {
+ let work = [{at: from, route: []}];
+ for (let i = 0; i < work.length; i++) {
+ let {at, route} = work[i];
+ for (let place of graph[at]) {
+ if (place == to) return route.concat(place);
+ if (!work.some(w => w.at == place)) {
+ work.push({at: place, route: route.concat(place)});
+ }
+ }
+ }
+}
+```
+
+The exploring has to be done in the right order—the places that were reached first have to be explored first. We can't immediately explore a place as soon as we reach it because that would mean places reached _from there_ would also be explored immediately, and so on, even though there may be other, shorter paths that haven't yet been explored.
+
+Therefore, the function keeps a _((work list))_. This is an array of places that should be explored next, along with the route that got us there. It starts with just the start position and an empty route.
+
+The search then operates by taking the next item in the list and exploring that, which means it looks at all roads going from that place. If one of them is the goal, a finished route can be returned. Otherwise, if we haven't looked at this place before, a new item is added to the list. If we have looked at it before, since we are looking at short routes first, we've found either a longer route to that place or one precisely as long as the existing one, and we don't need to explore it.
+
+You can visualize this as a web of known routes crawling out from the start location, growing evenly on all sides (but never tangling back into itself). As soon as the first thread reaches the goal location, that thread is traced back to the start, giving us our route.
+
+{{index "connected graph"}}
+
+Our code doesn't handle the situation where there are no more work items on the work list because we know that our graph is _connected_, meaning that every location can be reached from all other locations. We'll always be able to find a route between two points, and the search can't fail.
+
+```{includeCode: true}
+function goalOrientedRobot({place, parcels}, route) {
+ if (route.length == 0) {
+ let parcel = parcels[0];
+ if (parcel.place != place) {
+ route = findRoute(roadGraph, place, parcel.place);
+ } else {
+ route = findRoute(roadGraph, place, parcel.address);
+ }
+ }
+ return {direction: route[0], memory: route.slice(1)};
+}
+```
+
+{{index "goalOrientedRobot function"}}
+
+This robot uses its memory value as a list of directions to move in, just like the route-following robot. Whenever that list is empty, it has to figure out what to do next. It takes the first undelivered parcel in the set and, if that parcel hasn't been picked up yet, plots a route toward it. If the parcel _has_ been picked up, it still needs to be delivered, so the robot creates a route toward the delivery address instead.
+
+{{if interactive
+
+Let's see how it does.
+
+```{test: no, startCode: true}
+runRobotAnimation(VillageState.random(),
+ goalOrientedRobot, []);
+```
+
+if}}
+
+This robot usually finishes the task of delivering 5 parcels in about 16 turns. That's slightly better than `routeRobot` but still definitely not optimal. We'll continue refining it in the exercises.
+
+## Exercises
+
+### Measuring a robot
+
+{{index "measuring a robot (exercise)", testing, automation, "compareRobots function"}}
+
+It's hard to objectively compare ((robot))s by just letting them solve a few scenarios. Maybe one robot just happened to get easier tasks or the kind of tasks that it is good at, whereas the other didn't.
+
+Write a function `compareRobots` that takes two robots (and their starting memory). It should generate 100 tasks and let both of the robots solve each of these tasks. When done, it should output the average number of steps each robot took per task.
+
+For the sake of fairness, make sure you give each task to both robots, rather than generating different tasks per robot.
+
+{{if interactive
+
+```{test: no}
+function compareRobots(robot1, memory1, robot2, memory2) {
+ // Your code here
+}
+
+compareRobots(routeRobot, [], goalOrientedRobot, []);
+```
+if}}
+
+{{hint
+
+{{index "measuring a robot (exercise)", "runRobot function"}}
+
+You'll have to write a variant of the `runRobot` function that, instead of logging the events to the console, returns the number of steps the robot took to complete the task.
+
+Your measurement function can then, in a loop, generate new states and count the steps each of the robots takes. When it has generated enough measurements, it can use `console.log` to output the average for each robot, which is the total number of steps taken divided by the number of measurements.
+
+hint}}
+
+### Robot efficiency
+
+{{index "robot efficiency (exercise)"}}
+
+Can you write a robot that finishes the delivery task faster than `goalOrientedRobot`? If you observe that robot's behavior, what obviously stupid things does it do? How could those be improved?
+
+If you solved the previous exercise, you might want to use your `compareRobots` function to verify whether you improved the robot.
+
+{{if interactive
+
+```{test: no}
+// Your code here
+
+runRobotAnimation(VillageState.random(), yourRobot, memory);
+```
+
+if}}
+
+{{hint
+
+{{index "robot efficiency (exercise)"}}
+
+The main limitation of `goalOrientedRobot` is that it considers only one parcel at a time. It will often walk back and forth across the village because the parcel it happens to be looking at happens to be at the other side of the map, even if there are others much closer.
+
+One possible solution would be to compute routes for all packages and then take the shortest one. Even better results can be obtained, if there are multiple shortest routes, by preferring the ones that go to pick up a package instead of delivering a package.
+
+hint}}
+
+### Persistent group
+
+{{index "persistent group (exercise)", "persistent data structure", "Set class", "set (data structure)", "Group class", "PGroup class"}}
+
+Most data structures provided in a standard JavaScript environment aren't very well suited for persistent use. Arrays have `slice` and `concat` methods, which allow us to easily create new arrays without damaging the old one. But `Set`, for example, has no methods for creating a new set with an item added or removed.
+
+Write a new class `PGroup`, similar to the `Group` class from [Chapter ?](object#groups), which stores a set of values. Like `Group`, it has `add`, `delete`, and `has` methods. Its `add` method, however, should return a _new_ `PGroup` instance with the given member added and leave the old one unchanged. Similarly, `delete` should create a new instance without a given member.
+
+The class should work for values of any type, not just strings. It does _not_ have to be efficient when used with large numbers of values.
+
+{{index [interface, object]}}
+
+The ((constructor)) shouldn't be part of the class's interface (though you'll definitely want to use it internally). Instead, there is an empty instance, `PGroup.empty`, that can be used as a starting value.
+
+{{index singleton}}
+
+Why do you need only one `PGroup.empty` value rather than having a function that creates a new, empty map every time?
+
+{{if interactive
+
+```{test: no}
+class PGroup {
+ // Your code here
+}
+
+let a = PGroup.empty.add("a");
+let ab = a.add("b");
+let b = ab.delete("a");
+
+console.log(b.has("b"));
+// → true
+console.log(a.has("b"));
+// → false
+console.log(b.has("a"));
+// → false
+```
+
+if}}
+
+{{hint
+
+{{index "persistent map (exercise)", "Set class", [array, creation], "PGroup class"}}
+
+The most convenient way to represent the set of member values is still as an array, since arrays are easy to copy.
+
+{{index "concat method", "filter method"}}
+
+When a value is added to the group, you can create a new group with a copy of the original array that has the value added (for example, using `concat`). When a value is deleted, you filter it from the array.
+
+The class's ((constructor)) can take such an array as its argument and store it as the instance's (only) property. This array is never updated.
+
+{{index "static property"}}
+
+To add the `empty` property to the constructor, you can declare it as a static property.
+
+You need only one `empty` instance because all empty groups are the same and instances of the class don't change. You can create many different groups from that single empty group without affecting it.
+
+hint}}
diff --git a/08_error.md b/08_error.md
new file mode 100644
index 000000000..f191c9ae6
--- /dev/null
+++ b/08_error.md
@@ -0,0 +1,647 @@
+{{meta {load_files: ["code/chapter/08_error.js"]}}}
+
+# Bugs and Errors
+
+{{quote {author: "Brian Kernighan and P.J. Plauger", title: "The Elements of Programming Style", chapter: true}
+
+Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.
+
+quote}}
+
+{{figure {url: "img/chapter_picture_8.jpg", alt: "Illustration showing various insects and a centipede", chapter: framed}}}
+
+{{index "Kernighan, Brian", "Plauger, P.J.", debugging, "error handling"}}
+
+Flaws in computer programs are usually called _((bug))s_. It makes programmers feel good to imagine them as little things that just happen to crawl into our work. In reality, of course, we put them there ourselves.
+
+If a program is crystallized thought, we can roughly categorize bugs into those caused by the thoughts being confused and those caused by mistakes introduced while converting a thought to code. The former type is generally harder to diagnose and fix than the latter.
+
+## Language
+
+{{index parsing, analysis}}
+
+Many mistakes could be pointed out to us automatically by the computer if it knew enough about what we're trying to do. But here, JavaScript's looseness is a hindrance. Its concept of bindings and properties is vague enough that it will rarely catch ((typo))s before actually running the program. Even then, it allows you to do some clearly nonsensical things without complaint, such as computing `true * "monkey"`.
+
+{{index [syntax, error], [property, access]}}
+
+There are some things that JavaScript does complain about. Writing a program that does not follow the language's ((grammar)) will immediately make the computer complain. Other things, such as calling something that's not a function or looking up a property on an ((undefined)) value, will cause an error to be reported when the program tries to perform the action.
+
+{{index NaN, error}}
+
+Often, however, your nonsense computation will merely produce `NaN` (not a number) or an undefined value, while the program happily continues, convinced that it's doing something meaningful. The mistake will manifest itself only later, after the bogus value has traveled through several functions. It might not trigger an error at all, but silently cause the program's output to be wrong. Finding the source of such problems can be difficult.
+
+The process of finding mistakes—bugs—in programs is called _((debugging))_.
+
+## Strict mode
+
+{{index "strict mode", [syntax, error], function}}
+
+{{indexsee "use strict", "strict mode"}}
+
+JavaScript can be made a _little_ stricter by enabling _strict mode_. This can done by putting the string `"use strict"` at the top of a file or a function body. Here's an example:
+
+```{test: "error \"ReferenceError: counter is not defined\""}
+function canYouSpotTheProblem() {
+ "use strict";
+ for (counter = 0; counter < 10; counter++) {
+ console.log("Happy happy");
+ }
+}
+
+canYouSpotTheProblem();
+// → ReferenceError: counter is not defined
+```
+
+{{index ECMAScript, compatibility}}
+
+Code inside classes and modules (which we will discuss in [Chapter ?](modules)) is automatically strict. The old nonstrict behavior still exists only because some old code might depend on it, and the language designers work hard to avoid breaking any existing programs.
+
+{{index "let keyword", [binding, global]}}
+
+Normally, when you forget to put `let` in front of your binding, as with `counter` in the example, JavaScript quietly creates a global binding and uses that. In strict mode, an ((error)) is reported instead. This is very helpful. It should be noted, though, that this doesn't work when the binding in question already exists somewhere in scope. In that case, the loop will still quietly overwrite the value of the binding.
+
+{{index "this binding", "global object", undefined, "strict mode"}}
+
+Another change in strict mode is that the `this` binding holds the value `undefined` in functions that are not called as ((method))s. When making such a call outside of strict mode, `this` refers to the global scope object, which is an object whose properties are the global bindings. So if you accidentally call a method or constructor incorrectly in strict mode, JavaScript will produce an error as soon as it tries to read something from `this`, rather than happily writing to the global scope.
+
+For example, consider the following code, which calls a ((constructor)) function without the `new` keyword so that its `this` will _not_ refer to a newly constructed object:
+
+```
+function Person(name) { this.name = name; }
+let ferdinand = Person("Ferdinand"); // oops
+console.log(name);
+// → Ferdinand
+```
+
+{{index error}}
+
+The bogus call to `Person` succeeded, but returned an undefined value and created the global binding `name`. In strict mode, the result is different.
+
+```{test: "error \"TypeError: Cannot set properties of undefined (setting 'name')\""}
+"use strict";
+function Person(name) { this.name = name; }
+let ferdinand = Person("Ferdinand"); // forgot new
+// → TypeError: Cannot set property 'name' of undefined
+```
+
+We are immediately told that something is wrong. This is helpful.
+
+Fortunately, constructors created with the `class` notation will always complain if they are called without `new`, making this less of a problem even in nonstrict mode.
+
+{{index parameter, [binding, naming], "with statement"}}
+
+Strict mode does a few more things. It disallows giving a function multiple parameters with the same name and removes certain problematic language features entirely (such as the `with` statement, which is so wrong it is not further discussed in this book).
+
+{{index debugging}}
+
+In short, putting `"use strict"` at the top of your program rarely hurts and might help you spot a problem.
+
+## Types
+
+Some languages want to know the types of all your bindings and expressions before even running a program. They will tell you right away when a type is used in an inconsistent way. JavaScript considers types only when actually running the program, and even there often tries to implicitly convert values to the type it expects, so it's not much help.
+
+Still, types provide a useful framework for talking about programs. A lot of mistakes come from being confused about the kind of value that goes into or comes out of a function. If you have that information written down, you're less likely to get confused.
+
+You could add a comment like the following before the `findRoute` function from the previous chapter to describe its type:
+
+```
+// (graph: Object, from: string, to: string) => string[]
+function findRoute(graph, from, to) {
+ // ...
+}
+```
+
+There are a number of different conventions for annotating JavaScript programs with types.
+
+One thing about types is that they need to introduce their own complexity to be able to describe enough code to be useful. What do you think would be the type of the `randomPick` function that returns a random element from an array? You'd need to introduce a _((type variable))_, _T_, which can stand in for any type, so that you can give `randomPick` a type like `(T[]) → T` (function from an array of *T*s to a *T*).
+
+{{index "type checking", TypeScript}}
+
+{{id typing}}
+
+When the types of a program are known, it is possible for the computer to _check_ them for you, pointing out mistakes before the program is run. There are several JavaScript dialects that add types to the language and check them. The most popular one is called [TypeScript](https://www.typescriptlang.org/). If you are interested in adding more rigor to your programs, I recommend you give it a try.
+
+In this book, we will continue using raw, dangerous, untyped JavaScript code.
+
+## Testing
+
+{{index "test suite", "run-time error", automation, testing}}
+
+If the language is not going to do much to help us find mistakes, we'll have to find them the hard way: by running the program and seeing whether it does the right thing.
+
+Doing this by hand, again and again, is a really bad idea. Not only is it annoying but it also tends to be ineffective, since it takes too much time to exhaustively test everything every time you make a change.
+
+Computers are good at repetitive tasks, and testing is the ideal repetitive task. Automated testing is the process of writing a program that tests another program. Writing tests is a bit more work than testing manually, but once you've done it, you gain a kind of superpower: it takes you only a few seconds to verify that your program still behaves properly in all the situations you wrote tests for. When you break something, you'll immediately notice rather than randomly running into it at some later time.
+
+{{index "toUpperCase method"}}
+
+Tests usually take the form of little labeled programs that verify some aspect of your code. For example, a set of tests for the (standard, probably already tested by someone else) `toUpperCase` method might look like this:
+
+```
+function test(label, body) {
+ if (!body()) console.log(`Failed: ${label}`);
+}
+
+test("convert Latin text to uppercase", () => {
+ return "hello".toUpperCase() == "HELLO";
+});
+test("convert Greek text to uppercase", () => {
+ return "Χαίρετε".toUpperCase() == "ΧΑΊΡΕΤΕ";
+});
+test("don't convert case-less characters", () => {
+ return "مرحبا".toUpperCase() == "مرحبا";
+});
+```
+
+{{index "domain-specific language"}}
+
+Writing tests like this tends to produce rather repetitive, awkward code. Fortunately, there exist pieces of software that help you build and run collections of tests (_((test suites))_) by providing a language (in the form of functions and methods) suited to expressing tests and by outputting informative information when a test fails. These are usually called _((test runners))_.
+
+{{index "persistent data structure"}}
+
+Some code is easier to test than other code. Generally, the more external objects that the code interacts with, the harder it is to set up the context in which to test it. The style of programming shown in the [previous chapter](robot), which uses self-contained persistent values rather than changing objects, tends to be easy to test.
+
+## Debugging
+
+{{index debugging}}
+
+Once you notice there is something wrong with your program because it misbehaves or produces errors, the next step is to figure out _what_ the problem is.
+
+Sometimes it is obvious. The ((error)) message will point at a specific line of your program, and if you look at the error description and that line of code, you can often see the problem.
+
+{{index "run-time error"}}
+
+But not always. Sometimes the line that triggered the problem is simply the first place where a flaky value produced elsewhere gets used in an invalid way. If you have been solving the ((exercises)) in earlier chapters, you will probably have already experienced such situations.
+
+{{index "decimal number", "binary number"}}
+
+The following example program tries to convert a whole number to a string in a given base (decimal, binary, and so on) by repeatedly picking out the last ((digit)) and then dividing the number to get rid of this digit. But the strange output that it currently produces suggests that it has a ((bug)).
+
+```
+function numberToString(n, base = 10) {
+ let result = "", sign = "";
+ if (n < 0) {
+ sign = "-";
+ n = -n;
+ }
+ do {
+ result = String(n % base) + result;
+ n /= base;
+ } while (n > 0);
+ return sign + result;
+}
+console.log(numberToString(13, 10));
+// → 1.5e-3231.3e-3221.3e-3211.3e-3201.3e-3191.3e-3181.3…
+```
+
+{{index analysis}}
+
+Even if you see the problem already, pretend for a moment that you don't. We know that our program is malfunctioning, and we want to find out why.
+
+{{index "trial and error"}}
+
+This is where you must resist the urge to start making random changes to the code to see whether that makes it better. Instead, _think_. Analyze what is happening and come up with a ((theory)) of why it might be happening. Then make additional observations to test this theory—or, if you don't yet have a theory, make additional observations to help you come up with one.
+
+{{index "console.log", output, debugging, logging}}
+
+Putting a few strategic `console.log` calls into the program is a good way to get additional information about what the program is doing. In this case, we want `n` to take the values `13`, `1`, and then `0`. Let's write out its value at the start of the loop.
+
+```{lang: null}
+13
+1.3
+0.13
+0.013
+…
+1.5e-323
+```
+
+{{index rounding}}
+
+_Right_. Dividing 13 by 10 does not produce a whole number. Instead of `n /= base`, what we actually want is `n = Math.floor(n / base)` so that the number is properly "shifted" to the right.
+
+{{index "JavaScript console", "debugger statement"}}
+
+An alternative to using `console.log` to peek into the program's behavior is to use the _debugger_ capabilities of your browser. Browsers come with the ability to set a _((breakpoint))_ on a specific line of your code. When the execution of the program reaches a line with a breakpoint, it is paused, and you can inspect the values of bindings at that point. I won't go into details, as debuggers differ from browser to browser, but look in your browser's ((developer tools)) or search the web for instructions.
+
+Another way to set a breakpoint is to include a `debugger` statement (consisting simply of that keyword) in your program. If the ((developer tools)) of your browser are active, the program will pause whenever it reaches such a statement.
+
+## Error propagation
+
+{{index input, output, "run-time error", error, validation}}
+
+Not all problems can be prevented by the programmer, unfortunately. If your program communicates with the outside world in any way, it is possible to get malformed input, to become overloaded with work, or to have the network fail.
+
+{{index "error recovery"}}
+
+If you're programming only for yourself, you can afford to just ignore such problems until they occur. But if you build something that is going to be used by anybody else, you usually want the program to do better than just crash. Sometimes the right thing to do is take the bad input in stride and continue running. In other cases, it is better to report to the user what went wrong and then give up. In either situation the program has to actively do something in response to the problem.
+
+{{index "promptNumber function", validation}}
+
+Say you have a function `promptNumber` that asks the user for a number and returns it. What should it return if the user inputs "orange"?
+
+{{index null, undefined, "return value", "special return value"}}
+
+One option is to make it return a special value. Common choices for such values are `null`, `undefined`, or `-1`.
+
+```{test: no}
+function promptNumber(question) {
+ let result = Number(prompt(question));
+ if (Number.isNaN(result)) return null;
+ else return result;
+}
+
+console.log(promptNumber("How many trees do you see?"));
+```
+
+Now any code that calls `promptNumber` must check whether an actual number was read and, failing that, must somehow recover—maybe by asking again or by filling in a default value. Or it could again return a special value to _its_ caller to indicate that it failed to do what it was asked.
+
+{{index "error handling"}}
+
+In many situations, mostly when ((error))s are common and the caller should be explicitly taking them into account, returning a special value is a good way to indicate an error. It does, however, have its downsides. First, what if the function can already return every possible kind of value? In such a function, you'll have to do something like wrap the result in an object to be able to distinguish success from failure, the way the `next` method on the iterator interface does.
+
+```
+function lastElement(array) {
+ if (array.length == 0) {
+ return {failed: true};
+ } else {
+ return {value: array[array.length - 1]};
+ }
+}
+```
+
+{{index "special return value", readability}}
+
+The second issue with returning special values is that it can lead to awkward code. If a piece of code calls `promptNumber` 10 times, it has to check 10 times whether `null` was returned. If its response to finding `null` is to simply return `null` itself, callers of the function will in turn have to check for it, and so on.
+
+## Exceptions
+
+{{index "error handling"}}
+
+When a function cannot proceed normally, what we would often _like_ to do is just stop what we are doing and immediately jump to a place that knows how to handle the problem. This is what _((exception handling))_ does.
+
+{{index ["control flow", exceptions], "raising (exception)", "throw keyword", "call stack"}}
+
+Exceptions are a mechanism that makes it possible for code that runs into a problem to _raise_ (or _throw_) an exception. An exception can be any value. Raising one somewhat resembles a super-charged return from a function: it jumps out of not just the current function but also its callers, all the way down to the first call that started the current execution. This is called _((unwinding the stack))_. You may remember the stack of function calls mentioned in [Chapter ?](functions#stack). An exception zooms down this stack, throwing away all the call contexts it encounters.
+
+{{index "error handling", [syntax, statement], "catch keyword"}}
+
+If exceptions always zoomed right down to the bottom of the stack, they would not be of much use. They'd just provide a novel way to blow up your program. Their power lies in the fact that you can set "obstacles" along the stack to _catch_ the exception as it is zooming down. Once you've caught an exception, you can do something with it to address the problem and then continue to run the program.
+
+Here's an example:
+
+{{id look}}
+```
+function promptDirection(question) {
+ let result = prompt(question);
+ if (result.toLowerCase() == "left") return "L";
+ if (result.toLowerCase() == "right") return "R";
+ throw new Error("Invalid direction: " + result);
+}
+
+function look() {
+ if (promptDirection("Which way?") == "L") {
+ return "a house";
+ } else {
+ return "two angry bears";
+ }
+}
+
+try {
+ console.log("You see", look());
+} catch (error) {
+ console.log("Something went wrong: " + error);
+}
+```
+
+{{index "exception handling", block, "throw keyword", "try keyword", "catch keyword"}}
+
+The `throw` keyword is used to raise an exception. Catching one is done by wrapping a piece of code in a `try` block, followed by the keyword `catch`. When the code in the `try` block causes an exception to be raised, the `catch` block is evaluated, with the name in parentheses bound to the exception value. After the `catch` block finishes—or if the `try` block finishes without problems—the program proceeds beneath the entire `try/catch` statement.
+
+{{index debugging, "call stack", "Error type"}}
+
+In this case, we used the `Error` ((constructor)) to create our exception value. This is a ((standard)) JavaScript constructor that creates an object with a `message` property. Instances of `Error` also gather information about the call stack that existed when the exception was created, a so-called _((stack trace))_. This information is stored in the `stack` property and can be helpful when trying to debug a problem: it tells us the function where the problem occurred and which functions made the failing call.
+
+{{index "exception handling"}}
+
+Note that the `look` function completely ignores the possibility that `promptDirection` might go wrong. This is the big advantage of exceptions: error-handling code is necessary only at the point where the error occurs and at the point where it is handled. The functions in between can forget all about it.
+
+Well, almost...
+
+## Cleaning up after exceptions
+
+{{index "exception handling", "cleaning up", ["control flow", exceptions]}}
+
+The effect of an exception is another kind of control flow. Every action that might cause an exception, which is pretty much every function call and property access, might cause control to suddenly leave your code.
+
+This means when code has several side effects, even if its "regular" control flow looks like they'll always all happen, an exception might prevent some of them from taking place.
+
+{{index "banking example"}}
+
+Here is some really bad banking code:
+
+```{includeCode: true}
+const accounts = {
+ a: 100,
+ b: 0,
+ c: 20
+};
+
+function getAccount() {
+ let accountName = prompt("Enter an account name");
+ if (!Object.hasOwn(accounts, accountName)) {
+ throw new Error(`No such account: ${accountName}`);
+ }
+ return accountName;
+}
+
+function transfer(from, amount) {
+ if (accounts[from] < amount) return;
+ accounts[from] -= amount;
+ accounts[getAccount()] += amount;
+}
+```
+
+The `transfer` function transfers a sum of money from a given account to another, asking for the name of the other account in the process. If given an invalid account name, `getAccount` throws an exception.
+
+But `transfer` _first_ removes the money from the account and _then_ calls `getAccount` before it adds it to another account. If it is broken off by an exception at that point, it'll just make the money disappear.
+
+That code could have been written a little more intelligently, for example by calling `getAccount` before it starts moving money around. But often problems like this occur in more subtle ways. Even functions that don't look like they will throw an exception might do so in exceptional circumstances or when they contain a programmer mistake.
+
+One way to address this is to use fewer side effects. Again, a programming style that computes new values instead of changing existing data helps. If a piece of code stops running in the middle of creating a new value, no existing data structures were damaged, making it easier to recover.
+
+{{index block, "try keyword", "finally keyword"}}
+
+Since that isn't always practical, `try` statements have another feature: they may be followed by a `finally` block either instead of or in addition to a `catch` block. A `finally` block says "no matter _what_ happens, run this code after trying to run the code in the `try` block."
+
+```{includeCode: true}
+function transfer(from, amount) {
+ if (accounts[from] < amount) return;
+ let progress = 0;
+ try {
+ accounts[from] -= amount;
+ progress = 1;
+ accounts[getAccount()] += amount;
+ progress = 2;
+ } finally {
+ if (progress == 1) {
+ accounts[from] += amount;
+ }
+ }
+}
+```
+
+This version of the function tracks its progress, and if, when leaving, it notices that it was aborted at a point where it had created an inconsistent program state, it repairs the damage it did.
+
+Note that even though the `finally` code is run when an exception is thrown in the `try` block, it does not interfere with the exception. After the `finally` block runs, the stack continues unwinding.
+
+{{index "exception safety"}}
+
+Writing programs that operate reliably even when exceptions pop up in unexpected places is hard. Many people simply don't bother, and because exceptions are typically reserved for exceptional circumstances, the problem may occur so rarely that it is never even noticed. Whether that is a good thing or a really bad thing depends on how much damage the software will do when it fails.
+
+## Selective catching
+
+{{index "uncaught exception", "exception handling", "JavaScript console", "developer tools", "call stack", error}}
+
+When an exception makes it all the way to the bottom of the stack without being caught, it gets handled by the environment. What this means differs between environments. In browsers, a description of the error typically gets written to the JavaScript console (reachable through the browser's Tools or Developer menu). Node.js, the browserless JavaScript environment we will discuss in [Chapter ?](node), is more careful about data corruption. It aborts the whole process when an unhandled exception occurs.
+
+{{index crash, "error handling"}}
+
+For programmer mistakes, just letting the error go through is often the best you can do. An unhandled exception is a reasonable way to signal a broken program, and the JavaScript console will, on modern browsers, provide you with some information about which function calls were on the stack when the problem occurred.
+
+{{index "user interface"}}
+
+For problems that are _expected_ to happen during routine use, crashing with an unhandled exception is a terrible strategy.
+
+{{index [function, application], "exception handling", "Error type", [binding, undefined]}}
+
+Invalid uses of the language, such as referencing a nonexistent binding, looking up a property on `null`, or calling something that's not a function, will also result in exceptions being raised. Such exceptions can also be caught.
+
+{{index "catch keyword"}}
+
+When a `catch` body is entered, all we know is that _something_ in our `try` body caused an exception. But we don't know _what_ did or _which_ exception it caused.
+
+{{index "exception handling"}}
+
+JavaScript (in a rather glaring omission) doesn't provide direct support for selectively catching exceptions: either you catch them all or you don't catch any. This makes it tempting to _assume_ that the exception you get is the one you were thinking about when you wrote the `catch` block.
+
+{{index "promptDirection function"}}
+
+But it might not be. Some other ((assumption)) might be violated, or you might have introduced a bug that is causing an exception. Here is an example that _attempts_ to keep on calling `promptDirection` until it gets a valid answer:
+
+```{test: no}
+for (;;) {
+ try {
+ let dir = promtDirection("Where?"); // ← typo!
+ console.log("You chose ", dir);
+ break;
+ } catch (e) {
+ console.log("Not a valid direction. Try again.");
+ }
+}
+```
+
+{{index "infinite loop", "for loop", "catch keyword", debugging}}
+
+The `for (;;)` construct is a way to intentionally create a loop that doesn't terminate on its own. We break out of the loop only when a valid direction is given. Unfortunately, we misspelled `promptDirection`, which will result in an "undefined variable" error. Because the `catch` block completely ignores its exception value (`e`), assuming it knows what the problem is, it wrongly treats the binding error as indicating bad input. Not only does this cause an infinite loop but it also "buries" the useful error message about the misspelled binding.
+
+As a general rule, don't blanket-catch exceptions unless it is for the purpose of "routing" them somewhere—for example, over the network to tell another system that our program crashed. And even then, think carefully about how you might be hiding information.
+
+{{index "exception handling"}}
+
+We want to catch a _specific_ kind of exception. We can do this by checking in the `catch` block whether the exception we got is the one we are interested in, and if not, rethrow it. But how do we recognize an exception?
+
+We could compare its `message` property against the ((error)) message we happen to expect. But that's a shaky way to write code—we'd be using information that's intended for human consumption (the message) to make a programmatic decision. As soon as someone changes (or translates) the message, the code will stop working.
+
+{{index "Error type", "instanceof operator", "promptDirection function"}}
+
+Rather, let's define a new type of error and use `instanceof` to identify it.
+
+```{includeCode: true}
+class InputError extends Error {}
+
+function promptDirection(question) {
+ let result = prompt(question);
+ if (result.toLowerCase() == "left") return "L";
+ if (result.toLowerCase() == "right") return "R";
+ throw new InputError("Invalid direction: " + result);
+}
+```
+
+{{index "throw keyword", inheritance}}
+
+The new error class extends `Error`. It doesn't define its own constructor, which means that it inherits the `Error` constructor, which expects a string message as argument. In fact, it doesn't define anything at all—the class is empty. `InputError` objects behave like `Error` objects, except that they have a different class by which we can recognize them.
+
+{{index "exception handling"}}
+
+Now the loop can catch these more carefully.
+
+```{test: no}
+for (;;) {
+ try {
+ let dir = promptDirection("Where?");
+ console.log("You chose ", dir);
+ break;
+ } catch (e) {
+ if (e instanceof InputError) {
+ console.log("Not a valid direction. Try again.");
+ } else {
+ throw e;
+ }
+ }
+}
+```
+
+{{index debugging}}
+
+This will catch only instances of `InputError` and let unrelated exceptions through. If you reintroduce the typo, the undefined binding error will be properly reported.
+
+## Assertions
+
+{{index "assert function", assertion, debugging}}
+
+_Assertions_ are checks inside a program that verify that something is the way it is supposed to be. They are used not to handle situations that can come up in normal operation but to find programmer mistakes.
+
+If, for example, `firstElement` is described as a function that should never be called on empty arrays, we might write it like this:
+
+```
+function firstElement(array) {
+ if (array.length == 0) {
+ throw new Error("firstElement called with []");
+ }
+ return array[0];
+}
+```
+
+{{index validation, "run-time error", crash, assumption}}
+
+Now, instead of silently returning undefined (which you get when reading an array property that does not exist), this will loudly blow up your program as soon as you misuse it. This makes it less likely for such mistakes to go unnoticed and easier to find their cause when they occur.
+
+I do not recommend trying to write assertions for every possible kind of bad input. That'd be a lot of work and would lead to very noisy code. You'll want to reserve them for mistakes that are easy to make (or that you find yourself making).
+
+## Summary
+
+An important part of programming is finding, diagnosing, and fixing bugs. Problems can become easier to notice if you have an automated test suite or add assertions to your programs.
+
+Problems caused by factors outside the program's control should usually be actively planned for. Sometimes, when the problem can be handled locally, special return values are a good way to track them. Otherwise, exceptions may be preferable.
+
+Throwing an exception causes the call stack to be unwound until the next enclosing `try/catch` block or until the bottom of the stack. The exception value will be given to the `catch` block that catches it, which should verify that it is actually the expected kind of exception and then do something with it. To help address the unpredictable control flow caused by exceptions, `finally` blocks can be used to ensure that a piece of code _always_ runs when a block finishes.
+
+## Exercises
+
+### Retry
+
+{{index "primitiveMultiply (exercise)", "exception handling", "throw keyword"}}
+
+Say you have a function `primitiveMultiply` that in 20 percent of cases multiplies two numbers and in the other 80 percent of cases raises an exception of type `MultiplicatorUnitFailure`. Write a function that wraps this clunky function and just keeps trying until a call succeeds, after which it returns the result.
+
+{{index "catch keyword"}}
+
+Make sure you handle only the exceptions you are trying to handle.
+
+{{if interactive
+
+```{test: no}
+class MultiplicatorUnitFailure extends Error {}
+
+function primitiveMultiply(a, b) {
+ if (Math.random() < 0.2) {
+ return a * b;
+ } else {
+ throw new MultiplicatorUnitFailure("Klunk");
+ }
+}
+
+function reliableMultiply(a, b) {
+ // Your code here.
+}
+
+console.log(reliableMultiply(8, 8));
+// → 64
+```
+if}}
+
+{{hint
+
+{{index "primitiveMultiply (exercise)", "try keyword", "catch keyword", "throw keyword"}}
+
+The call to `primitiveMultiply` should definitely happen in a `try` block. The corresponding `catch` block should rethrow the exception when it is not an instance of `MultiplicatorUnitFailure` and ensure the call is retried when it is.
+
+To do the retrying, you can either use a loop that stops only when a call succeeds—as in the [`look` example](error#look) earlier in this chapter—or use ((recursion)) and hope you don't get a string of failures so long that it overflows the stack (which is a pretty safe bet).
+
+hint}}
+
+### The locked box
+
+{{index "locked box (exercise)"}}
+
+Consider the following (rather contrived) object:
+
+```
+const box = new class {
+ locked = true;
+ #content = [];
+
+ unlock() { this.locked = false; }
+ lock() { this.locked = true; }
+ get content() {
+ if (this.locked) throw new Error("Locked!");
+ return this.#content;
+ }
+};
+```
+
+{{index "private property", "access control"}}
+
+It is a ((box)) with a lock. There is an array in the box, but you can get at it only when the box is unlocked.
+
+{{index "finally keyword", "exception handling"}}
+
+Write a function called `withBoxUnlocked` that takes a function value as argument, unlocks the box, runs the function, and then ensures that the box is locked again before returning, regardless of whether the argument function returned normally or threw an exception.
+
+{{if interactive
+
+```
+const box = new class {
+ locked = true;
+ #content = [];
+
+ unlock() { this.locked = false; }
+ lock() { this.locked = true; }
+ get content() {
+ if (this.locked) throw new Error("Locked!");
+ return this.#content;
+ }
+};
+
+function withBoxUnlocked(body) {
+ // Your code here.
+}
+
+withBoxUnlocked(() => {
+ box.content.push("gold piece");
+});
+
+try {
+ withBoxUnlocked(() => {
+ throw new Error("Pirates on the horizon! Abort!");
+ });
+} catch (e) {
+ console.log("Error raised: " + e);
+}
+console.log(box.locked);
+// → true
+```
+
+if}}
+
+For extra points, make sure that if you call `withBoxUnlocked` when the box is already unlocked, the box stays unlocked.
+
+{{hint
+
+{{index "locked box (exercise)", "finally keyword", "try keyword"}}
+
+This exercise calls for a `finally` block. Your function should first unlock the box and then call the argument function from inside a `try` body. The `finally` block after it should lock the box again.
+
+To make sure we don't lock the box when it wasn't already locked, check its lock at the start of the function and unlock and lock it only when it started out locked.
+
+hint}}
diff --git a/08_error.txt b/08_error.txt
deleted file mode 100644
index d51ccb5f3..000000000
--- a/08_error.txt
+++ /dev/null
@@ -1,858 +0,0 @@
-:chap_num: 8
-:prev_link: 07_elife
-:next_link: 09_regexp
-:load_files: ["code/chapter/08_error.js"]
-
-= Bugs and Error Handling =
-
-[chapterquote="true"]
-[quote, Brian Kernighan and P.J. Plauger, The Elements of Programming Style]
-____
-Debugging is
-twice as hard as writing the code in the first place. Therefore, if
-you write the code as cleverly as possible, you are, by definition,
-not smart enough to debug it.
-____
-
-ifdef::html_target[]
-
-[chapterquote="true"]
-[quote, Master Yuan-Ma, The Book of Programming]
-____
-Yuan-Ma had written a small program that used many global variables
-and shoddy shortcuts. Reading it, a student asked, ‘You warned us
-against these techniques, yet I find them in your program. How can
-this be?’ The master said, ‘There is no need to fetch a water hose
-when the house is not on fire.’
-____
-
-endif::html_target[]
-
-(((Kernighan+++,+++ Brian)))(((Plaugher+++,+++ P.J.)))(((debugging)))(((error handling)))A program is crystallized thought.
-Sometimes those thoughts are confused. Other times, mistakes are
-introduced when converting thought into code. Either way, the result
-is a flawed program.
-
-(((input)))(((output)))Flaws in a program are usually called ((bug))s.
-Bugs can be programmer errors, or problems in other systems that the
-program interacts with. Some bugs are immediately apparent, while
-others are subtle and might remain hidden in a system for years.
-
-Often, problems only surface when a program encounters a situation
-that the programmer didn't originally consider. Sometimes, such
-situations are unavoidable. When the user is asked to input their age,
-and types “orange”, this puts our program in a difficult position. The
-situation has to be anticipated and handled somehow.
-
-== Programmer mistakes ==
-
-(((parsing)))(((analysis)))When it comes to programmer mistakes, our
-aim is simple. We want to find them and fix them. Such mistakes can
-range from simple ((typo))s that cause the computer to complain as
-soon as it lays eyes on our program, to subtle mistakes in our
-understanding of the way the program operates, causing incorrect
-outcomes only in very specific situations. Bugs of the latter type can
-take weeks to diagnose.
-
-(((programming language)))(((type)))(((static typing)))(((dynamic
-typing)))(((run-time error)))(((error)))The degree in which languages
-help you find such mistakes varies. Unsurprisingly, JavaScript is at
-the “hardly helps at all” end of that scale. Some languages want to
-know the types of all your variables and expressions before even
-running a program, and will tell you right away when a type is used in
-an inconsistent way. JavaScript only considers types when actually
-running the program, and even then, it allows you to do some clearly
-nonsensical things without complaint, such as `x = true * "monkey"`.
-
-(((syntax)))There are some things that JavaScript does complain about,
-though. Writing a program that is not syntactically valid will
-immediately trigger an error. Other things, like calling something
-that's not a function or looking up a ((property)) on an ((undefined))
-value, will cause an error to be reported when the program is running
-and encounters the nonsensical action.
-
-(((NaN)))(((error)))But often, your nonsense computation will simply
-produce a `NaN` (not a number) or undefined value. And the program
-happily continues, convinced that it's doing something meaningful. The
-mistake will only manifest itself later, after the bogus value has
-traveled though several functions. It might not trigger an error at
-all, but silently cause the program's output to be wrong. Finding the
-source of such problems can be difficult.
-
-(((debugging)))The process of finding mistakes—bugs—in programs is
-called _debugging_.
-
-== Strict mode ==
-
-indexsee:[use strict,strict mode]
-
-(((strict mode)))(((syntax)))(((function)))JavaScript can be made a
-_little_ more strict by enabling _strict mode_. This is done by
-putting the string `"use strict"` at the top of a file or a function
-body. Like this:
-
-// test: error "ReferenceError: counter is not defined"
-
-[source,javascript]
-----
-function canYouSpotTheProblem() {
- "use strict";
- for (counter = 0; counter < 10; counter++)
- console.log("Happy happy");
-}
-
-canYouSpotTheProblem();
-// → ReferenceError: counter is not defined
-----
-
-(((var keyword)))(((variable,global)))Normally, when you forget to put
-`var` in front of your variable, as with `counter` in the example,
-JavaScript quietly creates a global variable and uses that. In strict
-mode, however, an ((error)) is reported instead. This is very helpful.
-It should be noted, though, that this doesn't work when the variable
-in question already exists as a global variable, but only when
-assigning to it would have created it.
-
-(((this)))(((global object)))(((undefined)))(((strict mode)))Another
-change in strict mode is that the `this` binding holds the value
-`undefined` in functions that are not called as ((method))s. When
-making such a call outside of strict mode, `this` refers the global
-scope object. So if you accidentally call a method or constructor
-incorrectly in strict mode, JavaScript will produce an error as soon
-as it tries to read something from `this`, rather than happily working
-with the global object, creating and reading global variables.
-
-For example, consider the following code, which calls a
-((constructor)) without the `new` keyword, so that its `this` will
-_not_ refer to a newly constructed object.
-
-[source,javascript]
-----
-function Person(name) { this.name = name; }
-var ferdinand = Person("Ferdinand"); // oops
-console.log(name);
-// → Ferdinand
-----
-
-(((error)))So the bogus call to `Person` succeeded, but returned an
-undefined value and created the global variable `name`. In strict
-mode, the result is different.
-
-// test: error "TypeError: Cannot set property 'name' of undefined"
-
-[source,javascript]
-----
-"use strict";
-function Person(name) { this.name = name; }
-// Oops, forgot 'new'
-var ferdinand = Person("Ferdinand");
-// → TypeError: Cannot set property 'name' of undefined
-----
-
-We are immediately told that something is wrong. This is helpful.
-
-(((parameter)))(((variable,naming)))(((with statement)))Strict mode
-does a few more things. It disallows giving a function multiple
-parameters with the same name, and removes certain problematic
-language features entirely (such as the `with` statement, which is so
-misguided it is not further discussed in this book).
-
-(((debugging)))In short, putting a `"use strict"` at the top of your
-program rarely hurts, and might help you spot a problem.
-
-== Testing ==
-
-(((test suite)))(((run-time error)))If the language is not going to do
-much to help us find mistakes, we'll have to find them the hard way:
-by running the program and seeing if it does the right thing.
-
-Doing this by hand, again and again, is a sure way to drive yourself
-insane. Fortunately, it is often possible to write a second program
-that automates testing your actual program.
-
-(((Vector type)))As an example, we once again use the `Vector` type.
-
-// include_code
-
-[source,javascript]
-----
-function Vector(x, y) {
- this.x = x;
- this.y = y;
-}
-Vector.prototype.plus = function(other) {
- return new Vector(this.x + other.x, this.y + other.y);
-};
-----
-
-We will write a program to check that our implementation of `Vector`
-works as intended. Then, every time we change the implementation, we
-follow up by running the test program, so that we can be reasonably
-confident that we didn't break anything. When we add extra
-functionality (for example a new method) to the `Vector` type, we also
-add tests for the new feature.
-
-[source,javascript]
-----
-function testVector() {
- var p1 = new Vector(10, 20);
- var p2 = new Vector(-10, 5);
- var p3 = p1.plus(p2);
-
- if (p1.x !== 10) return "fail: x property";
- if (p1.y !== 20) return "fail: y property";
- if (p2.x !== -10) return "fail: negative x property";
- if (p3.x !== 0) return "fail: x from plus";
- if (p3.y !== 25) return "fail: y from plus";
- return "everything ok";
-}
-console.log(testVector());
-// → everything ok
-----
-
-(((test suite)))(((testing framework)))(((domain-specific
-language)))Writing tests like this tends to produce rather repetetive,
-awkward code. Fortunately, there exist pieces of software that help
-you build and run collections of tests (“test suites”) by providing a
-language (in the form of functions and methods) suited to expressing
-tests, and outputting informative information when a test fails. These
-are called _testing frameworks_.
-
-== Debugging ==
-
-(((debugging)))Once you notice that there is something wrong with your
-program because it misbehaves or produces errors, the next step is to
-figure out _what_ the problem is.
-
-Sometimes, it is obvious. The ((error)) message will point at a
-specific line of your program, and if you look at the error
-description and that line of code, you can often see the problem.
-
-(((run-time error)))But not always. Sometimes the line that triggered
-the problem is simply the first place where a bogus value produced
-elsewhere gets used in an invalid way. And sometimes there is no error
-message at all—just an invalid result. If you have been solving the
-((exercises)) in the earlier chapters, you will probably have already
-experienced such situations.
-
-(((decimal number)))(((binary number)))The following example program
-tries to convert a whole number to a string in any base (decimal,
-binary, etc.) by repeatedly picking out the last ((digit)), and then
-dividing the number to get rid of this digit. But the insane output
-that it currently produces suggests that it has a ((bug)).
-
-[source,javascript]
-----
-function numberToString(n, base) {
- var result = "", sign = "";
- if (n < 0) {
- sign = "-";
- n = -n;
- }
- do {
- result = String(n % base) + result;
- n /= base;
- } while (n > 0);
- return sign + result;
-}
-console.log(numberToString(13, 10));
-// → 1.5e-3231.3e-3221.3e-3211.3e-3201.3e-3191.3e-3181.3…
-----
-
-(((analysis)))Even if you see the problem already, pretend for a
-moment that you don't. We know that our program is malfunctioning, and
-we want to find out why.
-
-(((trial-and-error)))This is where you must resist the urge to start
-making random changes to the code. Instead, _think_. Analyze what is
-happening and come up with a ((theory)) of why it might be happening.
-Then, make additional observations to test this theory—or, if you
-don't yet have a theory, make additional observations that might help
-you come up with one.
-
-(((console.log)))(((output)))(((debugging)))(((logging)))Putting a few
-strategic `console.log` calls into the program is a good way to get
-additional information about what the program is doing. In this case,
-we want `n` to take the values `13`, `1`, and then `0`. Let's write
-out its value at the start of the loop.
-
-----
-13
-1.3
-0.13
-0.013
-…
-1.5e-323
-----
-
-(((rounding)))_Right_. Dividing 13 by 10 does not produce a whole
-number. Instead of `n /= base`, what we actually want is `n =
-Math.floor(n / base)`, so that the number is properly “shifted” to the
-right.
-
-(((JavaScript console)))(((breakpoint)))(((debugger statement)))An
-alternative to using `console.log` is to use the _debugger_
-capabilities of your browser. Modern browsers come with the ability to
-set a _breakpoint_ on a specific line of your code. This will cause
-the execution of the program to pause every time the line with the
-breakpoint is reached, and allow you to inspect the values of
-variables at that point. I won't go into details here since debuggers
-differ from browser to browser, but look in your browser's developer
-tools and search the Web for more information. Another way to set a
-breakpoint is to include a `debugger` statement (consisting of simply
-that keyword) in your program. If the ((developer tools)) of your
-browser are active, the program will pause whenever it reaches that
-statement, and you will be able to inspect its state.
-
-== Error propagation ==
-
-(((input)))(((output)))(((run-time
-error)))(((error)))(((validation)))Not all problems can be prevented
-by the programmer, unfortunately. If your program communicates with
-the outside world in any way, there is a chance that the input it gets
-will be invalid, or that other systems that it tries to talk to are
-broken or unreachable.
-
-(((error recovery)))Simple programs, or programs that only run under
-your supervision, can afford to just give up when such a problem
-occurs. You'll look into the problem, and try again. “Real”
-applications, on the other hand, are expected to not simply crash.
-Sometimes the right thing to do is take the bad input in stride and
-continue running. In other cases, it is better to report to the user
-what went wrong and then give up. But in either situation, the program
-has to actively do something in response to the problem.
-
-(((promptInteger function)))(((validation)))Say you have a function
-`promptInteger` that asks the user for a whole number and returns it.
-What should it return if the user inputs “orange”?
-
-(((null)))(((undefined)))(((return value)))(((special return
-value)))One option is to make it return a special value. Common
-choices for such values are `null` and `undefined`.
-
-// test: no
-
-[source,javascript]
-----
-function promptNumber(question) {
- var result = Number(prompt(question, ""));
- if (isNaN(result)) return null;
- else return result;
-}
-
-console.log(promptNumber("How many trees do you see?"));
-----
-
-This is a sound strategy. Now any code that calls `promptNumber` must
-check whether an actual number was read, and failing that, must
-somehow recover—maybe by asking again, or by filling in a default
-value. Or it could again return a special value to _its_ caller, to
-indicate that it failed to do what it was asked.
-
-(((error handling)))In many situations, mostly when ((error))s are
-common and the caller should be explicitly taking them into account,
-returning a special value is a perfectly fine way to indicate an
-error. It does, however, have its downsides. First, what if the
-function can already return every possible kind of value? For such a
-function, it is hard to find a special value that can be distinguished
-from a valid result.
-
-(((special return value)))(((readability)))The second issue with
-returning special values is that it can lead to some very cluttered
-code. If a piece of code calls `promptNumber` 10 times, it has to
-check 10 times whether `null` was returned. And if its response to
-finding `null` is to simply return `null` itself, the caller will in
-turn have to check for it, and so on.
-
-== Exceptions ==
-
-(((error handling)))When a function cannot proceed normally, what we
-would _like_ to do is just stop what we are doing and immediately jump
-back to a place that knows how to handle the problem. This is what
-_((exception handling))_ does.
-
-(((control flow)))(((raising (exception))))(((throw keyword)))(((call
-stack)))Exceptions are a mechanism that make it possible for code that
-runs into a problem to _raise_ (or _throw_) an exception, which is
-simply a value. Raising an exception somewhat resembles a
-super-charged return from a function: it jumps out of not just the
-current function but also out of its callers, all the way down to the
-first call that started the current execution. This is called
-_((unwinding the stack))_. You may remember the stack of function
-calls that was mentioned in link:03_functions.html#stack[Chapter 3].
-An exception zooms down this stack, throwing away all the call
-contexts it encounters.
-
-(((error handling)))(((syntax)))(((catch keyword)))If exceptions
-always zoomed right down to the bottom of the stack, they would not be
-of much use. They would just provide a novel way to blow up your
-program. Their power lies in the fact that you can set “obstacles”
-along the stack to _catch_ the exception as it is zooming down. Then
-you can do something with it, after which the program continues
-running at the point where the exception was caught.
-
-Here's an example:
-
-[[look]]
-[source,javascript]
-----
-function promptDirection(question) {
- var result = prompt(question, "");
- if (result.toLowerCase() == "left") return "L";
- if (result.toLowerCase() == "right") return "R";
- throw new Error("Invalid direction: " + result);
-}
-
-function look() {
- if (promptDirection("Which way?") == "L")
- return "a house";
- else
- return "two angry bears";
-}
-
-try {
- console.log("You see", look());
-} catch (error) {
- console.log("Something went wrong: " + error);
-}
-----
-
-(((exception handling)))(((block)))(((throw keyword)))(((try
-keyword)))(((catch keyword)))The `throw` keyword is used to raise an
-exception. Catching one is done by wrapping a piece of code in a `try`
-block, followed by the keyword `catch`. When the code in the `try`
-block causes an exception to be raised, the `catch` block is
-evaluated. The variable name (in parentheses) after `catch` will be
-bound to the exception value. After the `catch` block finishes—or if
-the `try` block finishes without problems—control proceeds beneath the
-entire try-catch statement.
-
-(((debugging)))(((call stack)))(((Error type)))(((stack
-trace)))In this case, we used the `Error` ((constructor)) to create
-our exception value. This is a ((standard)) JavaScript constructor
-that creates an object with a `message` property. In modern JavaScript
-environments, instances of this constructor also gather information
-about the call stack that existed when the exception was created, a
-so-called _stack trace_. This information is stored in the `stack`
-property, and can be very helpful when trying to debug a problem: it
-tells us the precise function where the problem occurred, and which
-other functions led up to the call that failed.
-
-(((exception handling)))Note that the function `look` completely
-ignores the possibility that `promptDirection` might go wrong. This is
-the big advantage of exceptions—error-handling code is necessary only
-at the point where the error occurs and at the point where it is
-handled. The functions in between can forget all about it.
-
-Well, almost...
-
-== Cleaning up after exceptions ==
-
-(((exception handling)))(((cleaning up)))(((withContext
-function)))(((dynamic scope)))Consider the following situation: a
-function, `withContext`, wants to make sure that, during its
-execution, the top-level variable `context` holds a specific context
-value. After it finishes, it restores this variable to its old value.
-
-// include_code
-
-[source,javascript]
-----
-var context = null;
-
-function withContext(newContext, body) {
- var oldContext = context;
- context = newContext;
- var result = body();
- context = oldContext;
- return result;
-}
-----
-
-What if `body` raises an exception? In that case, the call to
-`withContext` will be thrown off the stack by the exception, and
-`context` will never be set back to its old value.
-
-(((block)))(((try keyword)))(((finally keyword)))There is one more
-feature that `try` statements have. They may be followed by a
-`finally` block, either instead of, or in addition to, a `catch`
-block. A `finally` block means “No matter _what_ happens, run this
-code after trying to run the code in the `try` block”. If a function
-has to clean something up, the cleanup code should usually be put into
-a `finally` block.
-
-// include_code
-
-[source,javascript]
-----
-function withContext(newContext, body) {
- var oldContext = context;
- context = newContext;
- try {
- return body();
- } finally {
- context = oldContext;
- }
-}
-----
-
-(((withContext function)))Note that we no longer have to store the
-result of `body` (which we want to return) in a variable. Even if we
-return directly from the `try` block, the `finally` block will be run.
-Now we can do this, and be safe:
-
-// test: no
-
-[source,javascript]
-----
-try {
- withContext(5, function() {
- if (context < 10)
- throw new Error("Not enough context!");
- });
-} catch (e) {
- console.log("Ignoring: " + e);
-}
-// → Ignoring: Error: Not enough context!
-
-console.log(context);
-// → null
-----
-
-Even though the function called from `withContext` exploded,
-`withContext` itself still properly cleaned up the `context` variable.
-
-== Selective catching ==
-
-(((uncaught exception)))(((exception handling)))(((JavaScript
-console)))(((developer tools)))(((call stack)))(((error)))When an
-exception makes it all the way to the bottom of the stack without
-being caught, it gets handled by the environment. What this means
-differs between environments. In browsers, a description of the error
-typically gets written to the JavaScript console (reachable through
-the browser's “tools” or “developer” menu).
-
-(((crash)))(((error handling)))For programmer mistakes or problems
-that the program cannot possibly handle, just letting the error go
-through is often okay. An unhandled exception is a reasonable way to
-signal a broken program, and the JavaScript console will, on modern
-browsers, provide you with some information about which function calls
-were on the stack when the problem occurred.
-
-(((user interface)))For problems that are _expected_ to happen during
-routine use, crashing with an unhandled exception is not a very
-friendly response.
-
-(((syntax)))(((function,application)))(((exception handling)))(((Error
-type)))Invalid uses of the language, like referencing a nonexistent
-((variable)), looking up a property on `null`, or calling something
-that's not a function, will also result in exceptions being raised.
-Such exceptions can be caught just like your own exceptions.
-
-(((catch keyword)))When a `catch` body is entered, all we know is that
-_something_ in our `try` body caused an exception. But we don't know
-_what_, nor _which_ exception it caused.
-
-(((exception handling)))JavaScript (in a rather glaring omission)
-doesn't provide direct support for selectively catching exceptions:
-you either catch them all, or you don't catch any. This makes it very
-easy to _assume_ that the exception you get is the one you were
-thinking about when you wrote the `catch` block.
-
-(((promptDirection function)))But it might not be. Some other
-((assumption)) might be violated, or you might have introduced a bug
-somewhere that is causing an exception. Here is an example, which
-_attempts_ to keep on calling `promptDirection` until it gets a valid
-answer.
-
-// test: no
-
-[source,javascript]
-----
-for (;;) {
- try {
- var dir = promtDirection("Where?"); // ← typo!
- console.log("You chose ", dir);
- break;
- } catch (e) {
- console.log("Not a valid direction. Try again.");
- }
-}
-----
-
-(((infinite loop)))(((for loop)))(((catch keyword)))(((debugging)))The
-`for (;;)` construct is a way to intentionally create a loop that
-doesn't terminate on its own. We only break out of the loop when a
-valid direction is given. _But_, we misspelled `promptDirection`,
-which will result in an “undefined variable” error. Because the
-`catch` block completely ignores its exception value (`e`), assuming
-it knows what the problem is, it wrongly treats the variable error as
-indicating bad input. Not only does this cause an infinite loop, but
-it also “buries” the (very useful) error message about the misspelled
-variable.
-
-As a general rule, don't blanket-catch exceptions unless it is for the
-purpose of “routing” them somewhere—for example, over the network, to
-tell another system that our program crashed. And even then, think
-carefully about how you might be hiding information.
-
-(((exception handling)))So we want to catch a _specific_ kind of
-exception. We can do this by checking in the `catch` block whether the
-exception we got is the one we are interested in, and re-throwing it
-otherwise. But how do we recognize an exception?
-
-Of course, we could match its `message` property against the ((error))
-message we happen to expect. But that's a shaky way to write code—we'd
-be using information that's intended for human consumption (the
-message) to make a programmatic decision. As soon as someone changes
-(or translates) the message, the code will stop working.
-
-(((Error type)))(((instanceof operator)))Rather, let's define a new
-type of error, and use `instanceof` to identify it.
-
-// include_code
-
-[source,javascript]
-----
-function InputError(message) {
- this.message = message;
- this.stack = (new Error()).stack;
-}
-InputError.prototype = Object.create(Error.prototype);
-InputError.prototype.name = "InputError";
-----
-
-(((throw keyword)))(((inheritance)))The prototype is made to derive
-from `Error.prototype`, so that `instanceof Error` will also return
-true for `InputError` objects. It's also given a `name` property,
-since the standard error types (`Error`, `SyntaxError`,
-`ReferenceError`, and so on) also have such a property.
-
-(((call stack)))The assignment to the `stack` property tries to give
-this object a somewhat useful ((stack trace)), on platforms that
-support it, by creating a regular error object, and then using that
-object's `stack` property as its own.
-
-(((promptDirection function)))Now `promptDirection` can throw such an
-error.
-
-// include_code
-
-[source,javascript]
-----
-function promptDirection(question) {
- var result = prompt(question, "");
- if (result.toLowerCase() == "left") return "L";
- if (result.toLowerCase() == "right") return "R";
- throw new InputError("Invalid direction: " + result);
-}
-----
-
-(((exception handling)))And the loop can catch it more carefully.
-
-// test: no
-
-[source,javascript]
-----
-for (;;) {
- try {
- var dir = promptDirection("Where?");
- console.log("You chose ", dir);
- break;
- } catch (e) {
- if (e instanceof InputError)
- console.log("Not a valid direction. Try again.");
- else
- throw e;
- }
-}
-----
-
-(((debugging)))This will only catch instances of `InputError` and let
-unrelated exceptions through. If you reintroduce the typo, the
-undefined variable error will be properly reported.
-
-== Assertions ==
-
-(((assert function)))(((assertion)))(((debugging)))_Assertions_ are a
-tool to do basic sanity checking for programmer errors. Consider this
-helper function, `assert`:
-
-[source,javascript]
-----
-function AssertionFailed(message) {
- this.message = message;
-}
-AssertionFailed.prototype = Object.create(Error.prototype);
-
-function assert(test, message) {
- if (!test)
- throw new AssertionFailed(message);
-}
-
-function lastElement(array) {
- assert(array.length > 0, "empty array in lastElement");
- return array[array.length - 1];
-}
-----
-
-(((validation)))(((run-time
-error)))(((crash)))(((assumption)))(((array)))This provides a
-compact way to enforce expectations, helpfully blowing up the program
-if the stated condition does not hold. For instance, the `lastElement`
-function, which fetches the last element from an array, would return
-`undefined` on empty arrays if the assertion was omitted. Fetching the
-last element from an empty array does not make much sense, so it is
-almost certainly a programmer error to do so.
-
-(((assertion)))(((debugging)))Assertions are a way to make sure
-mistakes cause failures at the point of the mistake, rather than
-silently producing nonsense values that may go on to cause trouble in
-an unrelated part of the system.
-
-== Summary ==
-
-Mistakes and bad input are facts of life. Bugs in programs need to be
-found and fixed. They can become easier to notice by having automated
-test suites and adding assertions to your programs.
-
-Problems caused by factors outside the program's control should
-usually be handled gracefully. Sometimes, when the problem can be
-handled locally, special return values are a sane way to track them.
-Otherwise, exceptions are preferable.
-
-Throwing an exception causes the call stack to be unwound until the
-next enclosing `try`/`catch` block, or until the bottom of the stack.
-The exception value will be given to the `catch` block that catches
-it, which should verify that it is actually the expected kind of
-exception, and then do something with it. To deal with the
-unpredictable control flow caused by exceptions, `finally` blocks can
-be used to ensure a piece of code is _always_ run when a block
-finishes.
-
-== Exercises ==
-
-=== Retry ===
-
-(((primitiveMultiply (exercise))))(((exception handling)))(((throw
-keyword)))Say you have a function `primitiveMultiply` that, in 50% of
-cases, multiplies two numbers, and in the other 50% raises an
-exception of type `MultiplicatorUnitFailure`. Write a function that
-wraps this clunky function and just keeps trying until a call
-succeeds, returning the result.
-
-(((catch keyword)))Make sure that you only handle the exceptions you
-are trying to handle.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-function MultiplicatorUnitFailure() {}
-
-function primitiveMultiply(a, b) {
- if (Math.random() < 0.5)
- return a * b;
- else
- throw new MultiplicatorUnitFailure();
-}
-
-function reliableMultiply(a, b) {
- // Your code here.
-}
-
-console.log(reliableMultiply(8, 8));
-// → 64
-----
-endif::html_target[]
-
-!!solution!!
-
-(((primitiveMultiply (exercise))))(((try keyword)))(((catch
-keyword)))(((throw keyword)))The call to `primitiveMultiply` should
-obviously happen in a `try` block. The corresponding `catch` block
-should re-throw the exception when it is not an instance of
-`MultiplicatorUnitFailure`, and ensure the call is retried when it is.
-
-To do the retrying, you can either use a loop that breaks only when a
-call succeeds—as in the link:08_error.html#look[`look` example]
-earlier in this chapter—or use ((recursion)), and hope you don't get a
-string of failures so long that it overflows the stack (which is a
-pretty safe bet).
-
-!!solution!!
-
-=== The locked box ===
-
-(((locked box (exercise))))Consider the following (rather contrived)
-object:
-
-// include_code
-
-[source,javascript]
-----
-var box = {
- locked: true,
- unlock: function() { this.locked = false; },
- lock: function() { this.locked = true; },
- _content: [],
- get content() {
- if (this.locked) throw new Error("Locked!");
- return this._content;
- }
-};
-----
-
-(((private property)))(((access control)))It is a ((box)), with a
-lock. Inside is an array, but you can only get at it when the box is
-unlocked. Directly accessing the `_content` property is not allowed.
-
-(((finally keyword)))(((exception handling)))Write a function called
-`withBoxUnlocked` that takes a function value as argument, unlocks the
-box, runs the function, and then ensures that the box is locked again
-before returning, regardless of whether the argument function returned
-normally or threw an exception.
-
-ifdef::html_target[]
-
-[source,javascript]
-----
-function withBoxUnlocked(body) {
- // Your code here.
-}
-
-withBoxUnlocked(function() {
- box.content.push("gold piece");
-});
-
-try {
- withBoxUnlocked(function() {
- throw new Error("Pirates on the horizon! Abort!");
- });
-} catch (e) {
- console.log("Error raised:", e);
-}
-console.log(box.locked);
-// → true
-----
-
-For extra points, make sure that if you call `withBoxUnlocked` when
-the box is already unlocked, the box stays unlocked.
-
-endif::html_target[]
-
-!!solution!!
-
-(((locked box (exercise))))(((finally keyword)))(((try keyword)))This
-exercise calls for a `finally` block, as you probably guessed. Your
-function should first unlock the box, then call the argument function
-from inside a `try` body. The `finally` block after it should lock the
-box again.
-
-To make sure we don't lock the box when it wasn't already locked,
-check its lock at the start of the function, and only unlock and lock
-it when it started out locked.
-
-!!solution!!
diff --git a/09_regexp.md b/09_regexp.md
new file mode 100644
index 000000000..02eae5bbb
--- /dev/null
+++ b/09_regexp.md
@@ -0,0 +1,1005 @@
+# Regular Expressions
+
+{{quote {author: "Jamie Zawinski", chapter: true}
+
+Some people, when confronted with a problem, think 'I know, I'll use regular expressions.' Now they have two problems.
+
+quote}}
+
+{{index "Zawinski, Jamie"}}
+
+{{if interactive
+
+{{quote {author: "Master Yuan-Ma", title: "The Book of Programming", chapter: true}
+
+When you cut against the grain of the wood, much strength is needed. When you program against the grain of the problem, much code is needed.
+
+quote}}
+
+if}}
+
+{{figure {url: "img/chapter_picture_9.jpg", alt: "Illustration of a railroad system representing the syntactic structure of regular expressions", chapter: "square-framed"}}}
+
+{{index evolution, adoption, integration}}
+
+Programming ((tool))s and techniques survive and spread in a chaotic, evolutionary way. It's not always the best or most brilliant ones that win, but rather the ones that function well enough within the right niche or that happen to be integrated with another successful piece of technology.
+
+{{index "domain-specific language"}}
+
+In this chapter, I will discuss one such tool, _((regular expression))s_. Regular expressions are a way to describe ((pattern))s in string data. They form a small, separate language that is part of JavaScript and many other languages and systems.
+
+{{index [interface, design]}}
+
+Regular expressions are both terribly awkward and extremely useful. Their syntax is cryptic and the programming interface JavaScript provides for them is clumsy. But they are a powerful ((tool)) for inspecting and processing strings. Properly understanding regular expressions will make you a more effective programmer.
+
+## Creating a regular expression
+
+{{index ["regular expression", creation], "RegExp class", "literal expression", "slash character"}}
+
+A regular expression is a type of object. It can be either constructed with the `RegExp` constructor or written as a literal value by enclosing a pattern in forward slash (`/`) characters.
+
+```
+let re1 = new RegExp("abc");
+let re2 = /abc/;
+```
+
+Both of those regular expression objects represent the same ((pattern)): an _a_ character followed by a _b_ followed by a _c_.
+
+{{index ["backslash character", "in regular expressions"], "RegExp class"}}
+
+When using the `RegExp` constructor, the pattern is written as a normal string, so the usual rules apply for backslashes.
+
+{{index ["regular expression", escaping], [escaping, "in regexps"], "slash character"}}
+
+The second notation, where the pattern appears between slash characters, treats backslashes somewhat differently. First, since a forward slash ends the pattern, we need to put a backslash before any forward slash that we want to be _part_ of the pattern. In addition, backslashes that aren't part of special character codes (like `\n`) will be _preserved_, rather than ignored as they are in strings, and change the meaning of the pattern. Some characters, such as question marks and plus signs, have special meanings in regular expressions and must be preceded by a backslash if they are meant to represent the character itself.
+
+```
+let aPlus = /A\+/;
+```
+
+## Testing for matches
+
+{{index matching, "test method", ["regular expression", methods]}}
+
+Regular expression objects have a number of methods. The simplest one is `test`. If you pass it a string, it will return a ((Boolean)) telling you whether the string contains a match of the pattern in the expression.
+
+```
+console.log(/abc/.test("abcde"));
+// → true
+console.log(/abc/.test("abxde"));
+// → false
+```
+
+{{index pattern}}
+
+A ((regular expression)) consisting of only nonspecial characters simply represents that sequence of characters. If _abc_ occurs anywhere in the string we are testing against (not just at the start), `test` will return `true`.
+
+## Sets of characters
+
+{{index "regular expression", "indexOf method"}}
+
+Finding out whether a string contains _abc_ could just as well be done with a call to `indexOf`. Regular expressions are useful because they allow us to describe more complicated ((pattern))s.
+
+Say we want to match any ((number)). In a regular expression, putting a ((set)) of characters between square brackets makes that part of the expression match any of the characters between the brackets.
+
+Both of the following expressions match all strings that contain a ((digit)):
+
+```
+console.log(/[0123456789]/.test("in 1992"));
+// → true
+console.log(/[0-9]/.test("in 1992"));
+// → true
+```
+
+{{index "hyphen character"}}
+
+Within square brackets, a hyphen (`-`) between two characters can be used to indicate a ((range)) of characters, where the ordering is determined by the character's ((Unicode)) number. Characters 0 to 9 sit right next to each other in this ordering (codes 48 to 57), so `[0-9]` covers all of them and matches any ((digit)).
+
+{{index [whitespace, matching], "alphanumeric character", "period character"}}
+
+A number of common character groups have their own built-in shortcuts. Digits are one of them: `\d` means the same thing as `[0-9]`.
+
+{{index "newline character", [whitespace, matching]}}
+
+{{table {cols: [1, 5]}}}
+
+| `\d` | Any ((digit)) character
+| `\w` | An alphanumeric character ("((word character))")
+| `\s` | Any whitespace character (space, tab, newline, and similar)
+| `\D` | A character that is _not_ a digit
+| `\W` | A nonalphanumeric character
+| `\S` | A nonwhitespace character
+| `.` | Any character except for newline
+
+You could match a ((date)) and ((time)) format like 01-30-2003 15:20 with the following expression:
+
+```
+let dateTime = /\d\d-\d\d-\d\d\d\d \d\d:\d\d/;
+console.log(dateTime.test("01-30-2003 15:20"));
+// → true
+console.log(dateTime.test("30-jan-2003 15:20"));
+// → false
+```
+
+{{index ["backslash character", "in regular expressions"]}}
+
+That regular expression looks completely awful, doesn't it? Half of it is backslashes, producing a background noise that makes it hard to spot the actual ((pattern)) expressed. We'll see a slightly improved version of this expression [later](regexp#date_regexp_counted).
+
+{{index [escaping, "in regexps"], "regular expression", set}}
+
+These backslash codes can also be used inside ((square brackets)). For example, `[\d.]` means any digit or a period character. The period itself, between square brackets, loses its special meaning. The same goes for other special characters, such as the plus sign (`+`).
+
+{{index "square brackets", inversion, "caret character"}}
+
+To _invert_ a set of characters—that is, to express that you want to match any character _except_ the ones in the set—you can write a caret (`^`) character after the opening bracket.
+
+```
+let nonBinary = /[^01]/;
+console.log(nonBinary.test("1100100010100110"));
+// → false
+console.log(nonBinary.test("0111010112101001"));
+// → true
+```
+
+## International characters
+
+{{index internationalization, Unicode, ["regular expression", internationalization]}}
+
+Because of JavaScript's initial simplistic implementation and the fact that this simplistic approach was later set in stone as ((standard)) behavior, JavaScript's regular expressions are rather dumb about characters that do not appear in the English language. For example, as far as JavaScript's regular expressions are concerned, a "((word character))" is only one of the 26 characters in the Latin alphabet (uppercase or lowercase), decimal digits, and, for some reason, the underscore character. Things like _é_ or _β_, which most definitely are word characters, will not match `\w` (and _will_ match uppercase `\W`, the nonword category).
+
+{{index [whitespace, matching]}}
+
+By a strange historical accident, `\s` (whitespace) does not have this problem and matches all characters that the Unicode standard considers whitespace, including things like the ((nonbreaking space)) and the ((Mongolian vowel separator)).
+
+{{index "character category", [Unicode, property]}}
+
+It is possible to use `\p` in a regular expression to match all characters to which the Unicode standard assigns a given property. This allows us to match things like letters in a more cosmopolitan way. However, again due to compatibility with the original language standards, those are recognized only when you put a `u` character (for ((Unicode))) after the regular expression.
+
+{{table {cols: [1, 5]}}}
+
+| `\p{L}` | Any letter
+| `\p{N}` | Any numeric character
+| `\p{P}` | Any punctuation character
+| `\P{L}` | Any nonletter (uppercase P inverts)
+| `\p{Script=Hangul}` | Any character from the given script (see [Chapter ?](higher_order#scripts))
+
+Using `\w` for text processing that may need to handle non-English text (or even English text with borrowed words like “cliché”) is a liability, since it won't treat characters like “é” as letters. Though they tend to be a bit more verbose, `\p` property groups are more robust.
+
+```{test: never}
+console.log(/\p{L}/u.test("α"));
+// → true
+console.log(/\p{L}/u.test("!"));
+// → false
+console.log(/\p{Script=Greek}/u.test("α"));
+// → true
+console.log(/\p{Script=Arabic}/u.test("α"));
+// → false
+```
+
+{{index "Number function"}}
+
+On the other hand, if you are matching numbers in order to do something with them, you often do want `\d` for digits, since converting arbitrary numeric characters into a JavaScript number is not something that a function like `Number` can do for you.
+
+## Repeating parts of a pattern
+
+{{index ["regular expression", repetition]}}
+
+We now know how to match a single digit. What if we want to match a whole number—a ((sequence)) of one or more ((digit))s?
+
+{{index "plus character", repetition, "+ operator"}}
+
+When you put a plus sign (`+`) after something in a regular expression, it indicates that the element may be repeated more than once. Thus, `/\d+/` matches one or more digit characters.
+
+```
+console.log(/'\d+'/.test("'123'"));
+// → true
+console.log(/'\d+'/.test("''"));
+// → false
+console.log(/'\d*'/.test("'123'"));
+// → true
+console.log(/'\d*'/.test("''"));
+// → true
+```
+
+{{index "* operator", asterisk}}
+
+The star (`*`) has a similar meaning but also allows the pattern to match zero times. Something with a star after it never prevents a pattern from matching—it'll just match zero instances if it can't find any suitable text to match.
+
+{{index "British English", "American English", "question mark"}}
+
+A question mark (`?`) makes a part of a pattern _((optional))_, meaning it may occur zero times or one time. In the following example, the _u_ character is allowed to occur, but the pattern also matches when it is missing:
+
+```
+let neighbor = /neighbou?r/;
+console.log(neighbor.test("neighbour"));
+// → true
+console.log(neighbor.test("neighbor"));
+// → true
+```
+
+{{index repetition, [braces, "in regular expression"]}}
+
+To indicate that a pattern should occur a precise number of times, use braces. Putting `{4}` after an element, for example, requires it to occur exactly four times. It is also possible to specify a ((range)) this way: `{2,4}` means the element must occur at least twice and at most four times.
+
+{{id date_regexp_counted}}
+
+Here is another version of the ((date)) and ((time)) pattern that allows both single- and double-((digit)) days, months, and hours. It is also slightly easier to decipher.
+
+```
+let dateTime = /\d{1,2}-\d{1,2}-\d{4} \d{1,2}:\d{2}/;
+console.log(dateTime.test("1-30-2003 8:45"));
+// → true
+```
+
+You can also specify open-ended ((range))s when using braces by omitting the number after the comma. For example, `{5,}` means five or more times.
+
+## Grouping subexpressions
+
+{{index ["regular expression", grouping], grouping, [parentheses, "in regular expressions"]}}
+
+To use an operator like `*` or `+` on more than one element at a time, you must use parentheses. A part of a regular expression that is enclosed in parentheses counts as a single element as far as the operators following it are concerned.
+
+```
+let cartoonCrying = /boo+(hoo+)+/i;
+console.log(cartoonCrying.test("Boohoooohoohooo"));
+// → true
+```
+
+{{index crying}}
+
+The first and second `+` characters apply only to the second `o` in `boo` and `hoo`, respectively. The third `+` applies to the whole group `(hoo+)`, matching one or more sequences like that.
+
+{{index "case sensitivity", capitalization, ["regular expression", flags]}}
+
+The `i` at the end of the expression in the example makes this regular expression case insensitive, allowing it to match the uppercase _B_ in the input string, even though the pattern is itself all lowercase.
+
+## Matches and groups
+
+{{index ["regular expression", grouping], "exec method", [array, "RegExp match"]}}
+
+The `test` method is the absolute simplest way to match a regular expression. It tells you only whether it matched and nothing else. Regular expressions also have an `exec` (execute) method that will return `null` if no match was found and return an object with information about the match otherwise.
+
+```
+let match = /\d+/.exec("one two 100");
+console.log(match);
+// → ["100"]
+console.log(match.index);
+// → 8
+```
+
+{{index "index property", [string, indexing]}}
+
+An object returned from `exec` has an `index` property that tells us _where_ in the string the successful match begins. Other than that, the object looks like (and in fact is) an array of strings, whose first element is the string that was matched. In the previous example, this is the sequence of ((digit))s that we were looking for.
+
+{{index [string, methods], "match method"}}
+
+String values have a `match` method that behaves similarly.
+
+```
+console.log("one two 100".match(/\d+/));
+// → ["100"]
+```
+
+{{index grouping, "capture group", "exec method"}}
+
+When the regular expression contains subexpressions grouped with parentheses, the text that matched those groups will also show up in the array. The whole match is always the first element. The next element is the part matched by the first group (the one whose opening parenthesis comes first in the expression), then the second group, and so on.
+
+```
+let quotedText = /'([^']*)'/;
+console.log(quotedText.exec("she said 'hello'"));
+// → ["'hello'", "hello"]
+```
+
+{{index "capture group"}}
+
+When a group does not end up being matched at all (for example, when followed by a question mark), its position in the output array will hold `undefined`. When a group is matched multiple times (for example, when followed by a `+`), only the last match ends up in the array.
+
+```
+console.log(/bad(ly)?/.exec("bad"));
+// → ["bad", undefined]
+console.log(/(\d)+/.exec("123"));
+// → ["123", "3"]
+```
+
+If you want to use parentheses purely for grouping, without having them show up in the array of matches, you can put `?:` after the opening parenthesis.
+
+```
+console.log(/(?:na)+/.exec("banana"));
+// → ["nana"]
+```
+
+{{index "exec method", ["regular expression", methods], extraction}}
+
+Groups can be useful for extracting parts of a string. If we don't just want to verify whether a string contains a ((date)) but also extract it and construct an object that represents it, we can wrap parentheses around the digit patterns and directly pick the date out of the result of `exec`.
+
+But first we'll take a brief detour to discuss the built-in way to represent date and ((time)) values in JavaScript.
+
+## The Date class
+
+{{index constructor, "Date class"}}
+
+JavaScript has a standard `Date` class for representing ((date))s, or rather, points in ((time)). If you simply create a date object using `new`, you get the current date and time.
+
+```{test: no}
+console.log(new Date());
+// → Fri Feb 02 2024 18:03:06 GMT+0100 (CET)
+```
+
+{{index "Date class"}}
+
+You can also create an object for a specific time.
+
+```
+console.log(new Date(2009, 11, 9));
+// → Wed Dec 09 2009 00:00:00 GMT+0100 (CET)
+console.log(new Date(2009, 11, 9, 12, 59, 59, 999));
+// → Wed Dec 09 2009 12:59:59 GMT+0100 (CET)
+```
+
+{{index "zero-based counting", [interface, design]}}
+
+JavaScript uses a convention where month numbers start at zero (so December is 11), yet day numbers start at one. This is confusing and silly. Be careful.
+
+The last four arguments (hours, minutes, seconds, and milliseconds) are optional and taken to be zero when not given.
+
+{{index "getTime method", timestamp}}
+
+Timestamps are stored as the number of milliseconds since the start of 1970, in the UTC ((time zone)). This follows a convention set by "((Unix time))", which was invented around that time. You can use negative numbers for times before 1970. The `getTime` method on a date object returns this number. It is big, as you can imagine.
+
+```
+console.log(new Date(2013, 11, 19).getTime());
+// → 1387407600000
+console.log(new Date(1387407600000));
+// → Thu Dec 19 2013 00:00:00 GMT+0100 (CET)
+```
+
+{{index "Date.now function", "Date class"}}
+
+If you give the `Date` constructor a single argument, that argument is treated as such a millisecond count. You can get the current millisecond count by creating a new `Date` object and calling `getTime` on it or by calling the `Date.now` function.
+
+{{index "getFullYear method", "getMonth method", "getDate method", "getHours method", "getMinutes method", "getSeconds method", "getYear method"}}
+
+Date objects provide methods such as `getFullYear`, `getMonth`, `getDate`, `getHours`, `getMinutes`, and `getSeconds` to extract their components. Besides `getFullYear` there's also `getYear`, which gives you the year minus 1900 (such as `98` or `125`) and is mostly useless.
+
+{{index "capture group", "getDate method", [parentheses, "in regular expressions"]}}
+
+Putting parentheses around the parts of the expression that we are interested in, we can now create a date object from a string.
+
+```
+function getDate(string) {
+ let [_, month, day, year] =
+ /(\d{1,2})-(\d{1,2})-(\d{4})/.exec(string);
+ return new Date(year, month - 1, day);
+}
+console.log(getDate("1-30-2003"));
+// → Thu Jan 30 2003 00:00:00 GMT+0100 (CET)
+```
+
+{{index destructuring, "underscore character"}}
+
+The underscore (`_`) binding is ignored and used only to skip the full match element in the array returned by `exec`.
+
+## Boundaries and look-ahead
+
+{{index matching, ["regular expression", boundary]}}
+
+Unfortunately, `getDate` will also happily extract a date from the string `"100-1-30000"`. A match may happen anywhere in the string, so in this case, it'll just start at the second character and end at the second-to-last character.
+
+{{index boundary, "caret character", "dollar sign"}}
+
+If we want to enforce that the match must span the whole string, we can add the markers `^` and `$`. The caret matches the start of the input string, whereas the dollar sign matches the end. Thus `/^\d+$/` matches a string consisting entirely of one or more digits, `/^!/` matches any string that starts with an exclamation mark, and `/x^/` does not match any string (there cannot be an `x` before the start of the string).
+
+{{index "word boundary", "word character"}}
+
+There is also a `\b` marker that matches _word boundaries_, positions that have a word character on one side, and a nonword character on the other. Unfortunately, these use the same simplistic concept of word characters as `\w` and are therefore not very reliable.
+
+Note that these boundary markers don't match any actual characters. They just enforce that a given condition holds at the place where it appears in the pattern.
+
+{{index "look-ahead"}}
+
+_Look-ahead_ tests do something similar. They provide a pattern and will make the match fail if the input doesn't match that pattern, but don't actually move the match position forward. They are written between `(?=` and `)`.
+
+```
+console.log(/a(?=e)/.exec("braeburn"));
+// → ["a"]
+console.log(/a(?! )/.exec("a b"));
+// → null
+```
+
+The `e` in the first example is necessary to match, but is not part of the matched string. The `(?! )` notation expresses a _negative_ look-ahead. This matches only if the pattern in the parentheses _doesn't_ match, causing the second example to match only `a` characters that don't have a space after them.
+
+## Choice patterns
+
+{{index branching, ["regular expression", alternatives], "farm example"}}
+
+Say we want to know whether a piece of text contains not only a number but a number followed by one of the words _pig_, _cow_, or _chicken_, or any of their plural forms.
+
+We could write three regular expressions and test them in turn, but there is a nicer way. The ((pipe character)) (`|`) denotes a ((choice)) between the pattern to its left and the pattern to its right. We can use it in expressions like this:
+
+```
+let animalCount = /\d+ (pig|cow|chicken)s?/;
+console.log(animalCount.test("15 pigs"));
+// → true
+console.log(animalCount.test("15 pugs"));
+// → false
+```
+
+{{index [parentheses, "in regular expressions"]}}
+
+Parentheses can be used to limit the part of the pattern to which the pipe operator applies, and you can put multiple such operators next to each other to express a choice between more than two alternatives.
+
+## The mechanics of matching
+
+{{index ["regular expression", matching], [matching, algorithm], "search problem"}}
+
+Conceptually, when you use `exec` or `test`, the regular expression engine looks for a match in your string by trying to match the expression first from the start of the string, then from the second character, and so on until it finds a match or reaches the end of the string. It'll either return the first match that can be found or fail to find any match at all.
+
+{{index ["regular expression", matching], [matching, algorithm]}}
+
+To do the actual matching, the engine treats a regular expression something like a ((flow diagram)). This is the diagram for the livestock expression in the previous example:
+
+{{figure {url: "img/re_pigchickens.svg", alt: "Railroad diagram that first passes through a box labeled 'digit', which has a loop going back from after it to before it, and then a box for a space character. After that, the railroad splits in three, going through boxes for 'pig', 'cow', and 'chicken'. After those it rejoins, and goes through a box labeled 's', which, being optional, also has a railroad that passes it by. Finally, the line reaches the accepting state."}}}
+
+{{index traversal}}
+
+If we can find a path from the left side of the diagram to the right side, our expression matches. We keep a current position in the string, and every time we move through a box, we verify that the part of the string after our current position matches that box.
+
+{{id backtracking}}
+
+## Backtracking
+
+{{index ["regular expression", backtracking], "binary number", "decimal number", "hexadecimal number", "flow diagram", [matching, algorithm], backtracking}}
+
+The regular expression `/^([01]+b|[\da-f]+h|\d+)$/` matches either a binary number followed by a `b`, a hexadecimal number (that is, base 16, with the letters `a` to `f` standing for the digits 10 to 15) followed by an `h`, or a regular decimal number with no suffix character. This is the corresponding diagram:
+
+{{figure {url: "img/re_number.svg", alt: "Railroad diagram for the regular expression '^([01]+b|\\d+|[\\da-f]+h)$'"}}}
+
+{{index branching}}
+
+When matching this expression, the top (binary) branch will often be entered even though the input does not actually contain a binary number. When matching the string `"103"`, for example, it becomes clear only at the `3` that we are in the wrong branch. The string _does_ match the expression, just not the branch we are currently in.
+
+{{index backtracking, "search problem"}}
+
+So the matcher _backtracks_. When entering a branch, it remembers its current position (in this case, at the start of the string, just past the first boundary box in the diagram) so that it can go back and try another branch if the current one does not work out. For the string `"103"`, after encountering the `3` character, the matcher starts trying the branch for hexadecimal numbers, which fails again because there is no `h` after the number. It then tries the decimal number branch. This one fits, and a match is reported after all.
+
+{{index [matching, algorithm]}}
+
+The matcher stops as soon as it finds a full match. This means that if multiple branches could potentially match a string, only the first one (ordered by where the branches appear in the regular expression) is used.
+
+Backtracking also happens for ((repetition)) operators like + and `*`. If you match `/^.*x/` against `"abcxe"`, the `.*` part will first try to consume the whole string. The engine will then realize that it needs an `x` to match the pattern. Since there is no `x` past the end of the string, the star operator tries to match one character less. But the matcher doesn't find an `x` after `abcx` either, so it backtracks again, matching the star operator to just `abc`. _Now_ it finds an `x` where it needs it and reports a successful match from positions 0 to 4.
+
+{{index performance, complexity}}
+
+It is possible to write regular expressions that will do a _lot_ of backtracking. This problem occurs when a pattern can match a piece of input in many different ways. For example, if we get confused while writing a binary-number regular expression, we might accidentally write something like `/([01]+)+b/`.
+
+{{figure {url: "img/re_slow.svg", alt: "Railroad diagram for the regular expression '([01]+)+b'",width: "6cm"}}}
+
+{{index "inner loop", [nesting, "in regexps"]}}
+
+If that tries to match some long series of zeros and ones with no trailing _b_ character, the matcher first goes through the inner loop until it runs out of digits. Then it notices there is no _b_, so it backtracks one position, goes through the outer loop once, and gives up again, trying to backtrack out of the inner loop once more. It will continue to try every possible route through these two loops. This means the amount of work _doubles_ with each additional character. For even just a few dozen characters, the resulting match will take practically forever.
+
+## The replace method
+
+{{index "replace method", "regular expression"}}
+
+String values have a `replace` method that can be used to replace part of the string with another string.
+
+```
+console.log("papa".replace("p", "m"));
+// → mapa
+```
+
+{{index ["regular expression", flags], ["regular expression", global]}}
+
+The first argument can also be a regular expression, in which case the first match of the regular expression is replaced. When a `g` option (for _global_) is added after the regular expression, _all_ matches in the string will be replaced, not just the first.
+
+```
+console.log("Borobudur".replace(/[ou]/, "a"));
+// → Barobudur
+console.log("Borobudur".replace(/[ou]/g, "a"));
+// → Barabadar
+```
+
+{{index grouping, "capture group", "dollar sign", "replace method", ["regular expression", grouping]}}
+
+The real power of using regular expressions with `replace` comes from the fact that we can refer to matched groups in the replacement string. For example, say we have a big string containing the names of people, one name per line, in the format `Lastname, Firstname`. If we want to swap these names and remove the comma to get a `Firstname Lastname` format, we can use the following code:
+
+```
+console.log(
+ "Liskov, Barbara\nMcCarthy, John\nMilner, Robin"
+ .replace(/(\p{L}+), (\p{L}+)/gu, "$2 $1"));
+// → Barbara Liskov
+// John McCarthy
+// Robin Milner
+```
+
+The `$1` and `$2` in the replacement string refer to the parenthesized groups in the pattern. `$1` is replaced by the text that matched against the first group, `$2` by the second, and so on, up to `$9`. The whole match can be referred to with `$&`.
+
+{{index [function, "higher-order"], grouping, "capture group"}}
+
+It is possible to pass a function—rather than a string—as the second argument to `replace`. For each replacement, the function will be called with the matched groups (as well as the whole match) as arguments, and its return value will be inserted into the new string.
+
+Here's an example:
+
+```
+let stock = "1 lemon, 2 cabbages, and 101 eggs";
+function minusOne(match, amount, unit) {
+ amount = Number(amount) - 1;
+ if (amount == 1) { // only one left, remove the 's'
+ unit = unit.slice(0, unit.length - 1);
+ } else if (amount == 0) {
+ amount = "no";
+ }
+ return amount + " " + unit;
+}
+console.log(stock.replace(/(\d+) (\p{L}+)/gu, minusOne));
+// → no lemon, 1 cabbage, and 100 eggs
+```
+
+This code takes a string, finds all occurrences of a number followed by an alphanumeric word, and returns a string that has one less of every such quantity.
+
+The `(\d+)` group ends up as the `amount` argument to the function, and the `(\p{L}+)` group gets bound to `unit`. The function converts `amount` to a number—which always works, since it matched `\d+` earlier—and makes some adjustments in case there is only one or zero left.
+
+## Greed
+
+{{index greed, "regular expression"}}
+
+We can use `replace` to write a function that removes all ((comment))s from a piece of JavaScript ((code)). Here is a first attempt:
+
+```{test: wrap}
+function stripComments(code) {
+ return code.replace(/\/\/.*|\/\*[^]*\*\//g, "");
+}
+console.log(stripComments("1 + /* 2 */3"));
+// → 1 + 3
+console.log(stripComments("x = 10;// ten!"));
+// → x = 10;
+console.log(stripComments("1 /* a */+/* b */ 1"));
+// → 1 1
+```
+
+{{index "period character", "slash character", "newline character", "empty set", "block comment", "line comment"}}
+
+The part before the `|` operator matches two slash characters followed by any number of non-newline characters. The part for multiline comments is more involved. We use `[^]` (any character that is not in the empty set of characters) as a way to match any character. We cannot just use a period here because block comments can continue on a new line, and the period character does not match newline characters.
+
+But the output for the last line appears to have gone wrong. Why?
+
+{{index backtracking, greed, "regular expression"}}
+
+The `[^]*` part of the expression, as I described in the section on backtracking, will first match as much as it can. If that causes the next part of the pattern to fail, the matcher moves back one character and tries again from there. In the example, the matcher first tries to match the whole rest of the string and then moves back from there. It will find an occurrence of `*/` after going back four characters and match that. This is not what we wanted—the intention was to match a single comment, not to go all the way to the end of the code and find the end of the last block comment.
+
+Because of this behavior, we say the repetition operators (`+`, `*`, `?`, and `{}`) are _((greed))y_, meaning they match as much as they can and backtrack from there. If you put a ((question mark)) after them (`+?`, `*?`, `??`, `{}?`), they become nongreedy and start by matching as little as possible, matching more only when the remaining pattern does not fit the smaller match.
+
+And that is exactly what we want in this case. By having the star match the smallest stretch of characters that brings us to a `*/`, we consume one block comment and nothing more.
+
+```{test: wrap}
+function stripComments(code) {
+ return code.replace(/\/\/.*|\/\*[^]*?\*\//g, "");
+}
+console.log(stripComments("1 /* a */+/* b */ 1"));
+// → 1 + 1
+```
+
+A lot of ((bug))s in ((regular expression)) programs can be traced to unintentionally using a greedy operator where a nongreedy one would work better. When using a ((repetition)) operator, prefer the nongreedy variant.
+
+## Dynamically creating RegExp objects
+
+{{index ["regular expression", creation], "underscore character", "RegExp class"}}
+
+In some cases you may not know the exact ((pattern)) you need to match against when you are writing your code. Say you want to test for the user's name in a piece of text. You can build up a string and use the `RegExp` ((constructor)) on that.
+
+```
+let name = "harry";
+let regexp = new RegExp("(^|\\s)" + name + "($|\\s)", "gi");
+console.log(regexp.test("Harry is a dodgy character."));
+// → true
+```
+
+{{index ["regular expression", flags], ["backslash character", "in regular expressions"]}}
+
+When creating the `\s` part of the string, we have to use two backslashes because we are writing them in a normal string, not a slash-enclosed regular expression. The second argument to the `RegExp` constructor contains the options for the regular expression—in this case, `"gi"` for global and case insensitive.
+
+But what if the name is `"dea+hl[]rd"` because our user is a ((nerd))y teenager? That would result in a nonsensical regular expression that won't actually match the user's name.
+
+{{index ["backslash character", "in regular expressions"], [escaping, "in regexps"], ["regular expression", escaping]}}
+
+To work around this, we can add backslashes before any character that has a special meaning.
+
+```
+let name = "dea+hl[]rd";
+let escaped = name.replace(/[\\[.+*?(){|^$]/g, "\\$&");
+let regexp = new RegExp("(^|\\s)" + escaped + "($|\\s)",
+ "gi");
+let text = "This dea+hl[]rd guy is super annoying.";
+console.log(regexp.test(text));
+// → true
+```
+
+## The search method
+
+{{index ["regular expression", methods], "indexOf method", "search method"}}
+
+While the `indexOf` method on strings cannot be called with a regular expression, there is another method, `search`, that does expect a regular expression. Like `indexOf`, it returns the first index on which the expression was found, or -1 when it wasn't found.
+
+```
+console.log(" word".search(/\S/));
+// → 2
+console.log(" ".search(/\S/));
+// → -1
+```
+
+Unfortunately, there is no way to indicate that the match should start at a given offset (like we can with the second argument to `indexOf`), which would often be useful.
+
+## The lastIndex property
+
+{{index "exec method", "regular expression"}}
+
+The `exec` method similarly does not provide a convenient way to start searching from a given position in the string. But it does provide an *in*convenient way.
+
+{{index ["regular expression", matching], matching, "source property", "lastIndex property"}}
+
+Regular expression objects have properties. One such property is `source`, which contains the string that expression was created from. Another property is `lastIndex`, which controls, in some limited circumstances, where the next match will start.
+
+{{index [interface, design], "exec method", ["regular expression", global]}}
+
+Those circumstances are that the regular expression must have the global (`g`) or sticky (`y`) option enabled, and the match must happen through the `exec` method. Again, a less confusing solution would have been to just allow an extra argument to be passed to `exec`, but confusion is an essential feature of JavaScript's regular expression interface.
+
+```
+let pattern = /y/g;
+pattern.lastIndex = 3;
+let match = pattern.exec("xyzzy");
+console.log(match.index);
+// → 4
+console.log(pattern.lastIndex);
+// → 5
+```
+
+{{index "side effect", "lastIndex property"}}
+
+If the match was successful, the call to `exec` automatically updates the `lastIndex` property to point after the match. If no match was found, `lastIndex` is set back to 0, which is also the value it has in a newly constructed regular expression object.
+
+The difference between the global and the sticky options is that when sticky is enabled, the match will succeed only if it starts directly at `lastIndex`, whereas with global, it will search ahead for a position where a match can start.
+
+```
+let global = /abc/g;
+console.log(global.exec("xyz abc"));
+// → ["abc"]
+let sticky = /abc/y;
+console.log(sticky.exec("xyz abc"));
+// → null
+```
+
+{{index bug}}
+
+When using a shared regular expression value for multiple `exec` calls, these automatic updates to the `lastIndex` property can cause problems. Your regular expression might be accidentally starting at an index left over from a previous call.
+
+```
+let digit = /\d/g;
+console.log(digit.exec("here it is: 1"));
+// → ["1"]
+console.log(digit.exec("and now: 1"));
+// → null
+```
+
+{{index ["regular expression", global], "match method"}}
+
+Another interesting effect of the global option is that it changes the way the `match` method on strings works. When called with a global expression, instead of returning an array similar to that returned by `exec`, `match` will find _all_ matches of the pattern in the string and return an array containing the matched strings.
+
+```
+console.log("Banana".match(/an/g));
+// → ["an", "an"]
+```
+
+So be cautious with global regular expressions. The cases where they are necessary—calls to `replace` and places where you want to explicitly use `lastIndex`—are typically the situations where you want to use them.
+
+{{index "lastIndex property", "exec method", loop}}
+
+A common thing to do is to find all the matches of a regular expression in a string. We can do this by using the `matchAll` method.
+
+```
+let input = "A string with 3 numbers in it... 42 and 88.";
+let matches = input.matchAll(/\d+/g);
+for (let match of matches) {
+ console.log("Found", match[0], "at", match.index);
+}
+// → Found 3 at 14
+// Found 42 at 33
+// Found 88 at 40
+```
+
+{{index ["regular expression", global]}}
+
+This method returns an array of match arrays. The regular expression given to `matchAll` _must_ have `g` enabled.
+
+{{id ini}}
+## Parsing an INI file
+
+{{index comment, "file format", "enemies example", "INI file"}}
+
+To conclude the chapter, we'll look at a problem that calls for ((regular expression))s. Imagine we are writing a program to automatically collect information about our enemies from the ((internet)). (We will not actually write that program here, just the part that reads the ((configuration)) file. Sorry.) The configuration file looks like this:
+
+```{lang: "null"}
+searchengine=https://duckduckgo.com/?q=$1
+spitefulness=9.7
+
+; comments are preceded by a semicolon...
+; each section concerns an individual enemy
+[larry]
+fullname=Larry Doe
+type=kindergarten bully
+website=http://www.geocities.com/CapeCanaveral/11451
+
+[davaeorn]
+fullname=Davaeorn
+type=evil wizard
+outputdir=/home/marijn/enemies/davaeorn
+```
+
+{{index grammar}}
+
+The exact rules for this format—which is a widely used file format, usually called an _INI_ file—are as follows:
+
+- Blank lines and lines starting with semicolons are ignored.
+
+- Lines wrapped in `[` and `]` start a new ((section)).
+
+- Lines containing an alphanumeric identifier followed by an `=` character add a setting to the current section.
+
+- Anything else is invalid.
+
+Our task is to convert a string like this into an object whose properties hold strings for settings written before the first section header and subobjects for sections, with those subobjects holding the section's settings.
+
+{{index "carriage return", "line break", "newline character"}}
+
+Since the format has to be processed ((line)) by line, splitting up the file into separate lines is a good start. We saw the `split` method in [Chapter ?](data#split). Some operating systems, however, use not just a newline character to separate lines but a carriage return character followed by a newline (`"\r\n"`). Given that the `split` method also allows a regular expression as its argument, we can use a regular expression like `/\r?\n/` to split in a way that allows both `"\n"` and `"\r\n"` between lines.
+
+```{startCode: true}
+function parseINI(string) {
+ // Start with an object to hold the top-level fields
+ let result = {};
+ let section = result;
+ for (let line of string.split(/\r?\n/)) {
+ let match;
+ if (match = line.match(/^(\w+)=(.*)$/)) {
+ section[match[1]] = match[2];
+ } else if (match = line.match(/^\[(.*)\]$/)) {
+ section = result[match[1]] = {};
+ } else if (!/^\s*(;|$)/.test(line)) {
+ throw new Error("Line '" + line + "' is not valid.");
+ }
+ };
+ return result;
+}
+
+console.log(parseINI(`
+name=Vasilis
+[address]
+city=Tessaloniki`));
+// → {name: "Vasilis", address: {city: "Tessaloniki"}}
+```
+
+{{index "parseINI function", parsing}}
+
+The code goes over the file's lines and builds up an object. Properties at the top are stored directly into that object, whereas properties found in sections are stored in a separate section object. The `section` binding points at the object for the current section.
+
+There are two kinds of significant lines—section headers or property lines. When a line is a regular property, it is stored in the current section. When it is a section header, a new section object is created, and `section` is set to point at it.
+
+{{index "caret character", "dollar sign", boundary}}
+
+Note the recurring use of `^` and `$` to make sure the expression matches the whole line, not just part of it. Leaving these out results in code that mostly works but behaves strangely for some input, which can be a difficult bug to track down.
+
+{{index "if keyword", assignment, ["= operator", "as expression"]}}
+
+The pattern `if (match = string.match(...))` makes use of the fact that the value of an ((assignment)) expression (`=`) is the assigned value. You often aren't sure that your call to `match` will succeed, so you can access the resulting object only inside an `if` statement that tests for this. To not break the pleasant chain of `else if` forms, we assign the result of the match to a binding and immediately use that assignment as the test for the `if` statement.
+
+{{index [parentheses, "in regular expressions"]}}
+
+If a line is not a section header or a property, the function checks whether it is a comment or an empty line using the expression `/^\s*(;|$)/` to match lines that either contain only whitespace, or whitespace followed by a semicolon (making the rest of the line a comment). When a line doesn't match any of the expected forms, the function throws an exception.
+
+## Code units and characters
+
+Another design mistake that's been standardized in JavaScript regular expressions is that by default, operators like `.` or `?` work on code units (as discussed in [Chapter ?](higher_order#code_units)), not actual characters. This means characters that are composed of two code units behave strangely.
+
+```
+console.log(/🍎{3}/.test("🍎🍎🍎"));
+// → false
+console.log(/<.>/.test("<🌹>"));
+// → false
+console.log(/<.>/u.test("<🌹>"));
+// → true
+```
+
+The problem is that the 🍎 in the first line is treated as two code units, and `{3}` is applied only to the second unit. Similarly, the dot matches a single code unit, not the two that make up the rose ((emoji)).
+
+You must add the `u` (Unicode) option to your regular expression to make it treat such characters properly.
+
+```
+console.log(/🍎{3}/u.test("🍎🍎🍎"));
+// → true
+```
+
+{{id summary_regexp}}
+
+## Summary
+
+Regular expressions are objects that represent patterns in strings. They use their own language to express these patterns.
+
+{{table {cols: [1, 5]}}}
+
+| `/abc/` | A sequence of characters
+| `/[abc]/` | Any character from a set of characters
+| `/[^abc]/` | Any character _not_ in a set of characters
+| `/[0-9]/` | Any character in a range of characters
+| `/x+/` | One or more occurrences of the pattern `x`
+| `/x+?/` | One or more occurrences, nongreedy
+| `/x*/` | Zero or more occurrences
+| `/x?/` | Zero or one occurrence
+| `/x{2,4}/` | Two to four occurrences
+| `/(abc)/` | A group
+| `/a|b|c/` | Any one of several patterns
+| `/\d/` | Any digit character
+| `/\w/` | An alphanumeric character ("word character")
+| `/\s/` | Any whitespace character
+| `/./` | Any character except newlines
+| `/\p{L}/u` | Any letter character
+| `/^/` | Start of input
+| `/$/` | End of input
+| `/(?=a)/` | A look-ahead test
+
+A regular expression has a method `test` to test whether a given string matches it. It also has a method `exec` that, when a match is found, returns an array containing all matched groups. Such an array has an `index` property that indicates where the match started.
+
+Strings have a `match` method to match them against a regular expression and a `search` method to search for one, returning only the starting position of the match. Their `replace` method can replace matches of a pattern with a replacement string or function.
+
+Regular expressions can have options, which are written after the closing slash. The `i` option makes the match case insensitive. The `g` option makes the expression _global_, which, among other things, causes the `replace` method to replace all instances instead of just the first. The `y` option makes and expression sticky, which means that it will not search ahead and skip part of the string when looking for a match. The `u` option turns on Unicode mode, which enables `\p` syntax and fixes a number of problems around the handling of characters that take up two code units.
+
+Regular expressions are a sharp ((tool)) with an awkward handle. They simplify some tasks tremendously but can quickly become unmanageable when applied to complex problems. Part of knowing how to use them is resisting the urge to try to shoehorn things into them that they cannot cleanly express.
+
+## Exercises
+
+{{index debugging, bug}}
+
+It is almost unavoidable that, in the course of working on these exercises, you will get confused and frustrated by some regular expression's inexplicable ((behavior)). Sometimes it helps to enter your expression into an online tool like [_debuggex.com_](https://www.debuggex.com) to see whether its visualization corresponds to what you intended and to ((experiment)) with the way it responds to various input strings.
+
+### Regexp golf
+
+{{index "program size", "code golf", "regexp golf (exercise)"}}
+
+_Code golf_ is a term used for the game of trying to express a particular program in as few characters as possible. Similarly, _regexp golf_ is the practice of writing as tiny a regular expression as possible to match a given pattern and _only_ that pattern.
+
+{{index boundary, matching}}
+
+For each of the following items, write a ((regular expression)) to test whether the given pattern occurs in a string. The regular expression should match only strings containing the pattern. When your expression works, see whether you can make it any smaller.
+
+ 1. _car_ and _cat_
+ 2. _pop_ and _prop_
+ 3. _ferret_, _ferry_, and _ferrari_
+ 4. Any word ending in _ious_
+ 5. A whitespace character followed by a period, comma, colon, or semicolon
+ 6. A word longer than six letters
+ 7. A word without the letter _e_ (or _E_)
+
+Refer to the table in the [chapter summary](regexp#summary_regexp) for help. Test each solution with a few test strings.
+
+{{if interactive
+```
+// Fill in the regular expressions
+
+verify(/.../,
+ ["my car", "bad cats"],
+ ["camper", "high art"]);
+
+verify(/.../,
+ ["pop culture", "mad props"],
+ ["plop", "prrrop"]);
+
+verify(/.../,
+ ["ferret", "ferry", "ferrari"],
+ ["ferrum", "transfer A"]);
+
+verify(/.../,
+ ["how delicious", "spacious room"],
+ ["ruinous", "consciousness"]);
+
+verify(/.../,
+ ["bad punctuation ."],
+ ["escape the period"]);
+
+verify(/.../,
+ ["Siebentausenddreihundertzweiundzwanzig"],
+ ["no", "three small words"]);
+
+verify(/.../,
+ ["red platypus", "wobbling nest"],
+ ["earth bed", "bedrøvet abe", "BEET"]);
+
+
+function verify(regexp, yes, no) {
+ // Ignore unfinished exercises
+ if (regexp.source == "...") return;
+ for (let str of yes) if (!regexp.test(str)) {
+ console.log(`Failure to match '${str}'`);
+ }
+ for (let str of no) if (regexp.test(str)) {
+ console.log(`Unexpected match for '${str}'`);
+ }
+}
+```
+
+if}}
+
+### Quoting style
+
+{{index "quoting style (exercise)", "single-quote character", "double-quote character"}}
+
+Imagine you have written a story and used single ((quotation mark))s throughout to mark pieces of dialogue. Now you want to replace all the dialogue quotes with double quotes, while keeping the single quotes used in contractions like _aren't_.
+
+{{index "replace method"}}
+
+Think of a pattern that distinguishes these two kinds of quote usage and craft a call to the `replace` method that does the proper replacement.
+
+{{if interactive
+```{test: no}
+let text = "'I'm the cook,' he said, 'it's my job.'";
+// Change this call.
+console.log(text.replace(/A/g, "B"));
+// → "I'm the cook," he said, "it's my job."
+```
+if}}
+
+{{hint
+
+{{index "quoting style (exercise)", boundary}}
+
+The most obvious solution is to replace only quotes with a nonletter character on at least one side—something like `/\P{L}'|'\P{L}/u`. But you also have to take the start and end of the line into account.
+
+{{index grouping, "replace method", [parentheses, "in regular expressions"]}}
+
+In addition, you must ensure that the replacement also includes the characters that were matched by the `\P{L}` pattern so that those are not dropped. This can be done by wrapping them in parentheses and including their groups in the replacement string (`$1`, `$2`). Groups that are not matched will be replaced by nothing.
+
+hint}}
+
+### Numbers again
+
+{{index sign, "fractional number", [syntax, number], minus, "plus character", exponent, "scientific notation", "period character"}}
+
+Write an expression that matches only JavaScript-style ((number))s. It must support an optional minus _or_ plus sign in front of the number, the decimal dot, and exponent notation—`5e-3` or `1E10`—again with an optional sign in front of the exponent. Also note that it is not necessary for there to be digits in front of or after the dot, but the number cannot be a dot alone. That is, `.5` and `5.` are valid JavaScript numbers, but a lone dot isn't.
+
+{{if interactive
+```{test: no}
+// Fill in this regular expression.
+let number = /^...$/;
+
+// Tests:
+for (let str of ["1", "-1", "+15", "1.55", ".5", "5.",
+ "1.3e2", "1E-4", "1e+12"]) {
+ if (!number.test(str)) {
+ console.log(`Failed to match '${str}'`);
+ }
+}
+for (let str of ["1a", "+-1", "1.2.3", "1+1", "1e4.5",
+ ".5.", "1f5", "."]) {
+ if (number.test(str)) {
+ console.log(`Incorrectly accepted '${str}'`);
+ }
+}
+```
+
+if}}
+
+{{hint
+
+{{index ["regular expression", escaping], ["backslash character", "in regular expressions"]}}
+
+First, do not forget the backslash in front of the period.
+
+Matching the optional ((sign)) in front of the ((number)), as well as in front of the ((exponent)), can be done with `[+\-]?` or `(\+|-|)` (plus, minus, or nothing).
+
+{{index "pipe character"}}
+
+The more complicated part of the exercise is the problem of matching both `"5."` and `".5"` without also matching `"."`. For this, a good solution is to use the `|` operator to separate the two cases—either one or more digits optionally followed by a dot and zero or more digits _or_ a dot followed by one or more digits.
+
+{{index exponent, "case sensitivity", ["regular expression", flags]}}
+
+Finally, to make the _e_ case insensitive, either add an `i` option to the regular expression or use `[eE]`.
+
+hint}}
diff --git a/09_regexp.txt b/09_regexp.txt
deleted file mode 100644
index 492c4edc4..000000000
--- a/09_regexp.txt
+++ /dev/null
@@ -1,1323 +0,0 @@
-:chap_num: 9
-:prev_link: 08_error
-:next_link: 10_modules
-
-= Regular Expressions =
-
-[chapterquote="true"]
-[quote,Jamie Zawinski]
-____
-Some people, when confronted with a
-problem, think ‘I know, I'll use regular expressions.’ Now they have
-two problems.
-____
-
-ifdef::html_target[]
-
-[chapterquote="true"]
-[quote, Master Yuan-Ma, The Book of Programming]
-____
-Yuan-Ma said, ‘When you cut against the grain of the wood, much
-strength is needed. When you program against the grain of a problem,
-much code is needed.’
-____
-
-endif::html_target[]
-
-(((Zawinski+++,+++
-Jamie)))(((evolution)))(((adoption)))(((integration)))Programming
-((tool))s and techniques survive and spread in a chaotic, evolutionary
-way. It's not always the pretty or brilliant ones that win, but rather
-the ones that function well enough within the right niche—for example
-by being integrated with another successful piece of technology.
-
-(((domain-specific language)))In this chapter I will discuss one such
-tool, _((regular expression))s_. Regular expressions are a way to
-describe ((pattern))s in string data. They form a small, separate
-language that is part of JavaScript and many other languages and
-tools.
-
-(((interface,design)))Regular expressions are both terribly awkward
-and extremely useful. Their syntax is cryptic, and the programming
-((interface)) JavaScript provides for them is clumsy. But they are a
-very powerful ((tool)) for inspecting and processing strings. Properly
-understanding regular expressions will make you a more effective
-programmer.
-
-== Creating a regular expression ==
-
-(((regular expression,creation)))(((RegExp constructor)))(((literal
-expression)))(((slash character)))A regular expression is a type of
-object. It can either be constructed with the `RegExp` constructor or
-written as a literal value by enclosing the pattern in forward slash
-(`/`) characters.
-
-[source,javascript]
-----
-var re1 = new RegExp("abc");
-var re2 = /abc/;
-----
-
-Both of these regular expression objects represent the same
-((pattern)): an “a” character followed by a “b” followed by a “c”.
-
-(((backslash character)))(((RegExp constructor)))When using the
-`RegExp` constructor, the pattern is written as a normal string, so
-the usual rules apply for backslashes.
-
-(((regular expression,escaping)))(((escaping,in regexps)))(((slash
-character)))The second notation, where the pattern appears between
-slash characters, treats backslashes somewhat differently. Firstly,
-since a forward slash ends the pattern, we need to put a backslash
-before any forward slash that we want to be _part_ of the pattern. In
-addition, backslashes that aren't part of special character codes
-(like `\n`), will be _preserved_, rather than ignored as they are in
-strings, and change the meaning of the pattern. Some characters, such
-as question marks and plus signs, have special meanings in regular
-expressions, and must be preceded by a backslash if they are meant to
-represent the character itself.
-
-[source,javascript]
-----
-var eighteenPlus = /eighteen\+/;
-----
-
-Knowing precisely what characters to backslash-escape when writing
-regular expressions requires you to know every character with a
-special meaning. For the time being, this may not be realistic, so
-when in doubt, just put a backslash before any character that is not a
-letter, number, or ((whitespace)).
-
-== Testing for matches ==
-
-(((matching)))(((test method)))(((regular expression,methods)))Regular
-expression objects have a number of methods. The simplest one is
-`test`. If you pass it a string, it will return a ((Boolean)) telling
-you whether the string contains a match of the pattern in the
-expression.
-
-[source,javascript]
-----
-console.log(/abc/.test("abcde"));
-// → true
-console.log(/abc/.test("abxde"));
-// → false
-----
-
-(((pattern)))A ((regular expression)) consisting of only non-special
-characters simply represents that sequence of characters. If “abc”
-occurs anywhere in the string we are testing against (not just at the
-start), `test` will return `true`.
-
-== Matching a set of characters ==
-
-(((regular expression)))(((indexOf method)))Finding out whether a
-string contains “abc” could just as well be done with a call to
-`indexOf`. Regular expressions allow us to go beyond that, and express
-more complicated ((pattern))s.
-
-Say we want to match any ((number)). In a regular expression, putting
-a ((set)) of characters between square brackets makes that part of the
-expression match any of the characters between the brackets.
-
-Both expressions below match all strings that contain a ((digit)).
-
-[source,javascript]
-----
-console.log(/[0123456789]/.test("in 1992"));
-// → true
-console.log(/[0-9]/.test("in 1992"));
-// → true
-----
-
-(((dash character)))Within square brackets, a dash (`-`) between two
-characters can be used to indicate a ((range)) of characters, where
-the ordering is determined by the character's ((Unicode)) number.
-Characters “0” to “9” sit right next to each other in this ordering
-(codes 48 to 57), so `[0-9]` covers all of them, and matches any
-((digit)).
-
-(((whitespace)))(((alphanumeric character)))(((period
-character)))There are a number of common character groups that have
-their own built-in shortcuts. Digits are one of them: `\d` means the
-same thing as `[0-9]`.
-
-[cols="1,5"]
-|====
-|`\d` |Any ((digit)) character
-|`\w` |An alphanumeric character (“((word character))”)
-|`\s` |Any ((whitespace)) character (space, tab, newline, and similar)
-|`\D` |A characters that is _not_ a digit
-|`\W` |A non-alphanumeric character
-|`\S` |A non-whitespace character
-|`.` |Any character except for newline(((newline character)))
-|====
-
-So you could match a ((date)) and ((time)) format like “30-01-2003
-15:20” with the following expression:
-
-[source,javascript]
-----
-var dateTime = /\d\d-\d\d-\d\d\d\d \d\d:\d\d/;
-console.log(dateTime.test("30-01-2003 15:20"));
-// → true
-console.log(dateTime.test("30-jan-2003 15:20"));
-// → false
-----
-
-(((backslash character)))Looks completely awful, doesn't it? Way too
-many backslashes, producing a background noise that makes it hard to
-spot the actual ((pattern)) expressed. We'll see a slightly improved
-version of this expression
-link:09_regexp.html#date_regexp_counted[later on].
-
-(((escaping,in regexps)))(((regular expression)))(((set)))These
-backslash codes can also be used inside of ((square brackets)). For
-example `[\d.]` means any digit or a period character. But note that
-the period itself, when used between square brackets, loses its
-special meaning. The same goes for other special characters, such as
-`+`.
-
-(((square brackets)))(((inversion)))(((caret character)))To _invert_ a
-set of characters—that is, to express that you want to match any
-character _except_ the ones in the set—you can write a caret (`^`)
-character after the opening bracket.
-
-[source,javascript]
-----
-var notBinary = /[^01]/;
-console.log(notBinary.test("1100100010100110"));
-// → false
-console.log(notBinary.test("1100100010200110"));
-// → true
-----
-
-== Repeating parts of a pattern ==
-
-(((regular expression,repetition)))We now know how to match a single digit. What
-if we want to match a whole number—a ((sequence)) of one or more
-((digit))s?
-
-(((plus character)))(((repetition)))(((+ operator)))When you put a
-plus sign (`+`) after something in a regular expression, it indicates
-that the element may be repeated more than once. Thus, `/\d+/` matches
-one or more digit characters.
-
-[source,javascript]
-----
-console.log(/'\d+'/.test("'123'"));
-// → true
-console.log(/'\d+'/.test("''"));
-// → false
-console.log(/'\d*'/.test("'123'"));
-// → true
-console.log(/'\d*'/.test("''"));
-// → true
-----
-
-(((pass:[*] operator)))(((asterisk)))The star (`*`) has a similar
-meaning, but also allows the pattern to match zero times. Something
-with a star after it never prevents a pattern from matching—it'll just
-match zero instances if it can't find any suitable text to match.
-
-(((British English)))(((American English)))(((question mark)))A
-question mark makes a part of a pattern “((optional))”, meaning it may
-occur zero or one times. In the following example, the “u” character
-is allowed to occur, but the pattern also matches when it is missing.
-
-[source,javascript]
-----
-var neighbor = /neighbou?r/;
-console.log(neighbor.test("neighbour"));
-// → true
-console.log(neighbor.test("neighbor"));
-// → true
-----
-
-(((repetition)))(((curly braces)))To indicate that a pattern should
-occur a precise number of times, use curly braces. Putting `{4}` after
-an element, for example, requires it to occur exactly four times. It
-is also possible to specify a ((range)) this way: `{2,4}` means the
-element must occur at least twice, and at most four times.
-
-[[date_regexp_counted]]
-Here is another version of the ((date)) and ((time)) pattern that
-allows both single- and double-((digit)) days, months, and hours. It
-is also slightly more readable:
-
-[source,javascript]
-----
-var dateTime = /\d{1,2}-\d{1,2}-\d{4} \d{1,2}:\d{2}/;
-console.log(dateTime.test("30-1-2003 8:45"));
-// → true
-----
-
-You can also specify open-ended ((range))s when using ((curly
-braces)), by omitting the number on either side of the comma. So
-`{,5}` means zero to five times, and `{5,}` means five or more times.
-
-== Grouping sub-expressions ==
-
-(((regular expression,grouping)))(((grouping)))To use an operator like `*` or
-`+` on more than one element at a time, you can use ((parentheses)). A
-part of a regular expression that is surrounded in parentheses counts
-as a single element as far as the operators following it are
-concerned.
-
-[source,javascript]
-----
-var cartoonCrying = /boo+(hoo+)+/i;
-console.log(cartoonCrying.test("Boohoooohoohooo"));
-// → true
-----
-
-(((crying)))The first and second `+` characters apply only to the
-second “o” in “boo” and “hoo”, respectively. The third `+` applies to
-the whole group `(hoo+)`, matching one or more sequences like that.
-
-(((case-sensitivity)))(((capitalization)))(((regular
-expression,flags)))The “i” at the end of the expression in the example
-above makes this regular expression case-insensitive, allowing it to
-match the uppercase “B” in the input string, even though the pattern
-is itself all lowercase.
-
-== Matches and groups ==
-
-(((regular expression,grouping)))(((exec method)))(((array)))The `test` method
-is the absolute simplest way to match a regular expression. It only
-tells you whether it matched, and nothing else. Regular expressions
-also have an `exec` (execute) method that will return `null` if no
-match was found, and an object with information about the match
-otherwise.
-
-[source,javascript]
-----
-var match = /\d+/.exec("one two 100");
-console.log(match);
-// → ["100"]
-console.log(match.index);
-// → 8
-----
-
-(((index property)))(((string,indexing)))An object returned from
-`exec` has an `index` property that tells us _where_ in the string the
-successful match begins. Other than that, the object looks like (and
-in fact is) an array of strings, whose first element is the string
-that was matched—in the example above, this is the sequence of
-((digit))s that we were looking for.
-
-(((string,methods)))(((match method)))String values have a `match`
-method that behaves very similarly.
-
-[source,javascript]
-----
-console.log("one two 100".match(/\d+/));
-// → ["100"]
-----
-
-(((grouping)))(((capture group)))(((exec method)))When the regular
-expression contains sub-expressions grouped with parentheses, the text
-that matched those groups will also show up in the array. The whole
-match is always the first element. The next element is the part
-matched by the first group (the one whose opening parenthesis comes
-first in the expression). Then the second group, and so on.
-
-[source,javascript]
-----
-var quotedText = /'([^']*)'/;
-console.log(quotedText.exec("she said 'hello'"));
-// → ["'hello'", "hello"]
-----
-
-(((capture group)))When a group does not end up being matched at all
-(for example when followed by a question mark), its position in the
-output array will hold `undefined`. Similarly, when a group is matched
-multiple times, only the last match ends up in the array.
-
-[source,javascript]
-----
-console.log(/bad(ly)?/.exec("bad"));
-// → ["bad", undefined]
-console.log(/(\d)+/.exec("123"));
-// → ["123", "3"]
-----
-
-(((exec method)))(((regular
-expression,methods)))(((extraction)))Groups can be very useful for
-extracting parts of a string. If we don't just want to verify whether
-a string contains a ((date)), but also extract it and construct an
-object that represents it, we can wrap parentheses around the digit
-patterns and directly pick the date out of the result of `exec`.
-
-But first, a brief detour, in which we discuss the preferred way to
-store date and ((time)) values in JavaScript.
-
-== The date type ==
-
-(((constructor)))(((Date constructor)))JavaScript has a standard
-object type for representing ((date))s—or rather, points in ((time)).
-It is called `Date`. If you simply create a date object using `new`,
-you get the current date and time.
-
-// test: no
-
-[source,javascript]
-----
-console.log(new Date());
-// → Wed Dec 04 2013 14:24:57 GMT+0100 (CET)
-----
-
-(((Date constructor)))You can also create an object for a specific
-time.
-
-[source,javascript]
-----
-console.log(new Date(2009, 11, 9));
-// → Wed Dec 09 2009 00:00:00 GMT+0100 (CET)
-console.log(new Date(2009, 11, 9, 12, 59, 59, 999));
-// → Wed Dec 09 2009 12:59:59 GMT+0100 (CET)
-----
-
-(((zero-based counting)))(((interface,design)))JavaScript uses a
-convention where month numbers start at zero (so December is 11), yet
-day numbers start at one. This is confusing and silly. Be careful.
-
-The last four arguments (hours, minutes, seconds, and milliseconds)
-are optional, and taken to be zero when not given.
-
-(((getTime method)))Timestamps are stored as the number of
-milliseconds since the start of 1970, using negative numbers for
-times before 1970 (following a convention set by “((Unix time))”,
-which was invented that year). The `getTime` method on a date object
-returns this number. It is big, as you can imagine.
-
-[source,javascript]
-----
-console.log(new Date(2013, 11, 19).getTime());
-// → 1387407600000
-console.log(new Date(1387407600000));
-// → Thu Dec 19 2013 00:00:00 GMT+0100 (CET)
-----
-
-(((Date.now function)))(((Date constructor)))If you give the `Date`
-constructor a single argument, that argument is treated as such
-millisecond count. You can get the current millisecond count by
-creating a new `Date` object and calling `getTime` on it, but also by
-calling the `Date.now` function.
-
-(((getFullYear method)))(((getMonth method)))(((getDate
-method)))(((getHours method)))(((getMinutes method)))(((getSeconds
-method)))(((getYear method)))Date objects provide methods like
-`getFullYear`, `getMonth`, `getDate`, `getHours`, `getMinutes`, and
-`getSeconds` to extract their components. There's also `getYear`,
-which gives you a rather useless two-digit year value (such as `93` or
-`14`).
-
-(((capture group)))Putting ((parentheses)) around the parts of the
-expression that we are interested in, we can now easily create a date
-object from a string.
-
-[source,javascript]
-----
-function findDate(string) {
- var dateTime = /(\d{1,2})-(\d{1,2})-(\d{4})/;
- var match = dateTime.exec(string);
- return new Date(Number(match[3]),
- Number(match[2]),
- Number(match[1]));
-}
-console.log(findDate("30-1-2003"));
-// → Sun Mar 02 2003 00:00:00 GMT+0100 (CET)
-----
-
-== Word and string boundaries ==
-
-(((matching)))(((regular expression,boundary)))Unfortunately,
-`findDate` will also happily extract the nonsensical date “00-1-3000”
-from the string `"100-1-30000"`. A match may happen anywhere in the
-string, so in this case it'll just start at the second character and
-end at the second-to-last.
-
-(((boundary)))(((caret character)))(((dollar sign)))If we want to
-enforce that the match must span the whole string, we can add the
-markers `^` and `$`. The caret matches the start of the input string,
-while the dollar sign matches the end. So, `/^\d+$/` matches a string
-consisting entirely of one or more digits, `/^!/` matches any string
-that starts with an exclamation mark, and `/x^/` does not match any
-string (there can not be an “x” _before_ the start of the string).
-
-(((word boundary)))(((word character)))If, on the other hand, we just
-want to make sure the date starts and ends on a word boundary, we can
-use the marker `\b`. A word boundary can be the start or end of the
-string, or any point in the string that has a word character (as in
-`\w`) on one side, and a non-word character on the other.
-
-[source,javascript]
-----
-console.log(/cat/.test("concatenate"));
-// → true
-console.log(/\bcat\b/.test("concatenate"));
-// → false
-----
-
-(((matching)))Note that a boundary marker doesn't represent an actual
-character. It just enforces that the regular expression matches only
-when a certain condition holds at the place where it appears in the
-pattern.
-
-== Choice patterns ==
-
-(((branching)))(((regular expression,alternatives)))(((farm
-example)))Say we want to know whether a piece of text contains not
-only a number, but a number followed by one of the words “pig”, “cow”,
-or “chicken”, or any of their plural forms.
-
-We could write three regular expressions and test them in turn, but
-there is a nicer way. The ((pipe character)) (“++|++”) denotes a
-((choice)) between the pattern to its left and the pattern to its
-right. So I can say this:
-
-[source,javascript]
-----
-var animalCount = /\b\d+ (pig|cow|chicken)s?\b/;
-console.log(animalCount.test("15 pigs"));
-// → true
-console.log(animalCount.test("15 pigchickens"));
-// → false
-----
-
-(((parentheses)))Parentheses can be used to limit the part of the
-pattern that the pipe operator applies to, and you can put multiple
-such operators next to each other to express a choice between more
-than two patterns.
-
-== The mechanics of matching ==
-
-(((regular expression,matching)))(((matching,algorithm)))Regular
-expressions can be thought of as ((flow diagram))s. This is the
-diagram for the livestock expression in the previous example:
-
-image::img/re_pigchickens.svg[alt="Visualization of /\b\d+ (pig|cow|chicken)s?\b/"]
-
-(((traversal)))Our expression matches a string if we can find a path
-from the left-hand side of the diagram to the right-hand side. We keep
-a current position in the string, and every time we move through a
-box, we verify that the part of the string after our current position
-matches that box.
-
-So if we try to match `"the 3 pigs"` with our regular expression,
-our progress through the flow chart would look like this:
-
- - At position 4, there is a word ((boundary)), so we can move past
- the first box.
-
- - Still at position 4, we find a digit, so we can also move past the
- second box.
-
- - At position 5, one path loops back to before the second (digit) box,
- while the other moves forward through the box that holds a single space
- character. There is a space here, not a digit, so we must take the
- second path.
-
- - We are now at position 6 (the start of “pigs”) and at the three-way
- branch in the diagram. We don't see “cow” or “chicken” here, but we
- do see “pig”, so we take that branch.
-
- - At position 9, after the three-way branch, one path skips
- the “s” box and go straight to the final word boundary, while the other path
- matches an “s”. There is an “s” character here, not a word boundary,
- so we go through the “s” box.
-
- - We're at position 10 (the end of the string) and can only match a
- word ((boundary)). The end of a string counts as a word boundary,
- so we go through the last box and have successfully matched this
- string.
-
-(((regular
-expression,matching)))(((matching,algorithm)))(((searching)))Conceptually,
-a regular expression engine looks for a match in a string as follows:
-it starts at the start of the string and tries a match there. In this
-case, there _is_ a word boundary there, so it'd get past the first
-box—but there is no digit, so it'd fail at the second box. Then it
-moves on to the second character in the string, and tries to begin a
-new match there. And so on, until it finds a match, or reaches the end
-of the string and decides that there really is no match.
-
-[[backtracking]]
-== Backtracking ==
-
-(((regular expression,backtracking)))(((binary number)))(((decimal
-number)))(((hexadecimal number)))(((flow
-diagram)))(((matching,algorithm)))(((backtracking)))The regular
-expression `/\b([01]+b|\d+|[\da-f]h)\b/` matches either a binary
-number followed by a “b”, a regular decimal number with no suffix
-character, or a hexadecimal number (that is, base 16, with the letters
-“a” to “f” standing for the digits 10 to 15) followed by an “h”. This
-is the corresponding diagram:
-
-image::img/re_number.svg[alt="Visualization of /\b([01]+b|\d+|[\da-f]h)\b/"]
-
-(((branching)))When matching this expression, it will often happen
-that the top (binary) branch is entered even though the input does not
-actually contain a binary number. When matching the string `"103"`,
-for example, it only becomes clear at the “3” that we are in the wrong
-branch. The string _does_ match the expression, just not the branch we
-are currently in.
-
-(((backtracking)))(((searching)))So the matcher _backtracks_. When
-entering a branch, it remembers where its current position (in this
-case, at the start of the string, just past the first boundary box in
-the diagram), so that it can go back and try another branch if the
-current one does not work out. For the string `"103"`, after
-encountering the “3” character, it will start trying the branch for
-decimal numbers. This one matches, so a match is reported after all.
-
-(((matching,algorithm)))The matcher stops as soon as it finds a full
-match. This means that if multiple branches could potentially match a
-string, only the first one (ordered by where the branches appear in
-the regular expression) is used.
-
-Backtracking also happens for ((repetition)) operators like + and `*`.
-If you match `/^.*x/` against `"abcxe"`, the `.*` part will first try
-to consume the whole string. The engine will then realize that it
-needs an “x” to match the pattern. Since there is no “x” past the end
-of the string, the star operator tries to match one character less.
-But the matcher doesn't find an “x” after `abcx` either, so it
-backtracks again, matching the star operator to just `abc`. _Now_ it
-finds an “x” where it needs it, and reports a successful match from
-position 0 to 4.
-
-(((performance)))(((complexity)))It is possible to write regular
-expressions that will do a _lot_ of backtracking. This problem occurs
-when a pattern can match a piece of input in many different ways. For
-example, if we get confused while writing a binary-number regexp, we
-might accidentally write something like `/([01]+)+b/`.
-
-image::img/re_slow.svg[alt="Visualization of /([01]+)+b/",width="6cm"]
-
-(((inner loop)))(((nesting,in regexps)))If that tries to match some
-long series of zeroes and ones with no trailing “b” character, the
-matcher will first go through the inner loop until it runs out of
-digits. Then it notices there is no “b”, so it backtracks one
-position, goes through the outer loop once, and gives up again, trying
-to backtrack out of the inner loop once more. It will continue to try
-every possible route through these two loops. This means the amount of
-work _doubles_ with each additional character. For even just a few
-dozen characters, the resulting match will take practically forever.
-
-== The replace method ==
-
-(((replace method)))(((regular expression)))String values have a
-`replace` method, which can be used to replace a part of the string
-with another string.
-
-[source,javascript]
-----
-console.log("papa".replace("p", "m"));
-// → mapa
-----
-
-(((regular expression,flags)))(((regular expression,global)))The first
-argument can also be a regular expression, in which case the first
-match of the regular expression is replaced. When a “g” option (for
-“global”) is added to the regular expression, _all_ matches in the
-string will be replaced, not just the first.
-
-[source,javascript]
-----
-console.log("Borobudur".replace(/[ou]/, "a"));
-// → Barobudur
-console.log("Borobudur".replace(/[ou]/g, "a"));
-// → Barabadar
-----
-
-(((interface,design)))(((argument)))It would have been sensible if the
-choice between replacing one match or all matches was made through an
-additional argument to `replace`, or by providing a different method,
-`replaceAll`. But for some unfortunate reason, the choice relies on a
-property of the regular expression instead.
-
-(((grouping)))(((capture group)))(((dollar sign)))(((replace
-method)))(((regular expression,grouping)))The real power of using
-regular expressions with `replace` comes from the fact that we can
-refer back to matched groups in the replacement string. For example,
-say we have a big string containing the names of people, one name per
-line, in the format `Lastname, Firstname`. If we want to swap these
-names and remove the comma to get a simple `Firstname Lastname`
-format, we can use the following code:
-
-[source,javascript]
-----
-console.log(
- "Hopper, Grace\nMcCarthy, John\nRitchie, Dennis"
- .replace(/([\w ]+), ([\w ]+)/g, "$2 $1"));
-// → Grace Hopper
-// John McCarthy
-// Dennis Ritchie
-----
-
-The `$1` and `$2` in the replacement string refer to the parenthesized
-groups in the pattern. `$1` is replaced by the text that matched
-against the first group, `$2` by the second, and so on, up to `$9`.
-The whole match can be referred to with `$&`.
-
-(((function,higher-order)))(((grouping)))(((capture group)))It is also
-possible to pass a function, rather than a string, as the second
-argument to `replace`. For each replacement, the function will be
-called with the matched groups (as well as the whole match) as
-arguments, and its return value will be inserted into the new string.
-
-Here's a simple example:
-
-[source,javascript]
-----
-var s = "the cia and fbi";
-console.log(s.replace(/\b(fbi|cia)\b/g, function(str) {
- return str.toUpperCase();
-}));
-// → the CIA and FBI
-----
-
-And here's a more interesting one:
-
-[source,javascript]
-----
-var stock = "1 lemon, 2 cabbages, and 101 eggs";
-function minusOne(match, amount, unit) {
- amount = Number(amount) - 1;
- if (amount == 1) // only one left, remove the 's'
- unit = unit.slice(0, unit.length - 1);
- else if (amount == 0)
- amount = "no";
- return amount + " " + unit;
-}
-console.log(stock.replace(/(\d+) (\w+)/g, minusOne));
-// → no lemon, 1 cabbage, and 100 eggs
-----
-
-This takes a string, finds all occurrences of a number followed by an
-alphanumeric word, and returns a string wherein every such occurrence
-is decremented by one.
-
-The `(\d+)` group ends up as the `amount` argument to the function,
-and the `(\w+)` group gets bound to `unit`. The function converts
-`amount` to a number—which always works, since it matched `\d+`—and
-makes some adjustments in case there is only one or zero left.
-
-== Greed ==
-
-(((greed)))(((regular expression)))It isn't hard to use `replace` to
-write a function that removes all ((comment))s from a piece of
-JavaScript ((code)). Here is a first attempt:
-
-// test: wrap
-
-[source,javascript]
-----
-function stripComments(code) {
- return code.replace(/\/\/.*|\/\*[^]*\*\//g, "");
-}
-console.log(stripComments("1 + /* 2 */3"));
-// → 1 + 3
-console.log(stripComments("x = 10;// ten!"));
-// → x = 10;
-console.log(stripComments("1 /* a */+/* b */ 1"));
-// → 1 1
-----
-
-(((period character)))(((slash character)))(((newline
-character)))(((empty set)))(((block comment)))(((line comment)))The
-part before the _or_ operator simply matches two slash characters
-followed by any number of non-newline characters. The part for
-multi-line comments is more involved. We use `[^]` (any character that
-is not in the empty set of characters) as a way to match any
-character. We cannot just use a dot here because block comments can
-continue on a new line, and dots do not match the newline character.
-
-But the output of the last example appears to have gone wrong. Why?
-
-(((backtracking)))(((greed)))(((regular expression)))The `.*` part of
-the expression, as I described in the section on backtracking, will
-first match as much as it can. If that causes the next part of the
-pattern to fail, the matcher moves back one character and tries again
-from there. In the example, the matcher first tries to match the whole
-rest of the string, and then moves back from there. It will find an
-occurrence of `*/` after going back four characters, and match that.
-This is not what we wanted—the intention was to match a single
-comment, not to go all the way to the end of the code and find the end
-of the last block comment.
-
-Because of this behavior, we say the repetition operators (`+`, `*`,
-`?`, and `{}`) are _((greed))y_, meaning they match as much as they
-can and backtrack from there. If you put a ((question mark)) after
-them (`+?`, `*?`, `??`, `{}?`) they become non-greedy, and start by
-matching as little as possible, only matching more when the remaining
-pattern does not fit the smaller match.
-
-And that is exactly what we want in this case. By having the star
-match the smallest stretch of characters that brings us to a “++*/++”,
-we consume one block comment, and nothing more.
-
-// test: wrap
-
-[source,javascript]
-----
-function stripComments(code) {
- return code.replace(/\/\/.*|\/\*[^]*?\*\//g, "");
-}
-console.log(stripComments("1 /* a */+/* b */ 1"));
-// → 1 + 1
-----
-
-A lot of ((bug))s in ((regular expression)) programs can be traced to
-unintentionally using a greedy operator where a non-greedy one would
-work better. When using a ((repetition)) operator, consider the
-non-greedy variant first.
-
-== Dynamically creating RegExp objects ==
-
-(((regular expression,creation)))(((underscore character)))(((RegExp
-constructor)))There are cases where you might not know the exact
-((pattern)) you need to match against when you are writing your code.
-Say you want to look for the user's name in a piece of text, and
-enclose it in underscore characters to make it stand out. Since you
-will only known the name once the program is actually running, you
-can't use the slash-based notation.
-
-But you can build up a string and use the `RegExp` ((constructor)) on
-that. For example:
-
-[source,javascript]
-----
-var name = "harry";
-var text = "Harry is a suspicious character.";
-var regexp = new RegExp("\\b(" + name + ")\\b", "gi");
-console.log(text.replace(regexp, "_$1_"));
-// → _Harry_ is a suspicious character.
-----
-
-(((regular expression,flags)))(((backslash character)))When creating
-the `\b` ((boundary)) markers, we have to use two backslashes, because
-we are writing them in a normal string, not a slash-enclosed regular
-expression. The second argument to the `RegExp` constructor contains
-the options for the regular expression—in this case `"gi"` for global
-and case-insensitive.
-
-But what if the name is `"dea+hl[]rd"` because our user is a ((nerd))y
-teenager? That would result in a nonsensical regular expression, which
-won't actually match the user's name.
-
-(((backslash character)))(((escaping,in regexps)))(((regular
-expression,escaping)))To work around this, we can add backslashes
-before any character that we don't trust. Adding backslashes before
-alphabetic characters is a bad idea, because things like `\b` and `\n`
-have a special meaning. But escaping everything that's not
-alphanumeric or ((whitespace)) is safe.
-
-[source,javascript]
-----
-var name = "dea+hl[]rd";
-var text = "This dea+hl[]rd guy is super annoying.";
-var escaped = name.replace(/[^\w\s]/g, "\\$&");
-var regexp = new RegExp("\\b(" + escaped + ")\\b", "gi");
-console.log(text.replace(regexp, "_$1_"));
-// → This _dea+hl[]rd_ guy is super annoying.
-----
-
-== The search method ==
-
-(((searching)))(((regular expression,methods)))(((indexOf
-method)))(((search method)))The `indexOf` method on strings cannot be
-called with a regular expression. But there is another method,
-`search`, which does expect a regular expression. Like `indexOf`, it
-returns the first index on which the expression was found, or -1 when
-it wasn't found.
-
-[source,javascript]
-----
-console.log(" word".search(/\S/));
-// → 2
-console.log(" ".search(/\S/));
-// → -1
-----
-
-Unfortunately, there is no way to indicate that the match should start
-at a given offset (like we can with the second argument to `indexOf`),
-which would often be very useful.
-
-== The lastIndex property ==
-
-(((exec method)))(((regular expression)))The `exec` method similarly
-does not provide a convenient way to start searching from a given
-position in the string. But it does provide an __in__convenient way.
-
-(((regular expression,matching)))(((matching)))(((source
-property)))(((lastIndex property)))Regular expression objects have
-properties. One such property is `source`, which contains the string
-that expression was created from. Another property is `lastIndex`,
-which controls, in some limited circumstances, where the next match
-will start.
-
-(((interface,design)))(((exec method)))(((regular
-expression,global)))Those circumstances are that the regular
-expression must have the “global” (`g`) option enabled, and the match
-must happen through the `exec` method. Again, a more sane solution
-would have been to just allow an extra argument to be passed to
-`exec`, but sanity is not a defining characteristic of JavaScript's
-regular expression interface.
-
-[source,javascript]
-----
-var pattern = /y/g;
-pattern.lastIndex = 3;
-var match = pattern.exec("xyzzy");
-console.log(match.index);
-// → 4
-console.log(pattern.lastIndex);
-// → 5
-----
-
-(((side effect)))(((lastIndex property)))If the match was successful,
-the call to `exec` automatically updates the `lastIndex` property to
-point after the match. If no match was found, `lastIndex` is set back
-to zero, which is also the value it has in a newly constructed regular
-expression object.
-
-(((bug)))When using a global regular expression value for multiple
-`exec` calls, these automatic updates to the `lastIndex` property can
-cause problems. Your regular expression might be accidentally starting
-at an index that was left over from a previous call.
-
-[source,javascript]
-----
-var digit = /\d/g;
-console.log(digit.exec("here it is: 1"));
-// → ["1"]
-console.log(digit.exec("and now: 1"));
-// → null
-----
-
-(((regular expression,global)))(((match method)))Another interesting
-effect of the global option is that it changes the way the `match`
-method on strings works. When called with a global expression, instead
-of returning an array similar to that returned by `exec`, `match` will
-find _all_ matches of the pattern in the string, and return an array
-containing the matched strings.
-
-[source,javascript]
-----
-console.log("Banana".match(/an/g));
-// → ["an", "an"]
-----
-
-So be cautious with global regular expressions. The cases where they
-are necessary—calls to `replace` and places where you want to
-explicitly use ++lastIndex++—are typically the only places where you
-want to use them.
-
-=== Looping over matches ===
-
-(((lastIndex property)))(((exec method)))(((loop)))A common pattern is
-to scan through all occurrences of a pattern in a string, in a way
-that gives us access to the match object in the loop body, by using
-`lastIndex` and `exec`.
-
-[source,javascript]
-----
-var input = "A string with 3 numbers in it... 42 and 88.";
-var number = /\b(\d+)\b/g;
-var match;
-while (match = number.exec(input))
- console.log("Found", match[1], "at", match.index);
-// → Found 3 at 14
-// Found 42 at 33
-// Found 88 at 40
-----
-
-(((while loop)))(((= operator)))This makes use of the fact that the
-value of an ((assignment)) expression (`=`) is the assigned value. So
-by using `match = re.exec(input)` as the condition in the `while`
-statement, we perform the match at the start of each iteration, save
-its result in a ((variable)), and stop looping when no more matches
-are found.
-
-[[ini]]
-== Parsing an INI file ==
-
-(((comment)))(((file format)))(((enemies example)))(((ini file)))To
-conclude the chapter, we'll look at a problem that calls for ((regular
-expression))s. Imagine we are writing a program to automatically
-harvest information about our enemies from the ((Internet)). (We will
-not actually write that program here, just the part that reads the
-((configuration)) file. Sorry to disappoint.) The configuration file
-looks like this:
-
-[source,text/plain]
-----
-searchengine=http://www.google.com/search?q=$1
-spitefulness=9.7
-
-; comments are preceded by a semicolon...
-; each section concerns an individual enemy
-[larry]
-fullname=Larry Doe
-type=kindergarten bully
-website=http://www.geocities.com/CapeCanaveral/11451
-
-[gargamel]
-fullname=Gargamel
-type=evil sorcerer
-outputdir=/home/marijn/enemies/gargamel
-----
-
-(((grammar)))The exact rules for this format (which is actually a
-widely used format, usually called an _INI_ file) are as follows:
-
-- Blank lines and lines starting with semicolons are ignored.
-
-- Lines wrapped in `[` and `]` start a new ((section)).
-
-- Lines containing an alphanumeric identifier followed by an `=`
- character add a setting to the current section.
-
-- Anything else is invalid.
-
-Our task is to convert a string like this into an array of objects,
-each with a `name` property and an array of settings. We'll need one
-such object for each section, and one for the global settings at the
-top.
-
-(((carriage return)))(((line break)))(((newline character)))Since the
-format has to be processed ((line)) by line, splitting the file up
-into separate lines is a good start. We used `string.split("\n")` to
-do this in link:06_object.html#split[Chapter 6]. Some operating
-systems, however, use not just a newline character to separate lines
-but a carriage return character followed by a newline (`"\r\n"`).
-Given that the `split` method also allows a regular expression as its
-argument, we can split on a regular expression like `/\r?\n/` to split
-in a way that allows both `"\n"` and `"\r\n"` between lines.
-
-[source,javascript]
-----
-function parseINI(string) {
- // Start with an object to hold the top-level fields
- var currentSection = {name: null, fields: []};
- var categories = [currentSection];
-
- string.split(/\r?\n/).forEach(function(line) {
- var match;
- if (/^\s*(;.*)?$/.test(line)) {
- return;
- } else if (match = line.match(/^\[(.*)\]$/)) {
- currentSection = {name: match[1], fields: []};
- categories.push(currentSection);
- } else if (match = line.match(/^(\w+)=(.*)$/)) {
- currentSection.fields.push({name: match[1],
- value: match[2]});
- } else {
- throw new Error("Line '" + line + "' is invalid.");
- }
- });
-
- return categories;
-}
-----
-
-(((parseINI function)))(((parsing)))This code goes over every line in
-the file, updating the “current section” object as it goes along.
-First it checks whether the line can be ignored, using the expression
-`/^\s*(;.*)?$/`. Do you see how it works? The part between the
-((parentheses)) will match comments, and the `?` will make sure it
-also matches lines containing only whitespace.
-
-If the line is not a ((comment)), the code then checks whether the
-line starts a new ((section)). If so, it creates a new current section
-object, to which subsequent settings will be added.
-
-The last meaningful possibility is that the line is a normal setting,
-which the code adds to the current section object.
-
-If a ((line)) matches none of these forms, the function throws an
-error.
-
-(((caret character)))(((dollar sign)))(((boundary)))Note the recurring
-use of `^` and `$` to make sure the expression matches the whole line,
-not just part of it. Leaving these out results in code that mostly
-works but behaves strangely for some input, which can be a difficult
-bug to track down.
-
-(((if keyword)))(((assignment)))(((= operator)))The pattern `if (match
-= string.match(...))` is similar to the trick of using an assignment
-as the condition for `while`. You often aren't sure that your call to
-`match` will succeed, so you can only access the resulting object
-inside an `if` statement that tests for this. To not break the
-pleasant chain of `if` forms, we assign the result of the match to a
-variable, and immediately use that assignment as the test in the `if`
-statement.
-
-== International characters ==
-
-(((internationalization)))(((Unicode)))(((regular
-expression,internationalization)))Due to JavaScript's initial
-simplistic implementation, and the fact that this simplistic approach
-was later set in stone as ((standard)) behavior, JavaScript's regular
-expressions are rather dumb about characters that do not appear in the
-English language. For example, as far as JavaScript's regular
-expressions are concerned, a “((word character))” is only one of the
-26 characters in the Latin alphabet (upper- or lowercase), and, for
-some reason, the underscore character. Things like “é” or “β”, which
-most definitely are word characters, will not match `\w` (and _will_
-match upper-case `\W`, the non-word category).
-
-(((whitespace)))By a strange historical accident, `\s` (whitespace)
-does not have this problem, and matches all characters that the
-Unicode standard considers whitespace, including things like the
-((non-breaking space)) and the ((Mongolian vowel separator)).
-
-(((character category)))Some ((regular expression))
-((implementation))s in other programming languages have syntax to
-match specific ((Unicode)) character categories, such as “all
-uppercase letters”, “all punctuation”, or “control characters”. There
-are plans to add support for such categories JavaScript, but they
-unfortunately look like they won't be realized in the near ((future)).
-
-[[summary_regexp]]
-== Summary ==
-
-Regular expressions are objects that represent patterns in strings.
-They use their own syntax to express these patterns.
-
-[cols="1,5"]
-|====
-|`/abc/` |A sequence of characters
-|`/[abc]/` |Any character from a set of characters
-|`/[^abc]/` |Any character _not_ in a set of characters
-|`/[0-9]/` |Any character in a range of characters
-|`/x+/` |One or more occurrences of the pattern `x`
-|`/x+?/` |One or more occurrences, non-greedy
-|`/x*/` |Zero or more occurrences
-|`/x?/` |Zero or one occurrence
-|`/x{2,4}/` |Between two and four occurrences
-|`/(abc)/` |A group
-|++/a{brvbar}b{brvbar}c/++ |Any one of several patterns
-|`/\d/` |Any digit character
-|`/\w/` |An alphanumeric character (“word character”)
-|`/\s/` |Any whitespace character
-|`/./` |Any character except newlines
-|`/\b/` |A word boundary
-|`/^/` |Start of input
-|`/$/` |End of input
-|====
-
-A regular expression has a method `test` to test whether a given
-string matches it. It also has an `exec` method which, when a match is
-found, returns an array containing all matched groups. The array also
-has an `index` property that indicates where the match started.
-
-Strings have a `match` method to match them against a regular
-expression, and a `search` method search for one, returning only the
-starting position of the match. Their `replace` method can replace
-matches of a pattern with a replacement string. Alternatively, you can
-pass a function to `replace`, which will be used to build up a
-replacement string based on the match text and matched groups.
-
-Regular expressions can have options, which are written after
-the closing slash. The “i” option makes the match case-insensitive,
-while the “g” option makes the expression _global_ which, among other
-things, causes the `replace` method to replace all instances instead
-of just the first.
-
-The `RegExp` constructor can be used to create a regular expression
-value from a string.
-
-Regular expressions are a sharp ((tool)) with an awkward handle. They
-simplify some tasks tremendously, but can quickly become unmanageable
-when applied to complex problems. Part of knowing how to use them is
-resisting the urge to try and shoehorn things that they can not sanely
-express into them.
-
-== Exercises ==
-
-(((debugging)))(((bug)))It is almost unavoidable that, in the course
-of working on these exercises, you will get confused and frustrated by
-some regular expression's inexplicable ((behavior)). Sometimes it
-helps to enter your expression into an online tool like
-https://www.debuggex.com/[_debuggex.com_] to see whether its
-visualization corresponds to what you intended, and to ((experiment))
-with the way it responds to various input strings.
-
-=== Regexp golf ===
-
-(((program size)))(((code golf)))(((regexp golf (exercise))))“Code
-golf” is a term used for the game of trying to express a particular
-program in as few characters as possible. Similarly, “regexp golf” is
-the practice of writing as tiny a regular expression as possible to
-match a given pattern, and _only_ that pattern.
-
-(((boundary)))(((matching)))For each of the following items, write a ((regular
-expression)) to test whether any of the given sub-strings occur in a
-string. The regular expression should match only strings containing
-one the sub-strings described. Do not worry about word boundaries
-unless explicitly mentioned. When your expression works, see if you
-can make it any smaller.
-
- 1. “car” and “cat”
- 2. “pop” and “prop”
- 3. “ferret”, “ferry”, and “ferrari”
- 4. Any word _ending_ in “ious”
- 5. A whitespace character followed by a dot, comma, colon, or semicolon
- 6. A word longer than 6 letters
- 7. A word without the letter “e”
-
-Refer back to the table in the
-link:09_regexp.html#summary_regexp[chapter summary] to quickly look
-something up. Test each solution out with a few test strings.
-
-ifdef::html_target[]
-[source,javascript]
-----
-// Fill in the regular expressions
-
-verify(/.../,
- ["my car", "bad cats"],
- ["camper", "high art"]);
-
-verify(/.../,
- ["pop culture", "mad props"],
- ["plop"]);
-
-verify(/.../,
- ["ferret", "ferry", "ferrari"],
- ["ferrum", "transfer A"]);
-
-verify(/.../,
- ["how delicious", "spacious room"],
- ["ruinous", "consciousness"]);
-
-verify(/.../,
- ["bad punctuation ."],
- ["escape the dot"]);
-
-verify(/.../,
- ["hottentottententen"],
- ["no", "hotten totten tenten"]);
-
-verify(/.../,
- ["red platypus", "wobbling nest"],
- ["earth bed", "learning ape"]);
-
-
-function verify(regexp, yes, no) {
- // Ignore unfinished exercises
- if (regexp.source == "...") return;
- yes.forEach(function(s) {
- if (!regexp.test(s))
- console.log("Failure to match '" + s + "'");
- });
- no.forEach(function(s) {
- if (regexp.test(s))
- console.log("Unexpected match for '" + s + "'");
- });
-}
-----
-endif::html_target[]
-
-=== Quoting style ===
-
-(((quoting style (exercise))))(((single-quote
-character)))(((double-quote character)))Imagine you have written a
-story, and used single ((quotation mark))s throughout to mark pieces
-of dialogue. Now you want to replace all the dialogue quotes with
-double quotes, while keeping the single quotes used in contractions
-like “aren't”.
-
-(((replace method)))Think of a pattern that distinguishes these two
-kinds of quote usage and craft a call to the `replace` method that
-does the proper replacement.
-
-ifdef::html_target[]
-// test: no
-
-[source,javascript]
-----
-var text = "'I'm the cook,' he said, 'it's my job.'";
-// Change this call.
-console.log(text.replace(/A/g, "B"));
-// → "I'm the cook," he said, "it's my job."
-----
-endif::html_target[]
-
-!!solution!!
-
-(((quoting style (exercise))))(((boundary)))The most obvious solution
-is to only replace quotes with a non-word character on at least one
-side. Something like `/\W'|'\W/`. But you also have to take the start
-and end of the line into account.
-
-(((grouping)))(((replace method)))In addition, you must ensure that
-the replacement also includes the characters that were matched by the
-`\W` pattern, so that those are not dropped. This can be done by
-wrapping them in ((parentheses)), and including their groups in the
-replacement string (`$1`, `$2`). Groups that are not matched will be
-replaced by nothing.
-
-!!solution!!
-
-=== Numbers again ===
-
-(((number)))A series of ((digit))s can be matched by the simple
-regular expression `/\d+/`.
-
-(((sign)))(((fractional number)))(((syntax)))(((minus)))(((plus
-character)))(((exponent)))(((scientific notation)))(((period
-character)))Write an expression that matches only JavaScript-style
-numbers. It must support an optional minus _or_ plus sign in front of
-the number, the decimal dot, and exponent notation—`5e-3` or ++1E10++—
-again with an optional sign in front of the exponent. Also note that
-it is not necessary for there to be digits in front of or after the
-dot, but that the number cannot be a dot alone. That is, `.5` and `5.`
-are valid JavaScript numbers, but a lone dot _isn't_.
-
-ifdef::html_target[]
-// test: no
-
-[source,javascript]
-----
-// Fill in this regular expression.
-var number = /^...$/;
-
-// Tests:
-["1", "-1", "+15", "1.55", ".5", "5.", "1.3e2", "1E-4",
- "1e+12"].forEach(function(s) {
- if (!number.test(s))
- console.log("Failed to match '" + s + "'");
-});
-["1a", "+-1", "1.2.3", "1+1", "1e4.5", ".5.", "1f5",
- "."].forEach(function(s) {
- if (number.test(s))
- console.log("Incorrectly accepted '" + s + "'");
-});
-----
-endif::html_target[]
-
-!!solution!!
-
-(((regular expression,escaping)))(((backslash character)))First, do
-not forget the backslash in front of the dot.
-
-Matching the optional ((sign)) in front of the ((number)), as well as
-in front of the ((exponent)), can be done with `[+\-]?` or `(\+|-|)`
-(plus, minus, or nothing).
-
-(((pipe character)))The more complicated part of the exercise is the
-problem of matching both `"5."` and `".5"` without also matching
-`"."`. For this, a good solution is to use the `|` operator to
-separate the two cases—either one or more digits optionally followed
-by a dot and zero or more digits, _or_ a dot followed by one or more
-digits.
-
-(((exponent)))(((case-sensitivity)))(((regular
-expression,flags)))Finally, to make the “e” case-insensitive, either
-add an “i” option to the regular expression, or use `[eE]`.
-
-!!solution!!
diff --git a/10_modules.md b/10_modules.md
new file mode 100644
index 000000000..0f253a7e7
--- /dev/null
+++ b/10_modules.md
@@ -0,0 +1,487 @@
+{{meta {load_files: ["code/packages_chapter_10.js", "code/chapter/07_robot.js"]}}}
+
+# Modules
+
+{{quote {author: "Tef", title: "programming is terrible", chapter: true}
+
+Write code that is easy to delete, not easy to extend.
+
+quote}}
+
+{{index "Yuan-Ma", "Book of Programming"}}
+
+{{figure {url: "img/chapter_picture_10.jpg", alt: "Illustration of a complicated building built from modular pieces", chapter: framed}}}
+
+{{index organization, [code, "structure of"]}}
+
+Ideally, a program has a clear, straightforward structure. The way it works is easy to explain, and each part plays a well-defined role.
+
+{{index "organic growth"}}
+
+In practice, programs grow organically. Pieces of functionality are added as the programmer identifies new needs. Keeping such a program well structured requires constant attention and work. This is work that will pay off only in the future, the _next_ time someone works on the program, so it's tempting to neglect it and allow the various parts of the program to become deeply entangled.
+
+{{index readability, reuse, isolation}}
+
+This causes two practical issues. First, understanding an entangled system is hard. If everything can touch everything else, it is difficult to look at any given piece in isolation. You are forced to build up a holistic understanding of the entire thing. Second, if you want to use any of the functionality from such a program in another situation, rewriting it may be easier than trying to disentangle it from its context.
+
+The phrase "((big ball of mud))" is often used for such large, structureless programs. Everything sticks together, and when you try to pick out a piece, the whole thing comes apart, and you succeed only in making a mess.
+
+## Modular programs
+
+{{index dependency, [interface, module]}}
+
+_Modules_ are an attempt to avoid these problems. A ((module)) is a piece of program that specifies which other pieces it relies on and which functionality it provides for other modules to use (its _interface_).
+
+{{index "big ball of mud"}}
+
+Module interfaces have a lot in common with object interfaces, as we saw them in [Chapter ?](object#interface). They make part of the module available to the outside world and keep the rest private.
+
+{{index dependency}}
+
+But the interface that a module provides for others to use is only half the story. A good module system also requires modules to specify which code _they_ use from other modules. These relations are called _dependencies_. If module A uses functionality from module B, it is said to _depend_ on that module. When these are clearly specified in the module itself, they can be used to figure out which other modules need to be present to be able to use a given module and to automatically load dependencies.
+
+When the ways in which modules interact with each other are explicit, a system becomes more like ((LEGO)), where pieces interact through well-defined connectors, and less like mud, where everything mixes with everything else.
+
+{{id es}}
+
+## ES modules
+
+{{index "global scope", [binding, global]}}
+
+The original JavaScript language did not have any concept of a module. All scripts ran in the same scope, and accessing a function defined in another script was done by referencing the global bindings created by that script. This actively encouraged accidental, hard-to-see entanglement of code and invited problems like unrelated scripts trying to use the same binding name.
+
+{{index "ES modules"}}
+
+Since ECMAScript 2015, JavaScript supports two different types of programs. _Scripts_ behave in the old way: their bindings are defined in the global scope, and they have no way to directly reference other scripts. _Modules_ get their own separate scope and support the `import` and `export` keywords, which aren't available in scripts, to declare their dependencies and interface. This module system is usually called _ES modules_ (where _ES_ stands for ECMAScript).
+
+A modular program is composed of a number of such modules, wired together via their imports and exports.
+
+{{index "Date class", "weekDay module"}}
+
+The following example module converts between day names and numbers (as returned by `Date`'s `getDay` method). It defines a constant that is not part of its interface, and two functions that are. It has no dependencies.
+
+```
+const names = ["Sunday", "Monday", "Tuesday", "Wednesday",
+ "Thursday", "Friday", "Saturday"];
+
+export function dayName(number) {
+ return names[number];
+}
+export function dayNumber(name) {
+ return names.indexOf(name);
+}
+```
+
+The `export` keyword can be put in front of a function, class, or binding definition to indicate that that binding is part of the module's interface. This makes it possible for other modules to use that binding by importing it.
+
+```{test: no}
+import {dayName} from "./dayname.js";
+let now = new Date();
+console.log(`Today is ${dayName(now.getDay())}`);
+// → Today is Monday
+```
+
+{{index "import keyword", dependency, "ES modules"}}
+
+The `import` keyword, followed by a list of binding names in braces, makes bindings from another module available in the current module. Modules are identified by quoted strings.
+
+{{index [module, resolution], resolution}}
+
+How such a module name is resolved to an actual program differs by platform. The browser treats them as web addresses, whereas Node.js resolves them to files. When you run a module, all the other modules it depends on—and the modules _those_ depend on—are loaded, and the exported bindings are made available to the modules that import them.
+
+Import and export declarations cannot appear inside of functions, loops, or other blocks. They are immediately resolved when the module is loaded, regardless of how the code in the module executes. To reflect this, they must appear only in the outer module body.
+
+A module's interface thus consists of a collection of named bindings, which other modules that depend on the module can access. Imported bindings can be renamed to give them a new local name using `as` after their name.
+
+```
+import {dayName as nomDeJour} from "./dayname.js";
+console.log(nomDeJour(3));
+// → Wednesday
+```
+
+A module may also have a special export named `default`, which is often used for modules that only export a single binding. To define a default export, you write `export default` before an expression, a function declaration, or a class declaration.
+
+```
+export default ["Winter", "Spring", "Summer", "Autumn"];
+```
+
+Such a binding is imported by omitting the braces around the name of the import.
+
+```
+import seasonNames from "./seasonname.js";
+```
+
+To import all bindings from a module at the same time, you can use `import *`. You provide a name, and that name will be bound to an object holding all the module's exports. This can be useful when you are using a lot of different exports.
+
+```
+import * as dayName from "./dayname.js";
+console.log(dayName.dayName(3));
+// → Wednesday
+```
+
+## Packages
+
+{{index bug, dependency, structure, reuse}}
+
+One of the advantages of building a program out of separate pieces and being able to run some of those pieces on their own is that you might be able to use the same piece in different programs.
+
+{{index "parseINI function"}}
+
+But how do you set this up? Say I want to use the `parseINI` function from [Chapter ?](regexp#ini) in another program. If it is clear what the function depends on (in this case, nothing), I can just copy that module into my new project and use it. But then, if I find a mistake in the code, I'll probably fix it in whichever program I'm working with at the time and forget to also fix it in the other program.
+
+{{index duplication, "copy-paste programming"}}
+
+Once you start duplicating code, you'll quickly find yourself wasting time and energy moving copies around and keeping them up to date. That's where _((package))s_ come in. A package is a chunk of code that can be distributed (copied and installed). It may contain one or more modules and has information about which other packages it depends on. A package also usually comes with documentation explaining what it does so that people who didn't write it might still be able to use it.
+
+When a problem is found in a package or a new feature is added, the package is updated. Now the programs that depend on it (which may also be packages) can copy the new ((version)) to get the improvements that were made to the code.
+
+{{id modules_npm}}
+
+{{index installation, upgrading, "package manager", download, reuse}}
+
+Working in this way requires ((infrastructure)). We need a place to store and find packages and a convenient way to install and upgrade them. In the JavaScript world, this infrastructure is provided by ((NPM)) ([_https://npmjs.com_](https://npmjs.com)).
+
+NPM is two things: an online service where you can download (and upload) packages, and a program (bundled with Node.js) that helps you install and manage them.
+
+{{index "ini package"}}
+
+At the time of writing, there are more than three million different packages available on NPM. A large portion of those are rubbish, to be fair. But almost every useful, publicly available JavaScript package can be found on NPM. For example, an INI file parser, similar to the one we built in [Chapter ?](regexp), is available under the package name `ini`.
+
+{{index "command line"}}
+
+[Chapter ?](node) will show how to install such packages locally using the `npm` command line program.
+
+Having quality packages available for download is extremely valuable. It means that we can often avoid reinventing a program that 100 people have written before and get a solid, well-tested implementation at the press of a few keys.
+
+{{index maintenance}}
+
+Software is cheap to copy, so once someone has written it, distributing it to other people is an efficient process. Writing it in the first place _is_ work, though, and responding to people who have found problems in the code or who want to propose new features is even more work.
+
+By default, you own the ((copyright)) to the code you write, and other people may use it only with your permission. But because some people are just nice and because publishing good software can help make you a little bit famous among programmers, many packages are published under a ((license)) that explicitly allows other people to use it.
+
+Most code on ((NPM)) is licensed this way. Some licenses require you to also publish code that you build on top of the package under the same license. Others are less demanding, requiring only that you keep the license with the code as you distribute it. The JavaScript community mostly uses the latter type of license. When using other people's packages, make sure you are aware of their licenses.
+
+{{id modules_ini}}
+
+{{index "ini package"}}
+
+Now, instead of writing our own INI file parser, we can use one from ((NPM)).
+
+```
+import {parse} from "ini";
+
+console.log(parse("x = 10\ny = 20"));
+// → {x: "10", y: "20"}
+```
+
+{{id commonjs}}
+
+## CommonJS modules
+
+Before 2015, when the JavaScript language had no built-in module system, people were already building large systems in JavaScript. To make that workable, they _needed_ ((module))s.
+
+{{index [function, scope], [interface, module], [object, as module]}}
+
+The community designed its own improvised ((module system))s on top of the language. These use functions to create a local scope for the modules and regular objects to represent module interfaces.
+
+Initially, people just manually wrapped their entire module in an “((immediately invoked function
+expression))” to create the module's scope and assigned their interface objects to a single global
+variable.
+
+```
+const weekDay = function() {
+ const names = ["Sunday", "Monday", "Tuesday", "Wednesday",
+ "Thursday", "Friday", "Saturday"];
+ return {
+ name(number) { return names[number]; },
+ number(name) { return names.indexOf(name); }
+ };
+}();
+
+console.log(weekDay.name(weekDay.number("Sunday")));
+// → Sunday
+```
+
+{{index dependency, [interface, module]}}
+
+This style of modules provides ((isolation)), to a certain degree, but it does not declare dependencies. Instead, it just puts its interface into the ((global scope)) and expects its dependencies, if any, to do the same. This is not ideal.
+
+{{index "CommonJS modules"}}
+
+If we implement our own module loader, we can do better. The most widely used approach to bolted-on JavaScript modules is called _CommonJS modules_. ((Node.js)) used this module system from the start (though it now also knows how to load ES modules), and it is the module system used by many packages on ((NPM)).
+
+{{index "require function", [interface, module], "exports object"}}
+
+A CommonJS module looks like a regular script, but it has access to two bindings that it uses to interact with other modules. The first is a function called `require`. When you call this with the module name of your dependency, it makes sure the module is loaded and returns its interface. The second is an object named `exports`, which is the interface object for the module. It starts out empty and you add properties to it to define exported values.
+
+{{index "formatDate module", "Date class", "ordinal package", "date-names package"}}
+
+This CommonJS example module provides a date-formatting function. It uses two ((package))s from NPM—`ordinal` to convert numbers to strings like `"1st"` and `"2nd"`, and `date-names` to get the English names for weekdays and months. It exports a single function, `formatDate`, which takes a `Date` object and a ((template)) string.
+
+The template string may contain codes that direct the format, such as `YYYY` for the full year and `Do` for the ordinal day of the month. You could give it a string like `"MMMM Do YYYY"` to get output like `November 22nd 2017`.
+
+```
+const ordinal = require("ordinal");
+const {days, months} = require("date-names");
+
+exports.formatDate = function(date, format) {
+ return format.replace(/YYYY|M(MMM)?|Do?|dddd/g, tag => {
+ if (tag == "YYYY") return date.getFullYear();
+ if (tag == "M") return date.getMonth();
+ if (tag == "MMMM") return months[date.getMonth()];
+ if (tag == "D") return date.getDate();
+ if (tag == "Do") return ordinal(date.getDate());
+ if (tag == "dddd") return days[date.getDay()];
+ });
+};
+```
+
+{{index "destructuring binding"}}
+
+The interface of `ordinal` is a single function, whereas `date-names` exports an object containing multiple things—`days` and `months` are arrays of names. Destructuring is very convenient when creating bindings for imported interfaces.
+
+The module adds its interface function to `exports` so that modules that depend on it get access to it. We could use the module like this:
+
+```
+const {formatDate} = require("./format-date.js");
+
+console.log(formatDate(new Date(2017, 9, 13),
+ "dddd the Do"));
+// → Friday the 13th
+```
+
+CommonJS is implemented with a module loader that, when loading a module, wraps its code in a function (giving it its own local scope) and passes the `require` and `exports` bindings to that function as arguments.
+
+{{id require}}
+
+{{index "require function", "CommonJS modules", "readFile function"}}
+
+If we assume we have access to a `readFile` function that reads a file by name and gives us its content, we can define a simplified form of `require` like this:
+
+```{test: wrap, sandbox: require}
+function require(name) {
+ if (!(name in require.cache)) {
+ let code = readFile(name);
+ let exports = require.cache[name] = {};
+ let wrapper = Function("require, exports", code);
+ wrapper(require, exports);
+ }
+ return require.cache[name];
+}
+require.cache = Object.create(null);
+```
+
+{{id eval}}
+
+{{index "Function constructor", eval, security}}
+
+`Function` is a built-in JavaScript function that takes a list of arguments (as a comma-separated string) and a string containing the function body and returns a function value with those arguments and that body. This is an interesting concept—it allows a program to create new pieces of program from string data—but also a dangerous one, since if someone can trick your program into putting a string they provide into `Function`, they can make the program do anything they want.
+
+{{index [file, access]}}
+
+Standard JavaScript provides no such function as `readFile`, but different JavaScript environments, such as the browser and Node.js, provide their own ways of accessing files. The example just pretends that `readFile` exists.
+
+To avoid loading the same module multiple times, `require` keeps a store (cache) of already loaded modules. When called, it first checks whether the requested module has been loaded and, if not, loads it. This involves reading the module's code, wrapping it in a function, and calling it.
+
+{{index "ordinal package", "exports object", "module object", [interface, module]}}
+
+By defining `require` and `exports` as ((parameter))s for the generated wrapper function (and passing the appropriate values when calling it), the loader makes sure that these bindings are available in the module's ((scope)).
+
+An important difference between this system and ES modules is that ES module imports happen before a module's script starts running, whereas `require` is a normal function, invoked when the module is already running. Unlike `import` declarations, `require` calls _can_ appear inside functions, and the name of the dependency can be any expression that evaluates to a string, whereas `import` allows only plain quoted strings.
+
+The transition of the JavaScript community from CommonJS style to ES modules has been a slow and somewhat rough one. Fortunately we are now at a point where most of the popular packages on NPM provide their code as ES modules, and Node.js allows ES modules to import from CommonJS modules. While CommonJS code is still something you will run across, there is no real reason to write new programs in this style anymore.
+
+## Building and bundling
+
+{{index compilation, "type checking"}}
+
+Many JavaScript packages aren't technically written in JavaScript. Language extensions such as TypeScript, the type checking ((dialect)) mentioned in [Chapter ?](error#typing), are widely used. People also often start using planned new language features long before they have been added to the platforms that actually run JavaScript. To make this possible, they _compile_ their code, translating it from their chosen JavaScript dialect to plain old JavaScript—or even to a past version of JavaScript—so that ((browsers)) can run it.
+
+{{index latency, performance, [file, access], [network, speed]}}
+
+Including a modular program that consists of 200 different files in a ((web page)) produces its own problems. If fetching a single file over the network takes 50 milliseconds, loading the whole program takes 10 seconds, or maybe half that if you can load several files simultaneously. That's a lot of wasted time. Because fetching a single big file tends to be faster than fetching a lot of tiny ones, web programmers have started using tools that combine their programs (which they painstakingly split into modules) into a single big file before they publish it to the web. Such tools are called _((bundler))s_.
+
+{{index "file size"}}
+
+And we can go further. Apart from the number of files, the _size_ of the files also determines how fast they can be transferred over the network. Thus, the JavaScript community has invented _((minifier))s_. These are tools that take a JavaScript program and make it smaller by automatically removing comments and whitespace, renaming bindings, and replacing pieces of code with equivalent code that take up less space.
+
+{{index pipeline, tool}}
+
+It is not uncommon for the code that you find in an NPM package or that runs on a web page to have gone through _multiple_ stages of transformation—converting from modern JavaScript to historic JavaScript, combining the modules into a single file, and minifying the code. We won't go into the details of these tools in this book, since there are many of them, and which one is popular changes regularly. Just be aware that such things exist, and look them up when you need them.
+
+## Module design
+
+{{index [module, design], [interface, module], [code, "structure of"]}}
+
+Structuring programs is one of the subtler aspects of programming. Any nontrivial piece of functionality can be organized in various ways.
+
+Good program design is subjective—there are trade-offs involved, and matters of taste. The best way to learn the value of well-structured design is to read or work on a lot of programs and notice what works and what doesn't. Don't assume that a painful mess is “just the way it is”. You can improve the structure of almost everything by putting more thought into it.
+
+{{index [interface, module]}}
+
+One aspect of module design is ease of use. If you are designing something that is intended to be used by multiple people—or even by yourself, in three months when you no longer remember the specifics of what you did—it is helpful if your interface is simple and predictable.
+
+{{index "ini package", JSON}}
+
+That may mean following existing conventions. A good example is the `ini` package. This module imitates the standard `JSON` object by providing `parse` and `stringify` (to write an INI file) functions, and, like `JSON`, converts between strings and plain objects. The interface is small and familiar, and after you've worked with it once, you're likely to remember how to use it.
+
+{{index "side effect", "hard disk", composability}}
+
+Even if there's no standard function or widely used package to imitate, you can keep your modules predictable by using simple ((data structure))s and doing a single, focused thing. Many of the INI-file parsing modules on NPM provide a function that directly reads such a file from the hard disk and parses it, for example. This makes it impossible to use such modules in the browser, where we don't have direct filesystem access, and adds complexity that would have been better addressed by _composing_ the module with some file-reading function.
+
+{{index "pure function"}}
+
+This points to another helpful aspect of module design—the ease with which something can be composed with other code. Focused modules that compute values are applicable in a wider range of programs than bigger modules that perform complicated actions with side effects. An INI file reader that insists on reading the file from disk is useless in a scenario where the file's content comes from some other source.
+
+{{index "object-oriented programming"}}
+
+Relatedly, stateful objects are sometimes useful or even necessary, but if something can be done with a function, use a function. Several of the INI file readers on NPM provide an interface style that requires you to first create an object, then load the file into your object, and finally use specialized methods to get at the results. This type of thing is common in the object-oriented tradition, and it's terrible. Instead of making a single function call and moving on, you have to perform the ritual of moving your object through its various states. And because the data is now wrapped in a specialized object type, all code that interacts with it has to know about that type, creating unnecessary interdependencies.
+
+Often, defining new data structures can't be avoided—only a few basic ones are provided by the language standard, and many types of data have to be more complex than an array or a map. But when an array suffices, use an array.
+
+An example of a slightly more complex data structure is the graph from [Chapter ?](robot). There is no single obvious way to represent a ((graph)) in JavaScript. In that chapter, we used an object whose properties hold arrays of strings—the other nodes reachable from that node.
+
+There are several different pathfinding packages on ((NPM)), but none of them uses this graph format. They usually allow the graph's edges to have a weight, which is the cost or distance associated with it. That isn't possible in our representation.
+
+{{index "Dijkstra, Edsger", pathfinding, "Dijkstra's algorithm", "dijkstrajs package"}}
+
+For example, there's the `dijkstrajs` package. A well-known approach to pathfinding, quite similar to our `findRoute` function, is called _Dijkstra's algorithm_, after Edsger Dijkstra, who first wrote it down. The `js` suffix is often added to package names to indicate the fact that they are written in JavaScript. This `dijkstrajs` package uses a graph format similar to ours, but instead of arrays, it uses objects whose property values are numbers—the weights of the edges.
+
+If we wanted to use that package, we'd have to make sure that our graph was stored in the format it expects. All edges get the same weight, since our simplified model treats each road as having the same cost (one turn).
+
+```
+const {find_path} = require("dijkstrajs");
+
+let graph = {};
+for (let node of Object.keys(roadGraph)) {
+ let edges = graph[node] = {};
+ for (let dest of roadGraph[node]) {
+ edges[dest] = 1;
+ }
+}
+
+console.log(find_path(graph, "Post Office", "Cabin"));
+// → ["Post Office", "Alice's House", "Cabin"]
+```
+
+This can be a barrier to composition—when various packages are using different data structures to describe similar things, combining them is difficult. Therefore, if you want to design for composability, find out what ((data structure))s other people are using and, when possible, follow their example.
+
+{{index design}}
+
+Designing a fitting module structure for a program can be difficult. In the phase where you are still exploring the problem, trying different things to see what works, you might want to not worry about it too much, since keeping everything organized can be a big distraction. Once you have something that feels solid, that's a good time to take a step back and organize it.
+
+## Summary
+
+Modules provide structure to bigger programs by separating the code into pieces with clear interfaces and dependencies. The interface is the part of the module that's visible to other modules, and the dependencies are the other modules it makes use of.
+
+Because JavaScript historically did not provide a module system, the CommonJS system was built on top of it. Then at some point it _did_ get a built-in system, which now coexists uneasily with the CommonJS system.
+
+A package is a chunk of code that can be distributed on its own. NPM is a repository of JavaScript packages. You can download all kinds of useful (and useless) packages from it.
+
+## Exercises
+
+### A modular robot
+
+{{index "modular robot (exercise)", module, robot, NPM}}
+
+{{id modular_robot}}
+
+These are the bindings that the project from [Chapter ?](robot) creates:
+
+```{lang: "null"}
+roads
+buildGraph
+roadGraph
+VillageState
+runRobot
+randomPick
+randomRobot
+mailRoute
+routeRobot
+findRoute
+goalOrientedRobot
+```
+
+If you were to write that project as a modular program, what modules would you create? Which module would depend on which other module, and what would their interfaces look like?
+
+Which pieces are likely to be available prewritten on NPM? Would you prefer to use an NPM package or write them yourself?
+
+{{hint
+
+{{index "modular robot (exercise)"}}
+
+Here's what I would have done (but again, there is no single _right_ way to design a given module):
+
+{{index "dijkstrajs package"}}
+
+The code used to build the road graph lives in the `graph.js` module. Because I'd rather use `dijkstrajs` from NPM than our own pathfinding code, we'll make this build the kind of graph data that `dijkstrajs` expects. This module exports a single function, `buildGraph`. I'd have `buildGraph` accept an array of two-element arrays, rather than strings containing hyphens, to make the module less dependent on the input format.
+
+The `roads.js` module contains the raw road data (the `roads` array) and the `roadGraph` binding. This module depends on `./graph.js` and exports the road graph.
+
+{{index "random-item package"}}
+
+The `VillageState` class lives in the `state.js` module. It depends on the `./roads.js` module because it needs to be able to verify that a given road exists. It also needs `randomPick`. Since that is a three-line function, we could just put it into the `state.js` module as an internal helper function. But `randomRobot` needs it too. So we'd have to either duplicate it or put it into its own module. Since this function happens to exist on NPM in the `random-item` package, a reasonable solution is to just make both modules depend on that. We can add the `runRobot` function to this module as well, since it's small and closely related to state management. The module exports both the `VillageState` class and the `runRobot` function.
+
+Finally, the robots, along with the values they depend on, such as `mailRoute`, could go into an `example-robots.js` module, which depends on `./roads.js` and exports the robot functions. To make it possible for `goalOrientedRobot` to do route-finding, this module also depends on `dijkstrajs`.
+
+By offloading some work to ((NPM)) modules, the code became a little smaller. Each individual module does something rather simple and can be read on its own. Dividing code into modules also often suggests further improvements to the program's design. In this case, it seems a little odd that the `VillageState` and the robots depend on a specific road graph. It might be a better idea to make the graph an argument to the state's constructor and make the robots read it from the state object—this reduces dependencies (which is always good) and makes it possible to run simulations on different maps (which is even better).
+
+Is it a good idea to use NPM modules for things that we could have written ourselves? In principle, yes—for nontrivial things like the pathfinding function you are likely to make mistakes and waste time writing them yourself. For tiny functions like `random-item`, writing them yourself is easy enough. But adding them wherever you need them does tend to clutter your modules.
+
+However, you should also not underestimate the work involved in _finding_ an appropriate NPM package. And even if you find one, it might not work well or may be missing some feature you need. On top of that, depending on NPM packages means you have to make sure they are installed, you have to distribute them with your program, and you might have to periodically upgrade them.
+
+So again, this is a trade-off, and you can decide either way depending on how much a given package actually helps you.
+
+hint}}
+
+### Roads module
+
+{{index "roads module (exercise)"}}
+
+Write an ES module based on the example from [Chapter ?](robot) that contains the array of roads and exports the graph data structure representing them as `roadGraph`. It depends on a module `./graph.js` that exports a function `buildGraph`, used to build the graph. This function expects an array of two-element arrays (the start and end points of the roads).
+
+{{if interactive
+
+```{test: no}
+// Add dependencies and exports
+
+const roads = [
+ "Alice's House-Bob's House", "Alice's House-Cabin",
+ "Alice's House-Post Office", "Bob's House-Town Hall",
+ "Daria's House-Ernie's House", "Daria's House-Town Hall",
+ "Ernie's House-Grete's House", "Grete's House-Farm",
+ "Grete's House-Shop", "Marketplace-Farm",
+ "Marketplace-Post Office", "Marketplace-Shop",
+ "Marketplace-Town Hall", "Shop-Town Hall"
+];
+```
+
+if}}
+
+{{hint
+
+{{index "roads module (exercise)", "destructuring binding", "exports object"}}
+
+Since this is an ES module, you have to use `import` to access the graph module. That was described as exporting a `buildGraph` function, which you can pick out of its interface object with a destructuring `const` declaration.
+
+To export `roadGraph`, you put the keyword `export` before its definition. Because `buildGraph` takes a data structure that doesn't precisely match `roads`, the splitting of the road strings must happen in your module.
+
+hint}}
+
+### Circular dependencies
+
+{{index dependency, "circular dependency", "require function"}}
+
+A circular dependency is a situation where module A depends on B, and B also, directly or indirectly, depends on A. Many module systems simply forbid this because whichever order you choose for loading such modules, you cannot make sure that each module's dependencies have been loaded before it runs.
+
+((CommonJS modules)) allow a limited form of cyclic dependencies. As long as the modules don't access each other's interface until after they finish loading, cyclic dependencies are okay.
+
+The `require` function given [earlier in this chapter](modules#require) supports this type of dependency cycle. Can you see how it handles cycles?
+
+{{hint
+
+{{index overriding, "circular dependency", "exports object"}}
+
+The trick is that `require` adds the interface object for a module to its cache _before_ it starts loading the module. That way, if any `require` call made while it is running tries to load it, it is already known, and the current interface will be returned, rather than starting to load the module once more (which would eventually overflow the stack).
+
+hint}}
diff --git a/10_modules.txt b/10_modules.txt
deleted file mode 100644
index bcdfa7d26..000000000
--- a/10_modules.txt
+++ /dev/null
@@ -1,931 +0,0 @@
-:chap_num: 10
-:prev_link: 09_regexp
-:next_link: 11_language
-:load_files: ["code/chapter/10_modules.js", "js/loadfile.js"]
-
-= Modules =
-
-ifdef::html_target[]
-
-[chapterquote="true"]
-[quote, Master Yuan-Ma, The Book of Programming]
-____
-A beginning programmer writes her programs like an ant builds her
-hill, one piece at a time, without thought for the bigger structure.
-Her programs will be like loose sand. They may stand for a while, but
-growing too big they fall apart.
-
-Realizing this problem, the programmer will start to spend a lot of
-time thinking about structure. Her programs will be rigidly
-structured, like rock sculptures. They are solid, but when they must
-change, violence must be done to them.
-
-The master programmer knows when to apply structure and when to leave
-things in their simple form. Her programs are like clay, solid yet
-malleable.
-____
-
-endif::html_target[]
-
-(((organization)))(((code structure)))Every program has a shape. On
-a small scale, this shape is determined by its division into
-((function))s, and the blocks inside those functions. Programmers have
-a lot of freedom in the shape they give their programs. Shape follows
-more from the ((taste)) of the programmer than from the program's
-intended functionality.
-
-(((readability)))When looking at a larger program in its entirety,
-individual functions start to blend into the background. Such a
-program can be made more readable if we have larger unit of
-organization.
-
-_Modules_ divide programs into clusters of code that, by _some_
-criterion, belong together. This chapter explores some of the benefits
-that such division provides, and shows techniques for building
-((module))s in JavaScript.
-
-== Why modules help ==
-
-(((book analogy)))(((organization)))There are number of reasons why
-authors divide their books into ((chapter))s and sections. These
-divisions make it easier for a reader to see how the book is built up,
-and to find specific parts that they are interested in. They also help
-the _author_, by providing a clear focus for every section.
-
-The benefits of organizing a program into several ((file))s or
-((module))s are similar. Structure helps people who aren't yet
-familiar with the code find what they are looking for, and makes it
-easier for the programmer to keep things that are closely related
-close together.
-
-(((project chapter)))(((readability)))(((interconnection)))Some
-programs are even organized along the model of a traditional ((text)),
-with a well-defined order in which the reader is encouraged to go
-through the program, and lots of prose (comments) providing a coherent
-description of the code. This make reading the program a lot less
-intimidating—reading unknown code is usually intimidating—but has the
-downside of being more work to set up. It also makes the program more
-difficult to change, because prose tends to be more tightly
-interconnected than code. This style is called _((literate
-programming))_. The “project” chapters of this book can be considered
-literate programs.
-
-(((minimalism)))(((evolution)))(((structure)))(((organization)))As a
-general rule, structuring things costs energy. In the early stages of
-a project, when you are not quite sure yet what goes where or what
-kind of ((module))s the program needs at all, I endorse a minimalist,
-structureless attitude. Just put everything wherever it is convenient
-to put it until the code stabilizes. That way, you won't be wasting
-time moving pieces of the program back and forth, and won't
-accidentally lock yourself into a structure that does not actually fit
-your program.
-
-=== Namespacing ===
-
-(((encapsulation)))(((isolation)))(((global
-scope)))(((local scope)))Most modern ((programming language))s have a
-((scope)) level between _global_ (everyone can see it) and _local_
-(only this function can see it). JavaScript does not. Thus, by
-default, everything that needs to be visible outside of the scope of a
-top-level function is visible _everywhere_.
-
-(((namespace pollution)))Namespace pollution, the problem of a lot of
-unrelated code having to share a single set of global variable names,
-was mentioned in link:04_data.html#namespace_pollution[Chapter 4],
-where the `Math` object was given as an example of an object that acts
-like a module by grouping math-related functionality.
-
-(((function,as namespace)))Though JavaScript provides no actual
-((module)) construct yet, objects can be used to create publicly
-accessible sub-((namespace))s, and functions can be used to create an
-isolated, private namespace inside of a module. Later in this chapter,
-I will discuss way to build reasonably convenient, namespace-isolating
-modules on top of the primitive concepts that JavaScript gives us.
-
-=== Reuse ===
-
-(((version control)))(((bug)))(((copy-paste programming)))(((ini
-file)))(((dependency)))(((structure)))In a “flat” project, which isn't
-structured as a set of ((module))s, it is not apparent which parts of
-the code are needed to use a particular function. In my program for
-spying on my enemies (see link:09_regexp.html#ini[Chapter 9]), I wrote
-a function for reading configuration files. If I want to use that
-function in another project, I must go and copy out the part of the
-old program that look like they are relevant to the functionality that
-I need, and paste them into my new program. Then, if I find a mistake
-in that code, I'll only fix it in whichever program that I'm working
-with at the time, and forget to also fix it in the other program.
-
-(((duplication)))Once you have lots of such shared, duplicated pieces
-of code, you will find yourself wasting a lot of time and energy on
-moving them around and keeping them up to date.
-
-(((reuse)))Putting pieces of functionality that stand on their own
-into separate files and modules makes them easier to track, update,
-and share, because all the various pieces of code that want to use the
-module load it from the same actual file.
-
-(((dependency)))(((library)))(((installation)))(((upgrading)))This
-idea gets even more powerful when the relations between modules—which
-other modules each module depends on—are explicitly stated. You can
-then automate the process of installing and upgrading external modules
-(_libraries_).
-
-(((package manager)))(((download)))(((reuse)))Taking this idea even
-further, imagine an online service that tracks and distributes
-hundreds of thousands of such libraries, allowing you to search for
-the functionality you need, and, once you find it, set up your project
-to automatically download it.
-
-[[modules_npm]]
-(((NPM)))This service exists. It is called _NPM_
-(http://npmjs.org[_npmjs.org_]). NPM consists of an online database of
-modules, and a tool for downloading and upgrading the modules your
-program depends on. It grew out of ((Node.js)), the browser-less
-JavaScript environment we will discuss in
-link:20_node.html#node[Chapter 20], but can also be useful when
-programming for the browser.
-
-=== Decoupling ===
-
-(((isolation)))(((decoupling)))(((backwards
-compatibility)))Another important role of modules is isolating pieces
-of code from each other, in the same way that the object interfaces
-from link:06_object.html#interface[Chapter 6] do. A well-designed
-module will provide an interface for external code to use. As the
-module gets updated with ((bug))-fixes and new functionality, the
-existing ((interface)) stays the same (it is _stable_), so that other
-modules can use the new, improved version without any changes to
-themselves.
-
-(((stability)))Note that a stable interface does not mean no new
-functions, methods, or variables are added. It just means that
-existing functionality isn't removed and its meaning is not changed.
-
-(((implementation detail)))(((encapsulation)))A good ((module))
-((interface)) should allow the module to grow without breaking the old
-interface. This means exposing as few of the module's internal
-concepts as possible, while also making the “language” that the
-interface exposes powerful and flexible enough to be applicable in a
-wide range of situations.
-
-(((interface,design)))For interfaces that expose a single, focused
-concept, like a configuration file reader, this design comes
-naturally. For others, like a text editor, which has many different
-aspects that external code might need to access (content, styling,
-user actions, and so on), it requires careful design.
-
-== Using functions as namespaces ==
-
-(((namespace)))(((function,as namespace)))Functions are the only things in
-JavaScript that create a new ((scope)). So if we want our ((module))s
-to have their own scope, we will have to base them on functions.
-
-(((weekday example)))(((Date type)))(((getDay method)))Consider this
-trivial module for associating names with day-of-the-week numbers, as
-returned by a `Date` object's `getDay` method:
-
-[source,javascript]
-----
-var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
- "Thursday", "Friday", "Saturday"];
-function dayName(number) {
- return names[number];
-}
-
-console.log(dayName(1));
-// → Monday
-----
-
-(((access control)))(((encapsulation)))The `dayName` function is part
-of the module's ((interface)), but the `names` variable is not. We
-would prefer _not_ to spill it into the ((global scope)).
-
-We can do this:
-
-[source,javascript]
-----
-var dayName = function() {
- var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
- "Thursday", "Friday", "Saturday"];
- return function(number) {
- return names[number];
- };
-}();
-
-console.log(dayName(3));
-// → Wednesday
-----
-
-(((anonymous function)))Now `names` is a local variable in an
-(unnamed) function. This function is created and immediately called,
-and its return value (the actual `dayName` function) is stored in a
-variable. We could have pages and pages of code in this function, with
-a hundred local variables, and they would all be internal to our
-module—visible to the module itself, but not to outside code.
-
-(((isolation)))(((side effect)))We can use a similar pattern to
-isolate code from the outside world entirely. The module below logs a
-value to the console, but does not actually provide any values for
-other modules to use.
-
-[source,javascript]
-----
-(function() {
- function square(x) { return x * x; }
- var hundred = 100;
-
- console.log(square(hundred));
-})();
-// → 10000
-----
-
-(((namespace pollution)))This code simply outputs the square of one
-hundred, but in the real world it could be a module that adds a method
-to some ((prototype)), or sets up a widget on a web page. It is
-wrapped in a function to prevent the variables it uses internally from
-polluting the ((global scope)).
-
-(((parsing)))(((function keyword)))Why did we wrap the namespace
-function in a pair of ((parentheses))? This has to do with a quirk in
-JavaScript's ((syntax)). If an _((expression))_ starts with the
-keyword `function`, it is a function expression. However, if a
-_((statement))_ starts with `function`, it is a function
-_declaration_, which requires a name and, not being an expression,
-cannot be called by writing parentheses after it. You can think of the
-extra wrapping parentheses as a trick to force the function to be
-interpreted as an expression.
-
-== Objects as interfaces ==
-
-(((interface)))Now imagine that we want to add another function to our
-day-of-the-week module, one that that goes from a day name to a
-number. We can't simply return the function anymore, but must wrap the
-two functions in an object.
-
-[source,javascript]
-----
-var weekDay = function() {
- var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
- "Thursday", "Friday", "Saturday"];
- return {
- name: function(number) { return names[number]; },
- number: function(name) { return names.indexOf(name); }
- };
-}();
-
-console.log(weekDay.name(weekDay.number("Sunday")));
-// → Sunday
-----
-
-(((exporting)))(((exports object)))(((this)))For bigger ((module))s,
-gathering all the _exported_ values into an object at the end of the
-function becomes awkward, since many of the exported functions are
-likely to be big, and you'd prefer to write them somewhere else, near
-related internal code. A convenient alternative is to declare an
-object (conventionally named `exports`) and add properties to that
-whenever we are defining something that needs to be exported. In the
-example below, the module function takes its interface object as
-argument, allowing code outside of the function to create it and store
-it in a variable. (Outside of a function, `this` refers to the global
-scope object).
-
-[source,javascript]
-----
-(function(exports) {
- var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
- "Thursday", "Friday", "Saturday"];
-
- exports.name = function(number) {
- return names[number];
- };
- exports.number = function(name) {
- return names.indexOf(name);
- };
-})(this.weekDay = {});
-
-console.log(weekDay.name(weekDay.number("Saturday")));
-// → Saturday
-----
-
-== Detaching from the global scope ==
-
-(((variable,global)))The above pattern is commonly used by JavaScript
-modules intended for the ((browser)). The module will claim a single
-global variable, and wrap its code in a function in order to have its
-own private ((namespace)). But this pattern still causes problems if
-multiple modules happen to claim the same name, or if you want to load
-two ((version))s of a module alongside each other.
-
-(((module loader)))(((require
-function)))(((CommonJS)))(((dependency)))With a little plumbing, we
-can create a system that allows one ((module)) to directly ask for the
-((interface)) object of another module, without going through the
-global scope. Our goal is a `require` function which, when given a
-module name, will load that module's file (from disk or the Web,
-depending on the platform we are running on), and return the
-appropriate interface value.
-
-This approach solves the problems mentioned above, and has the added
-benefit of making you program's dependencies explicit, making it
-harder to accidentally make use of some module without stating that
-you need it.
-
-(((readFile function)))(((require function)))For `require` we need two
-things. First, we want a function `readFile`, which returns the
-content of a given file as a string. (A single such function is not
-present in ((standard)) JavaScript, but different JavaScript
-environments, such as the browser and Node.js, provide their own ways
-of accessing ((file))s. For now, let's just pretend we have this
-function.) Secondly, we need to be able to actually execute this
-string as JavaScript code.
-
-[[eval]]
-== Evaluating data as code ==
-
-(((evaluation)))(((interpretation)))There are several ways to take
-data (a string of code) and run it as part of the current program.
-
-(((isolation)))(((eval)))The most obvious way is the special operator
-`eval`, which will execute a string of code in the _current_ scope.
-This is usually a bad idea, because it breaks some of the sane
-properties that scopes normally have, such as being isolated from the
-outside world.
-
-[source,javascript]
-----
-function evalAndReturnX(code) {
- eval(code);
- return x;
-}
-
-console.log(evalAndReturnX("var x = 2"));
-// → 2
-----
-
-(((Function constructor)))A better way of interpreting data as code is
-to use the `Function` constructor. This takes two arguments: a string
-containing a comma-separated list of argument names, and a string
-containing the function's body.
-
-[source,javascript]
-----
-var plusOne = new Function("n", "return n + 1;");
-console.log(plusOne(4));
-// → 5
-----
-
-This is precisely what we need for our modules. We can wrap a module's
-code in a function, with that function's scope becoming our module
-((scope)).
-
-[[commonjs]]
-== Require ==
-
-(((require function)))(((CommonJS)))The following is a very minimal
-implementation of `require`:
-
-// test: wrap
-
-[source,javascript]
-----
-function require(name) {
- var code = new Function("exports", readFile(name));
- var exports = {};
- code(exports);
- return exports;
-}
-
-console.log(require("weekDay").name(1));
-// → Monday
-----
-
-(((weekday example)))(((exports object)))(((Function
-constructor)))Since the `new Function` constructor wraps the module
-code in a function, we don't have to write a wrapping ((namespace))
-function in the module file itself. And since we make `exports` an
-argument to the module function, the module does not have to declare
-it. This removes a lot of clutter from our example module:
-
-[source,javascript]
-----
-var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
- "Thursday", "Friday", "Saturday"];
-
-exports.name = function(number) {
- return names[number];
-};
-exports.number = function(name) {
- return names.indexOf(name);
-};
-----
-
-(((require function)))When using this pattern, a ((module)) typically
-starts with a few variable declarations that load the modules it
-depends on.
-
-// test: no
-
-[source,javascript]
-----
-var weekDay = require("weekDay");
-var today = require("today");
-
-console.log(weekDay.name(today.dayNumber()));
-----
-
-(((efficiency)))The simplistic implementation of `require` given above
-has several problems. For one, it will load and run a module every
-time it is ++require++d, so if several modules have the same
-dependency, or a `require` call is put inside of a function that will
-be called multiple times, time and energy will be wasted.
-
-(((cache)))This can be solved by storing the modules that have already
-been loaded in an object, and simply returning the existing value when
-one is loaded multiple times.
-
-(((exports object)))(((exporting)))The second problem is that it is
-not possible for a module to directly export value other than the
-`exports` object, such as a function. For example, a module might want
-to only export the constructor of the object type it defines. Right
-now, it can not do that, because `require` always uses the `exports`
-object it creates as the exported value.
-
-(((module object)))The traditional solution for this is to provide
-modules with another variable, `module`, which is an object that has a
-property `exports`. This property initially points at the empty object
-created by `require`, but can be overwritten with another value in
-order to export something else.
-
-// test: wrap
-// include_code
-
-[source,javascript]
-----
-function require(name) {
- if (name in require.cache)
- return require.cache[name];
-
- var code = new Function("exports, module", readFile(name));
- var exports = {}, mod = {exports: exports};
- code(exports, mod);
-
- require.cache[name] = module.exports;
- return module.exports;
-}
-require.cache = Object.create(null);
-----
-
-(((require function)))We now have a module system that uses a single
-global variable (`require`) to allow modules to find and use each
-other without going through the ((global scope)).
-
-This style of module system is called _((CommonJS)) modules_, after
-the pseudo-((standard)) that first specified it. It is built into the
-((Node.js)) system. Real implementations do a lot more than the
-example I showed. Most importantly, they have a much more intelligent
-way of going from a module name to an actual piece of code, allowing
-both pathnames relative to the current file, and module names that
-point directly to locally installed modules.
-
-[[amd]]
-== Slow-loading modules ==
-
-(((loading)))(((synchronous I/O)))(((blocking)))(((World Wide
-Web)))Though it is possible to use the CommonJS module style when
-writing JavaScript for the ((browser)), it is somewhat involved. The
-reason for this is that reading a file (module) from the Web is a lot
-slower than reading it from the hard disk. While a script is running
-in the browser, nothing else can happen to the website on which it
-runs, for reasons that will become clear in
-link:14_event.html#timeline[Chapter 14]. This means that if every
-`require` call went and fetched something from some far-away web
-server, the page would freeze for a painfully long time while loading
-its scripts.
-
-(((Browserify)))(((require function)))(((preprocessing)))One way to
-work around this problem is to run a program like
-http://browserify.org[_Browserify_] on your code before you serve it
-on a web page. This will look for calls to `require` and gather the
-dependencies it finds together into a big file. When you actually
-serve the code, you only have to call `require` once with this big
-file.
-
-(((dependency)))(((asynchronous I/O)))Another solution is to wrap the
-code that makes up your module in a function, so that the ((module
-loader)) can first load its dependencies in the background, and then
-call the function, initializing the ((module)), when the dependencies
-have been loaded. That is what the _Asynchronous Module Definition_
-(_((AMD))_) module system does.
-
-(((weekday example)))Our trivial program with dependencies would look
-like this in AMD:
-
-// test: no
-
-[source,javascript]
-----
-define(["weekDay", "today"], function(weekDay, today) {
- console.log(weekDay.name(today.dayNumber()));
-});
-----
-
-(((define function)))(((asynchronous programming)))The `define`
-function is central to this approach. It takes first an array of
-module names, and then a function that takes one argument for each
-dependency. It will load the dependencies (if they haven't already
-been loaded) in the background, allowing the page to continue working
-while the files are being fetched. Once all dependencies are loaded,
-`define` will call the function it was given, with the ((interface))s
-of those dependencies as arguments.
-
-(((weekday example)))(((define function)))The modules that are loaded
-this way must themselves contain a call to `define`. The value used as
-their interface is whatever was returned by the function passed to
-`define`. Here is the `weekDay` module again:
-
-[source,javascript]
-----
-define([], function() {
- var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
- "Thursday", "Friday", "Saturday"];
- return {
- name: function(number) { return names[number]; },
- number: function(name) { return names.indexOf(name); }
- };
-});
-----
-
-(((define function)))(((backgroundReadFile function)))In order to be
-able to show a minimal implementation of `define`, we will pretend we
-have a `backgroundReadFile` function, which takes a file name and a
-function, and will call the function with the content of the file as
-soon as it has finished loading it. (link:17_http.html#getURL[Chapter
-17] will explain how to write that function.)
-
-For the purpose of keeping track of modules while they are being
-loaded, the implementation of `define` will use objects that describe
-the state of modules, telling us whether they are available yet, and
-providing their interface when they are.
-
-The `getModule` function, when given a name, will return such an
-object, and ensure that the module is scheduled to be loaded. It uses
-a ((cache)) object to avoid loading the same module twice.
-
-// include_code
-
-[source,javascript]
-----
-var defineCache = Object.create(null);
-var currentMod = null;
-
-function getModule(name) {
- if (name in defineCache)
- return defineCache[name];
-
- var mod = {exports: null,
- loaded: false,
- onLoad: []};
- defineCache[name] = mod;
- backgroundReadFile(name, function(code) {
- currentMod = mod;
- new Function("", code)();
- });
- return mod;
-}
-----
-
-(((define function)))We assume the loaded file also contains a
-(single) call to `define`. The `currentMod` variable is used to tell
-this call about the module object that is currently being loaded, so
-that it can update this object when it finishes loading. We will come
-back to this mechanism in a moment.
-
-(((dependency)))(((Function constructor)))(((asynchronous
-programming)))(((event handling)))The `define` function itself uses
-`getModule` to fetch or create the module objects for the current
-module's dependencies. Its task is to schedule the `moduleFunction`
-(the function that contains the module's actual code) to be run
-whenever those dependencies are loaded. For this purpose, it defines a
-function `whenDepsLoaded`, that is added to the `onLoad` array of all
-dependencies that are not yet loaded. This function immediately
-returns if there are still unloaded dependencies, so it will only do
-actual work once, when the last dependency has finished loading. It is
-also called immediately, from `define` itself, in case there are no
-dependencies that need to be loaded.
-
-// include_code
-
-[source,javascript]
-----
-function define(depNames, moduleFunction) {
- var myMod = currentMod;
- var deps = depNames.map(getModule);
-
- deps.forEach(function(mod) {
- if (!mod.loaded)
- mod.onLoad.push(whenDepsLoaded);
- });
-
- function whenDepsLoaded() {
- if (!deps.every(function(m) { return m.loaded; }))
- return;
-
- var args = deps.map(function(m) { return m.exports; });
- var exports = moduleFunction.apply(null, args);
- if (myMod) {
- myMod.exports = exports;
- myMod.loaded = true;
- myMod.onLoad.every(function(f) { f(); });
- }
- }
- whenDepsLoaded();
-}
-----
-
-(((define function)))When all dependencies are available,
-`whenDepsLoaded` calls the function that holds the module, giving it
-the dependencies' interfaces as arguments.
-
-The first thing `define` does is store the value that `currentMod` had
-when it was called in a variable `myMod`. Remember that `getModule`,
-just before evaluating the code for a module, stored the corresponding
-module object in `currentMod`. This allows `whenDepsLoaded` to store
-the return value of the module function in that module's `exports`
-property, set the module's `loaded` property to true, and call all the
-functions that are waiting for the module to load.
-
-(((asynchronous programming)))This code is a lot harder to follow than
-the `require` function. Its execution does not follow a simple,
-predictable path. Instead, multiple operations are set up to happen at
-some unspecified time in the ((future)), which obscures the way the
-code executes.
-
-A real ((AMD)) implementation is, again, quite a lot more clever about
-resolving module names to actual URLs, and generally more robust than
-the one above. The _((RequireJS))_ (_requirejs.org_) project provides
-a popular implementation of this style of ((module loader)).
-
-== Interface design ==
-
-(((interface,design)))Designing interfaces for modules and object
-types is one of the subtler aspects of programming. Any non-trivial
-piece functionality can be modeled in various ways. Finding a way that
-works well requires insight and foresight.
-
-The best way to learn the value of good interface design is to use
-lots of interfaces, some good, some bad. Experience will teach
-you what works and what doesn't. Never assume that a painful interface
-is “just the way it is”. Fix it, or wrap it in a new interface that
-works better for you.
-
-=== Predictability ===
-
-(((documentation)))(((predictability)))(((convention)))If programmers
-can predict the way your interface works, they (or you) won't get
-sidetracked as often by the need to look up how to use it. Thus, try
-to follow conventions. When there is another module or part of the
-standard JavaScript environment that does something similar to what
-you are implementing, it might be a good idea to make your interface
-resemble the existing interface. That way, it'll feel familiar to
-people who know the existing interface.
-
-(((cleverness)))Another area where predictability is important is the
-actual _behavior_ of your code. It can be tempting to make an
-unnecessarily clever interface with the justification that it's more
-convenient to use. For example, you could accept all kinds of
-different types and combinations of arguments, and do the “right
-thing” for all of them. Or you could provide dozens of specialized
-convenience functions that provide slightly different flavors of your
-module's functionality. These might make code that builds on your
-interface slightly shorter, but they will also make it much harder for
-people to build a clear ((mental model)) of the module's behavior.
-
-=== Composability ===
-
-(((composability)))In your interfaces, try to use the simplest ((data
-structure))s possible, and make functions do a single, clear thing.
-Whenever practical, make them ((pure function))s (see
-link:03_functions.html#pure[Chapter 3]).
-
-(((array-like object)))For example, it is not uncommon for modules to
-provide their own array-like collection objects, with their own
-interface for counting and extracting elements. Such objects won't
-have `map` or `forEach` methods, and any existing function that
-expects a real array won't be able to work with them. This is an
-example of poor __composability__—the module cannot be easily composed
-with other code.
-
-(((encapsulation)))(((spell-check example)))Another example would be a
-module for spell-checking text, which we might need when we want to
-write a text editor. The spell-checker could be made to operate
-directly on whichever complicated ((data structure))s the editor uses,
-and directly call internal functions in the editor to have the user
-choose between spelling suggestions. If we go that way, the module
-cannot be used with any other programs. On the other hand, if we
-define the spell-checking interface so that you can pass it a simple
-string and it will return the position in the string where it found a
-possible misspelling, along with an array of suggested corrections,
-then we have an interface that could also be composed with other
-systems, because strings and arrays are always available in
-JavaScript.
-
-=== Layered interfaces ===
-
-(((simplicity)))(((complexity)))(((layering)))(((interface
-design)))When designing an interface for a complex piece of
-functionality—sending email, for example—you often run into a dilemma.
-On the one hand, you do not want to overload the user of your
-interface with details. They shouldn't have to study your interface
-for 20 minutes before they can send an email. On the other hand, you
-do not want to hide all the details either—when people need to do
-complicated things with your module, they should be able to.
-
-Often the solution is to provide two interfaces: a detailed
-_low-level_ one for complex situations and a simple _high-level_ one
-for routine use. The second can usually be built very easily using the
-tools provided by the first. In the email module, the high-level
-interface could just be a function that takes a message, a sender
-address, and a receiver address, and sends the email. The low-level
-interface would allow full control over email headers, attachments,
-sending HTML mail, and so on.
-
-== Summary ==
-
-Modules provide structure to bigger programs by separating the code
-into different files and namespaces. Giving these modules well-defined
-interfaces makes them easier to use and reuse,
-and makes it possible to continue using them as the module
-itself evolves.
-
-Though the JavaScript language itself is characteristically unhelpful
-when it comes to modules, the flexible functions and objects it
-provides make it possible to define rather nice module systems.
-Function scopes can be used as internal namespaces for the module, and
-objects can be used to store sets of exported values.
-
-There are two popular, well-defined approaches to such modules. One is
-called _CommonJS Modules_, and revolves around a `require` function
-that fetches a module by name and returns its interface. The other is
-called _AMD_, and uses a `define` function that takes an array of
-module names and a function, and, after loading the modules, runs the
-function with their interfaces as arguments.
-
-== Exercises ==
-
-=== Month names ===
-
-(((Date type)))(((weekday example)))(((month name (exercise))))Write a
-simple module similar to the `weekDay` module, which can convert month
-numbers (zero-based, as in the `Date` type) to names, and names back
-to numbers. Give it its own namespace, since it will need an internal
-array of month names, and use plain JavaScript, without any module
-loader system.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Your code here.
-
-console.log(month.name(2));
-// → March
-console.log(month.number("November"));
-// → 10
-----
-endif::html_target[]
-
-!!solution!!
-
-(((month name (exercise))))This follows the `weekDay` module almost
-exactly. A function expression, called immediately, wraps the variable
-that holds the array of names, along with the two functions that must
-be exported. The functions are put in an object, and returned. The
-returned interface object is stored in the `month` variable.
-
-!!solution!!
-
-=== A return to electronic life ===
-
-(((electronic life)))(((module)))Hoping that
-link:07_elife.html#elife[Chapter 7] is still somewhat fresh in your
-mind, think back to the system designed in that chapter and come up
-with a way to separate the code into modules. To refresh your memory,
-these are the functions and types defined in that chapter, in order of
-appearance.
-
-----
-Vector
-Grid
-directions
-randomElement
-BouncingCritter
-elementFromChar
-World
-charFromElement
-Wall
-View
-directionNames
-WallFollower
-dirPlus
-LifelikeWorld
-Plant
-PlantEater
-SmartPlantEater
-Tiger
-----
-
-(((book analogy)))Don't exaggerate and create too many modules. A book
-that starts a new chapter for every page would probably get on your
-nerves, if only because of all the space wasted on titles. Similarly,
-having to open ten files to read a tiny project isn't helpful. Aim for
-three to five modules.
-
-(((encapsulation)))You can choose to have some functions become
-internal to their module, and thus inaccessible to other modules.
-
-There is no single correct solution here. Module organization is
-largely a matter of ((taste)).
-
-!!solution!!
-
-Here is what I came up with. I've put parentheses around internal
-functions.
-
-----
-Module "grid"
- Vector
- Grid
- directions
-
-Module "world"
- (randomElement)
- (elementFromChar)
- (charFromElement)
- View
- World
- LifelikeWorld
- directions [re-exported]
-
-Module "simple_ecosystem"
- (randomElement) [duplicated]
- (directionNames)
- (dirPlus)
- Wall
- BouncingCritter
- WallFollower
-
-Module "ecosystem"
- Wall [duplicated]
- Plant
- PlantEater
- SmartPlantEater
- Tiger
-----
-
-(((exporting)))I have re-exported the `directions` array from the
-`grid` module from `world`, so that modules built on that (the
-ecosystems) don't have to know or worry about the existence of the
-`grid` module.
-
-(((duplication)))I also duplicated two generic and tiny helper values
-(`randomElement` and `Wall`) since they are used as internal details
-in different contexts, and do not belong in the interfaces for these
-modules.
-
-!!solution!!
-
-=== Circular dependencies ===
-
-(((dependency)))(((circular dependency)))(((require function)))A
-tricky subject in dependency management is circular dependencies,
-where module A depends on B, and B also depends on A. Many module
-systems simply forbid this. ((CommonJS)) modules allow a limited form:
-it works as long as the modules do not replace their default `exports`
-object with another value, and only start accessing each other's
-interface after they finish loading.
-
-Can you think of a way in which support for this feature could be
-implemented? Look back to the definition of `require`, and consider
-what the function would have to do to allow this.
-
-!!solution!!
-
-(((overriding)))(((circular dependency)))(((exports object)))The trick
-is to add the `exports` object created for a module to `require`'s
-((cache)) _before_ actually running the module. This means the module
-will not yet have had a chance to override `module.exports`, so we do
-not know whether it may want to export some other value. After
-loading, the cache object is overridden with `module.exports`, which
-may be a different value.
-
-But if, in the course of loading the module, a second module is loaded
-that asks for the first module, its default `exports` object, likely
-still empty at this point, will be in the cache, and the second module
-will receive a reference to it. If it doesn't try to do anything with
-the object until the first module has finished loading, things will
-work.
-
-!!solution!!
diff --git a/11_async.md b/11_async.md
new file mode 100644
index 000000000..ee30fd7fd
--- /dev/null
+++ b/11_async.md
@@ -0,0 +1,856 @@
+{{meta {load_files: ["code/hangar2.js", "code/chapter/11_async.js"], zip: "node/html"}}}
+
+# Asynchronous Programming
+
+{{quote {author: "Laozi", title: "Tao Te Ching", chapter: true}
+
+Who can wait quietly while the mud settles?\
+Who can remain still until the moment of action?
+
+quote}}
+
+{{index "Laozi"}}
+
+{{figure {url: "img/chapter_picture_11.jpg", alt: "Illustration showing two crows on a tree branch", chapter: framed}}}
+
+The central part of a computer, the part that carries out the individual steps that make up our programs, is called the _((processor))_. The programs we have seen so far will keep the processor busy until they have finished their work. The speed at which something like a loop that manipulates numbers can be executed depends pretty much entirely on the speed of the computer's processor and memory.
+
+{{index [memory, speed], [network, speed]}}
+
+But many programs interact with things outside of the processor. For example, they may communicate over a computer network or request data from the ((hard disk))—which is a lot slower than getting it from memory.
+
+When such a thing is happening, it would be a shame to let the processor sit idle—there might be some other work it could do in the meantime. In part, this is handled by your operating system, which will switch the processor between multiple running programs. But that doesn't help when we want a _single_ program to be able to make progress while it is waiting for a network request.
+
+## Asynchronicity
+
+{{index "synchronous programming"}}
+
+In a _synchronous_ programming model, things happen one at a time. When you call a function that performs a long-running action, it returns only when the action has finished and it can return the result. This stops your program for the time the action takes.
+
+{{index "asynchronous programming"}}
+
+An _asynchronous_ model allows multiple things to happen at the same time. When you start an action, your program continues to run. When the action finishes, the program is informed and gets access to the result (for example, the data read from disk).
+
+We can compare synchronous and asynchronous programming using a small example: a program that makes two requests over the ((network)) and then combines the results.
+
+{{index "synchronous programming"}}
+
+In a synchronous environment, where the request function returns only after it has done its work, the easiest way to perform this task is to make the requests one after the other. This has the drawback that the second request will be started only when the first has finished. The total time taken will be at least the sum of the two response times.
+
+{{index parallelism}}
+
+The solution to this problem, in a synchronous system, is to start additional ((thread))s of control. A _thread_ is another running program whose execution may be interleaved with other programs by the operating system—since most modern computers contain multiple processors, multiple threads may even run at the same time, on different processors. A second thread could start the second request, and then both threads wait for their results to come back, after which they resynchronize to combine their results.
+
+{{index CPU, blocking, "asynchronous programming", timeline, "callback function"}}
+
+In the following diagram, the thick lines represent time the program spends running normally, and the thin lines represent time spent waiting for the network. In the synchronous model, the time taken by the network is _part_ of the timeline for a given thread of control. In the asynchronous model, starting a network action allows the program to continue running while the network communication happens alongside it, notifying the program when it is finished.
+
+{{figure {url: "img/control-io.svg", alt: "Diagram of showing control flow in synchronous and asynchronous programs. The first part shows a synchronous program, where the program's active and waiting phases all happen on a single, sequential line. The second part shows a multi-threaded synchronous program, with two parallel lines, on which the waiting parts happen alongside each other, causing the program to finish faster. The last part shows an asynchronous program, where the multiple asynchronous actions branch off from the main program, which at some point stops, and then resumes whenever the first thing it was waiting for finishes.",width: "8cm"}}}
+
+{{index ["control flow", asynchronous], "asynchronous programming", verbosity, performance}}
+
+Another way to describe the difference is that waiting for actions to finish is _implicit_ in the synchronous model, while it is _explicit_—under our control—in the asynchronous one.
+
+Asynchronicity cuts both ways. It makes expressing programs that do not fit the straight-line model of control easier, but it can also make expressing programs that do follow a straight line more awkward. We'll see some ways to reduce this awkwardness later in the chapter.
+
+Both prominent JavaScript programming platforms—((browser))s and ((Node.js))—make operations that might take a while asynchronous, rather than relying on ((thread))s. Since programming with threads is notoriously hard (understanding what a program does is much more difficult when it's doing multiple things at once), this is generally considered a good thing.
+
+## Callbacks
+
+{{indexsee [function, callback], "callback function"}}
+
+One approach to ((asynchronous programming)) is to make functions that need to wait for something take an extra argument, a _((callback function))_. The asynchronous function starts a process, sets things up so that the callback function is called when the process finishes, and then returns.
+
+{{index "setTimeout function", waiting}}
+
+As an example, the `setTimeout` function, available both in Node.js and in browsers, waits a given number of milliseconds and then calls a function.
+
+```{test: no}
+setTimeout(() => console.log("Tick"), 500);
+```
+
+Waiting is not generally important work, but it can be very useful when you need to arrange for something to happen at a certain time or check whether some action is taking longer than expected.
+
+{{index "readTextFile function"}}
+
+Another example of a common asynchronous operation is reading a file from a device's storage. Imagine you have a function `readTextFile` that reads a file's content as a string and passes it to a callback function.
+
+```
+readTextFile("shopping_list.txt", content => {
+ console.log(`Shopping List:\n${content}`);
+});
+// → Shopping List:
+// → Peanut butter
+// → Bananas
+```
+
+The `readTextFile` function is not part of standard JavaScript. We will see how to read files in the browser and in Node.js in later chapters.
+
+Performing multiple asynchronous actions in a row using callbacks means that you have to keep passing new functions to handle the ((continuation)) of the computation after the actions. An asynchronous function that compares two files and produces a boolean indicating whether their content is the same might look like this:
+
+```
+function compareFiles(fileA, fileB, callback) {
+ readTextFile(fileA, contentA => {
+ readTextFile(fileB, contentB => {
+ callback(contentA == contentB);
+ });
+ });
+}
+```
+
+This style of programming is workable, but the indentation level increases with each asynchronous action because you end up in another function. Doing more complicated things, such as wrapping asynchronous actions in a loop, can get awkward.
+
+In a way, asynchronicity is _contagious_. Any function that calls a function that works asynchronously must itself be asynchronous, using a callback or similar mechanism to deliver its result. Calling a callback is somewhat more involved and error prone than simply returning a value, so needing to structure large parts of your program that way is not great.
+
+## Promises
+
+A slightly different way to build an asynchronous program is to have asynchronous functions return an object that represents its (future) result instead of passing around callback functions. This way, such functions actually return something meaningful, and the shape of the program more closely resembles that of synchronous programs.
+
+{{index "Promise class", "asynchronous programming", "resolving (a promise)", "then method", "callback function"}}
+
+This is what the standard class `Promise` is for. A _promise_ is a receipt representing a value that may not be available yet. It provides a `then` method that allows you to register a function that should be called when the action for which it is waiting finishes. When the promise is _resolved_, meaning its value becomes available, such functions (there can be multiple) are called with the result value. It is possible to call `then` on a promise that has already resolved—your function will still be called.
+
+{{index "Promise.resolve function"}}
+
+The easiest way to create a promise is by calling `Promise.resolve`. This function ensures that the value you give it is wrapped in a promise. If it's already a promise, it is simply returned. Otherwise, you get a new promise that immediately resolves with your value as its result.
+
+```
+let fifteen = Promise.resolve(15);
+fifteen.then(value => console.log(`Got ${value}`));
+// → Got 15
+```
+
+{{index "Promise class"}}
+
+To create a promise that does not immediately resolve, you can use `Promise` as a constructor. It has a somewhat odd interface: the constructor expects a function as its argument, which it immediately calls, passing it a function that it can use to resolve the promise.
+
+For example, this is how you could create a promise-based interface for the `readTextFile` function:
+
+{{index "textFile function"}}
+
+```
+function textFile(filename) {
+ return new Promise(resolve => {
+ readTextFile(filename, text => resolve(text));
+ });
+}
+
+textFile("plans.txt").then(console.log);
+```
+
+Note how, in contrast to callback-style functions, this asynchronous function returns a meaningful value—a promise to give you the contents of the file at some point in the future.
+
+{{index "then method"}}
+
+A useful thing about the `then` method is that it itself returns another promise. This one resolves to the value returned by the callback function or, if that returned value is a promise, to the value that promise resolves to. Thus, you can “chain” multiple calls to `then` together to set up a sequence of asynchronous actions.
+
+This function, which reads a file full of filenames and returns the content of a random file in that list, shows this kind of asynchronous promise pipeline:
+
+```
+function randomFile(listFile) {
+ return textFile(listFile)
+ .then(content => content.trim().split("\n"))
+ .then(ls => ls[Math.floor(Math.random() * ls.length)])
+ .then(filename => textFile(filename));
+}
+```
+
+The function returns the result of this chain of `then` calls. The initial promise fetches the list of files as a string. The first `then` call transforms that string into an array of lines, producing a new promise. The second `then` call picks a random line from that, producing a third promise that yields a single filename. The final `then` call reads this file, so the result of the function as a whole is a promise that returns the content of a random file.
+
+In this code, the functions used in the first two `then` calls return a regular value that will immediately be passed into the promise returned by `then` when the function returns. The last `then` call returns a promise (`textFile(filename)`), making it an actual asynchronous step.
+
+It would also have been possible to perform all these steps inside a single `then` callback, since only the last step is actually asynchronous. But the kind of `then` wrappers that only do some synchronous data transformation are often useful, such as when you want to return a promise that produces a processed version of some asynchronous result.
+
+```
+function jsonFile(filename) {
+ return textFile(filename).then(JSON.parse);
+}
+
+jsonFile("package.json").then(console.log);
+```
+
+Generally, it is useful to think of a promise as a device that lets code ignore the question of when a value is going to arrive. A normal value has to actually exist before we can reference it. A promised value is a value that _might_ already be there or might appear at some point in the future. Computations defined in terms of promises, by wiring them together with `then` calls, are executed asynchronously as their inputs become available.
+
+## Failure
+
+{{index "exception handling"}}
+
+Regular JavaScript computations can fail by throwing an exception. Asynchronous computations often need something like that. A network request may fail, a file may not exist, or some code that is part of the asynchronous computation may throw an exception.
+
+{{index "callback function", error}}
+
+One of the most pressing problems with the callback style of asynchronous programming is that it makes it extremely difficult to ensure failures are properly reported to the callbacks.
+
+A common convention is to use the first argument to the callback to indicate that the action failed, and the second to pass the value produced by the action when it was successful.
+
+```
+someAsyncFunction((error, value) => {
+ if (error) handleError(error);
+ else processValue(value);
+});
+```
+
+Such callback functions must always check whether they received an exception and make sure that any problems they cause, including exceptions thrown by functions they call, are caught and given to the right function.
+
+{{index "rejecting (a promise)", "resolving (a promise)", "then method"}}
+
+Promises make this easier. They can be either resolved (the action finished successfully) or rejected (it failed). Resolve handlers (as registered with `then`) are called only when the action is successful, and rejections are propagated to the new promise returned by `then`. When a handler throws an exception, this automatically causes the promise produced by its `then` call to be rejected. If any element in a chain of asynchronous actions fails, the outcome of the whole chain is marked as rejected, and no success handlers are called beyond the point where it failed.
+
+{{index "Promise.reject function", "Promise class"}}
+
+Much like resolving a promise provides a value, rejecting one also provides a value, usually called the _reason_ of the rejection. When an exception in a handler function causes the rejection, the exception value is used as the reason. Similarly, when a handler returns a promise that is rejected, that rejection flows into the next promise. There's a `Promise.reject` function that creates a new, immediately rejected promise.
+
+{{index "catch method"}}
+
+To explicitly handle such rejections, promises have a `catch` method that registers a handler to be called when the promise is rejected, similar to how `then` handlers handle normal resolution. It's also very much like `then` in that it returns a new promise, which resolves to the original promise's value when that resolves normally and to the result of the `catch` handler otherwise. If a `catch` handler throws an error, the new promise is also rejected.
+
+{{index "then method"}}
+
+As a shorthand, `then` also accepts a rejection handler as a second argument, so you can install both types of handlers in a single method call: `.then(acceptHandler, rejectHandler)`.
+
+A function passed to the `Promise` constructor receives a second argument, alongside the resolve function, which it can use to reject the new promise.
+
+{{index "textFile function"}}
+
+When our `readTextFile` function encounters a problem, it passes the error to its callback function as a second argument. Our `textFile` wrapper should actually check that argument so that a failure causes the promise it returns to be rejected.
+
+```{includeCode: true}
+function textFile(filename) {
+ return new Promise((resolve, reject) => {
+ readTextFile(filename, (text, error) => {
+ if (error) reject(error);
+ else resolve(text);
+ });
+ });
+}
+```
+
+The chains of promise values created by calls to `then` and `catch` thus form a pipeline through which asynchronous values or failures move. Since such chains are created by registering handlers, each link has a success handler or a rejection handler (or both) associated with it. Handlers that don't match the type of outcome (success or failure) are ignored. Handlers that do match are called, and their outcome determines what kind of value comes next—success when they return a non-promise value, rejection when they throw an exception, and the outcome of the promise when they return a promise.
+
+```{test: no}
+new Promise((_, reject) => reject(new Error("Fail")))
+ .then(value => console.log("Handler 1:", value))
+ .catch(reason => {
+ console.log("Caught failure " + reason);
+ return "nothing";
+ })
+ .then(value => console.log("Handler 2:", value));
+// → Caught failure Error: Fail
+// → Handler 2: nothing
+```
+
+The first `then` handler function isn't called because at that point of the pipeline the promise holds a rejection. The `catch` handler handles that rejection and returns a value, which is given to the second `then` handler function.
+
+{{index "uncaught exception", "exception handling"}}
+
+Much like an uncaught exception is handled by the environment, JavaScript environments can detect when a promise rejection isn't handled and will report this as an error.
+
+## Carla
+
+{{index "Carla the crow"}}
+
+It's a sunny day in Berlin. The runway of the old, decommissioned airport is teeming with cyclists and inline skaters. In the grass near a garbage container, a flock of crows noisily mills about, trying to convince a group of tourists to part with their sandwiches.
+
+One of the crows stands out—a large scruffy female with a few white feathers in her right wing. She is baiting people with a skill and confidence that suggest she's been doing this for a long time. When an elderly man is distracted by the antics of another crow, she casually swoops in, snatches his half-eaten bun from his hand, and sails away.
+
+Contrary to the rest of the group, who look like they are happy to spend the day goofing around here, the large crow looks purposeful. Carrying her loot, she flies straight toward the roof of the hangar building, disappearing into an air vent.
+
+Inside the building, you can hear an odd tapping sound—soft, but persistent. It comes from a narrow space under the roof of an unfinished stairwell. The crow is sitting there, surrounded by her stolen snacks, half a dozen smartphones (several of which are turned on), and a mess of cables. She rapidly taps the screen of one of the phones with her beak. Words are appearing on it. If you didn't know better, you'd think she was typing.
+
+This crow is known to her peers as “cāāw-krö”. But since those sounds are poorly suited for human vocal chords, we'll refer to her as Carla.
+
+Carla is a somewhat peculiar crow. In her youth, she was fascinated by human language, eavesdropping on people until she had a good grasp of what they were saying. Later in life, her interest shifted to human technology, and she started stealing phones to study them. Her current project is learning to program. The text she is typing in her hidden lab is, in fact, a piece of asynchronous JavaScript code.
+
+## Breaking In
+
+{{index "Carla the crow"}}
+
+Carla loves the internet. Annoyingly, the phone she is working on is about to run out of prepaid data. The building has a wireless network, but it requires a code to access.
+
+Fortunately, the wireless routers in the building are 20 years old and poorly secured. Doing some research, Carla finds out that the network authentication mechanism has a flaw she can use. When joining the network, a device must send along the correct six-digit passcode. The access point will reply with a success or failure message depending on whether the right code is provided. However, when sending a partial code (say, only three digits), the response is different based on whether those digits are the correct start of the code or not. Sending incorrect numbers immediately returns a failure message. When sending the correct ones, the access point waits for more digits.
+
+This makes it possible to greatly speed up the guessing of the number. Carla can find the first digit by trying each number in turn, until she finds one that doesn't immediately return failure. Having one digit, she can find the second digit in the same way, and so on, until she knows the entire passcode.
+
+Assume Carla has a `joinWifi` function. Given the network name and the passcode (as a string), the function tries to join the network, returning a promise that resolves if successful and rejects if the authentication failed. The first thing she needs is a way to wrap a promise so that it automatically rejects after it takes too much time, to allow the program to quickly move on if the access point doesn't respond.
+
+```{includeCode: true}
+function withTimeout(promise, time) {
+ return new Promise((resolve, reject) => {
+ promise.then(resolve, reject);
+ setTimeout(() => reject("Timed out"), time);
+ });
+}
+```
+
+This makes use of the fact that a promise can be resolved or rejected only once. If the promise given as its argument resolves or rejects first, that result will be the result of the promise returned by `withTimeout`. If, on the other hand, the `setTimeout` fires first, rejecting the promise, any further resolve or reject calls are ignored.
+
+To find the whole passcode, the program needs to repeatedly look for the next digit by trying each digit. If authentication succeeds, we know we have found what we are looking for. If it immediately fails, we know that digit was wrong and must try the next digit. If the request times out, we have found another correct digit and must continue by adding another digit.
+
+Because you cannot wait for a promise inside a `for` loop, Carla uses a recursive function to drive this process. On each call, this function gets the code as we know it so far, as well as the next digit to try. Depending on what happens, it may return a finished code or call through to itself, to either start cracking the next position in the code or to try again with another digit.
+
+```{includeCode: true}
+function crackPasscode(networkID) {
+ function nextDigit(code, digit) {
+ let newCode = code + digit;
+ return withTimeout(joinWifi(networkID, newCode), 50)
+ .then(() => newCode)
+ .catch(failure => {
+ if (failure == "Timed out") {
+ return nextDigit(newCode, 0);
+ } else if (digit < 9) {
+ return nextDigit(code, digit + 1);
+ } else {
+ throw failure;
+ }
+ });
+ }
+ return nextDigit("", 0);
+}
+```
+
+The access point tends to respond to bad authentication requests in about 20 milliseconds, so to be safe, this function waits for 50 milliseconds before timing out a request.
+
+```
+crackPasscode("HANGAR 2").then(console.log);
+// → 555555
+```
+
+Carla tilts her head and sighs. This would have been more satisfying if the code had been a bit harder to guess.
+
+## Async functions
+
+{{index "Promise class", recursion}}
+
+Even with promises, this kind of asynchronous code is annoying to write. Promises often need to be tied together in verbose, arbitrary-looking ways. To create an asynchronous loop, Carla was forced to introduce a recursive function.
+
+{{index "synchronous programming", "asynchronous programming"}}
+
+The thing the cracking function actually does is completely linear—it always waits for the previous action to complete before starting the next one. In a synchronous programming model, it'd be more straightforward to express.
+
+{{index "async function", "await keyword"}}
+
+The good news is that JavaScript allows you to write pseudosynchronous code to describe asynchronous computation. An `async` function implicitly returns a promise and can, in its body, `await` other promises in a way that _looks_ synchronous.
+
+{{index "findInStorage function"}}
+
+We can rewrite `crackPasscode` like this:
+
+```
+async function crackPasscode(networkID) {
+ for (let code = "";;) {
+ for (let digit = 0;; digit++) {
+ let newCode = code + digit;
+ try {
+ await withTimeout(joinWifi(networkID, newCode), 50);
+ return newCode;
+ } catch (failure) {
+ if (failure == "Timed out") {
+ code = newCode;
+ break;
+ } else if (digit == 9) {
+ throw failure;
+ }
+ }
+ }
+ }
+}
+```
+
+This version more clearly shows the double loop structure of the function (the inner loop tries digit 0 to 9 and the outer loop adds digits to the passcode).
+
+{{index "async function", "return keyword", "exception handling"}}
+
+An `async` function is marked by the word `async` before the `function` keyword. Methods can also be made `async` by writing `async` before their name. When such a function or method is called, it returns a promise. As soon as the function returns something, that promise is resolved. If the body throws an exception, the promise is rejected.
+
+{{index "await keyword", ["control flow", asynchronous]}}
+
+Inside an `async` function, the word `await` can be put in front of an expression to wait for a promise to resolve and only then continue the execution of the function. If the promise rejects, an exception is raised at the point of the `await`.
+
+Such a function no longer runs from start to completion in one go like a regular JavaScript function. Instead, it can be _frozen_ at any point that has an `await` and can be resumed at a later time.
+
+For most asynchronous code, this notation is more convenient than directly using promises. You do still need an understanding of promises, since in many cases you'll still interact with them directly. But when wiring them together, `async` functions are generally more pleasant to write than chains of `then` calls.
+
+{{id generator}}
+
+## Generators
+
+{{index "async function"}}
+
+This ability of functions to be paused and then resumed again is not exclusive to `async` functions. JavaScript also has a feature called _((generator))_ functions. These are similar, but without the promises.
+
+When you define a function with `function*` (placing an asterisk after the word `function`), it becomes a generator. When you call a generator, it returns an ((iterator)), which we already saw in [Chapter ?](object).
+
+```
+function* powers(n) {
+ for (let current = n;; current *= n) {
+ yield current;
+ }
+}
+
+for (let power of powers(3)) {
+ if (power > 50) break;
+ console.log(power);
+}
+// → 3
+// → 9
+// → 27
+```
+
+{{index "next method", "yield keyword"}}
+
+Initially, when you call `powers`, the function is frozen at its start. Every time you call `next` on the iterator, the function runs until it hits a `yield` expression, which pauses it and causes the yielded value to become the next value produced by the iterator. When the function returns (the one in the example never does), the iterator is done.
+
+Writing iterators is often much easier when you use generator functions. The iterator for the `Group` class (from the exercise in [Chapter ?](object#group_iterator)) can be written with this generator:
+
+{{index "Group class"}}
+
+```
+Group.prototype[Symbol.iterator] = function*() {
+ for (let i = 0; i < this.members.length; i++) {
+ yield this.members[i];
+ }
+};
+```
+
+```{hidden: true, includeCode: true}
+class Group {
+ constructor() { this.members = []; }
+ add(m) { this.members.add(m); }
+}
+```
+
+{{index [state, in iterator]}}
+
+There's no longer a need to create an object to hold the iteration state—generators automatically save their local state every time they yield.
+
+Such `yield` expressions may occur only directly in the generator function itself and not in an inner function you define inside of it. The state a generator saves, when yielding, is only its _local_ environment and the position where it yielded.
+
+{{index "await keyword"}}
+
+An `async` function is a special type of generator. It produces a promise when called, which is resolved when it returns (finishes) and rejected when it throws an exception. Whenever it yields (awaits) a promise, the result of that promise (value or thrown exception) is the result of the `await` expression.
+
+## A Corvid Art Project
+
+{{index "Carla the crow"}}
+
+One morning, Carla wakes up to unfamiliar noise from the tarmac outside of her hangar. Hopping onto the edge of the roof, she sees the humans are setting up for something. There's a lot of electric cabling, a stage, and some kind of big black wall being built up.
+
+Being a curious crow, Carla takes a closer look at the wall. It appears to consist of a number of large glass-fronted devices wired up to cables. On the back, the devices say “LedTec SIG-5030”.
+
+A quick internet search turns up a user manual for these devices. They appear to be traffic signs, with a programmable matrix of amber LED lights. The intent of the humans is probably to display some kind of information on them during their event. Interestingly, the screens can be programmed over a wireless network. Could it be they are connected to the building's local network?
+
+Each device on a network gets an _IP address_, which other devices can use to send it messages. We talk more about that in [Chapter ?](browser). Carla notices that her own phones all get addresses like `10.0.0.20` or `10.0.0.33`. It might be worth trying to send messages to all such addresses and see if any one of them responds to the interface described in the manual for the signs.
+
+[Chapter ?](http) shows how to make real requests on real networks. In this chapter, we'll use a simplified dummy function called `request` for network communication. This function takes two arguments—a network address and a message, which may be anything that can be sent as JSON—and returns a promise that either resolves to a response from the machine at the given address, or rejects if there was a problem.
+
+According to the manual, you can change what is displayed on a SIG-5030 sign by sending it a message with content like `{"command": "display", "data": [0, 0, 3, …]}`, where `data` holds one number per LED dot, providing its brightness—0 means off, 3 means maximum brightness. Each sign is 50 lights wide and 30 lights high, so an update command should send 1,500 numbers.
+
+This code sends a display update message to all addresses on the local network, to see what sticks. Each of the numbers in an IP address can go from 0 to 255. In the data it sends, it activates a number of lights corresponding to the network address's last number.
+
+```
+for (let addr = 1; addr < 256; addr++) {
+ let data = [];
+ for (let n = 0; n < 1500; n++) {
+ data.push(n < addr ? 3 : 0);
+ }
+ let ip = `10.0.0.${addr}`;
+ request(ip, {command: "display", data})
+ .then(() => console.log(`Request to ${ip} accepted`))
+ .catch(() => {});
+}
+```
+
+Since most of these addresses won't exist or will not accept such messages, the `catch` call makes sure network errors don't crash the program. The requests are all sent out immediately, without waiting for other requests to finish, in order to not waste time when some of the machines don't answer.
+
+Having fired off her network scan, Carla heads back outside to see the result. To her delight, all of the screens are now showing a stripe of light in their upper-left corners. They _are_ on the local network, and they _do_ accept commands. She quickly notes the numbers shown on each screen. There are nine screens, arranged three high and three wide. They have the following network addresses:
+
+```{includeCode: true}
+const screenAddresses = [
+ "10.0.0.44", "10.0.0.45", "10.0.0.41",
+ "10.0.0.31", "10.0.0.40", "10.0.0.42",
+ "10.0.0.48", "10.0.0.47", "10.0.0.46"
+];
+```
+
+Now this opens up possibilities for all kinds of shenanigans. She could show “crows rule, humans drool” on the wall in giant letters. But that feels a bit crude. Instead, she plans to show a video of a flying crow covering all of the screens at night.
+
+Carla finds a fitting video clip, in which a second and a half of footage can be repeated to create a looping video showing a crow's wingbeat. To fit the nine screens (each of which can show 50×30 pixels), Carla cuts and resizes the videos to get a series of 150×90 images, 10 per second. Those are then each cut into nine rectangles, and processed so that the dark spots on the video (where the crow is) show a bright light, and the light spots (no crow) are left dark, which should create the effect of an amber crow flying against a black background.
+
+She has set up the `clipImages` variable to hold an array of frames, where each frame is represented with an array of nine sets of pixels—one for each screen—in the format that the signs expect.
+
+To display a single frame of the video, Carla needs to send a request to all the screens at once. But she also needs to wait for the result of these requests, both in order to not start sending the next frame before the current one has been properly sent and in order to notice when requests are failing.
+
+{{index "Promise.all function"}}
+
+`Promise` has a static method `all` that can be used to convert an array of promises into a single promise that resolves to an array of results. This provides a convenient way to have some asynchronous actions happen alongside each other, wait for them all to finish, and then do something with their results (or at least wait for them to make sure they don't fail).
+
+```{includeCode: true}
+function displayFrame(frame) {
+ return Promise.all(frame.map((data, i) => {
+ return request(screenAddresses[i], {
+ command: "display",
+ data
+ });
+ }));
+}
+```
+
+This maps over the images in `frame` (which is an array of display data arrays) to create an array
+of request promises. It then returns a promise that combines all of those.
+
+In order to be able to stop a playing video, the process is wrapped in a class. This class has an asynchronous `play` method that returns a promise that resolves only when the playback is stopped again via the `stop` method.
+
+```{includeCode: true}
+function wait(time) {
+ return new Promise(accept => setTimeout(accept, time));
+}
+
+class VideoPlayer {
+ constructor(frames, frameTime) {
+ this.frames = frames;
+ this.frameTime = frameTime;
+ this.stopped = true;
+ }
+
+ async play() {
+ this.stopped = false;
+ for (let i = 0; !this.stopped; i++) {
+ let nextFrame = wait(this.frameTime);
+ await displayFrame(this.frames[i % this.frames.length]);
+ await nextFrame;
+ }
+ }
+
+ stop() {
+ this.stopped = true;
+ }
+}
+```
+
+The `wait` function wraps `setTimeout` in a promise that resolves after the given number of milliseconds. This is useful for controlling the speed of the playback.
+
+```{startCode: true}
+let video = new VideoPlayer(clipImages, 100);
+video.play().catch(e => {
+ console.log("Playback failed: " + e);
+});
+setTimeout(() => video.stop(), 15000);
+```
+
+For the entire week that the screen wall stands, every evening, when it is dark, a huge glowing orange bird mysteriously appears on it.
+
+## The event loop
+
+{{index "asynchronous programming", scheduling, "event loop", timeline}}
+
+An asynchronous program starts by running its main script, which will often set up callbacks to be called later. That main script, as well as the callbacks, run to completion in one piece, uninterrupted. But between them, the program may sit idle, waiting for something to happen.
+
+{{index "setTimeout function"}}
+
+So callbacks are not directly called by the code that scheduled them. If I call `setTimeout` from within a function, that function will have returned by the time the callback function is called. And when the callback returns, control does not go back to the function that scheduled it.
+
+{{index "Promise class", "catch keyword", "exception handling"}}
+
+Asynchronous behavior happens on its own empty function ((call stack)). This is one of the reasons that, without promises, managing exceptions across asynchronous code is so hard. Since each callback starts with a mostly empty stack, your `catch` handlers won't be on the stack when they throw an exception.
+
+```
+try {
+ setTimeout(() => {
+ throw new Error("Woosh");
+ }, 20);
+} catch (e) {
+ // This will not run
+ console.log("Caught", e);
+}
+```
+
+{{index thread, queue}}
+
+No matter how closely together events—such as timeouts or incoming requests—happen, a JavaScript environment will run only one program at a time. You can think of this as it running a big loop _around_ your program, called the _event loop_. When there's nothing to be done, that loop is paused. But as events come in, they are added to a queue, and their code is executed one after the other. Because no two things run at the same time, slow-running code can delay the handling of other events.
+
+This example sets a timeout but then dallies until after the timeout's intended point of time, causing the timeout to be late.
+
+```
+let start = Date.now();
+setTimeout(() => {
+ console.log("Timeout ran at", Date.now() - start);
+}, 20);
+while (Date.now() < start + 50) {}
+console.log("Wasted time until", Date.now() - start);
+// → Wasted time until 50
+// → Timeout ran at 55
+```
+
+{{index "resolving (a promise)", "rejecting (a promise)", "Promise class"}}
+
+Promises always resolve or reject as a new event. Even if a promise is already resolved, waiting for it will cause your callback to run after the current script finishes, rather than right away.
+
+```
+Promise.resolve("Done").then(console.log);
+console.log("Me first!");
+// → Me first!
+// → Done
+```
+
+In later chapters we'll see various other types of events that run on the event loop.
+
+## Asynchronous bugs
+
+{{index "asynchronous programming", [state, transitions]}}
+
+When your program runs synchronously, in a single go, there are no state changes happening except those that the program itself makes. For asynchronous programs this is different—they may have _gaps_ in their execution during which other code can run.
+
+Let's look at an example. This is a function that tries to report the size of each file in an array of files, making sure to read them all at the same time rather than in sequence.
+
+{{index "fileSizes function"}}
+
+```{includeCode: true}
+async function fileSizes(files) {
+ let list = "";
+ await Promise.all(files.map(async fileName => {
+ list += fileName + ": " +
+ (await textFile(fileName)).length + "\n";
+ }));
+ return list;
+}
+```
+
+{{index "async function"}}
+
+The `async fileName =>` part shows how ((arrow function))s can also be made `async` by putting the word `async` in front of them.
+
+{{index "Promise.all function"}}
+
+The code doesn't immediately look suspicious... it maps the `async` arrow function over the array of names, creating an array of promises, and then uses `Promise.all` to wait for all of these before returning the list they build up.
+
+But this program is entirely broken. It'll always return only a single line of output, listing the file that took the longest to read.
+
+{{if interactive
+
+```
+fileSizes(["plans.txt", "shopping_list.txt"])
+ .then(console.log);
+```
+
+if}}
+
+Can you work out why?
+
+{{index "+= operator"}}
+
+The problem lies in the `+=` operator, which takes the _current_ value of `list` at the time the statement starts executing and then, when the `await` finishes, sets the `list` binding to be that value plus the added string.
+
+{{index "await keyword"}}
+
+But between the time the statement starts executing and the time it finishes, there's an asynchronous gap. The `map` expression runs before anything has been added to the list, so each of the `+=` operators starts from an empty string and ends up, when its storage retrieval finishes, setting `list` to the result of adding its line to the empty string.
+
+{{index "side effect"}}
+
+This could have easily been avoided by returning the lines from the mapped promises and calling `join` on the result of `Promise.all`, instead of building up the list by changing a binding. As usual, computing new values is less error prone than changing existing values.
+
+{{index "fileSizes function"}}
+
+```
+async function fileSizes(files) {
+ let lines = files.map(async fileName => {
+ return fileName + ": " +
+ (await textFile(fileName)).length;
+ });
+ return (await Promise.all(lines)).join("\n");
+}
+```
+
+Mistakes like this are easy to make, especially when using `await`, and you should be aware of where the gaps in your code occur. An advantage of JavaScript's _explicit_ asynchronicity (whether through callbacks, promises, or `await`) is that spotting these gaps is relatively easy.
+
+## Summary
+
+Asynchronous programming makes it possible to express waiting for long-running actions without freezing the whole program. JavaScript environments typically implement this style of programming using callbacks, functions that are called when the actions complete. An event loop schedules such callbacks to be called when appropriate, one after the other, so that their execution does not overlap.
+
+Programming asynchronously is made easier by promises, objects that represent actions that might complete in the future, and `async` functions, which allow you to write an asynchronous program as if it were synchronous.
+
+## Exercises
+
+### Quiet Times
+
+{{index "quiet times (exercise)", "security camera", "Carla the crow", "async function"}}
+
+There's a security camera near Carla's lab that's activated by a motion sensor. It is connected to the network and starts sending out a video stream when it is active. Because she'd rather not be discovered, Carla has set up a system that notices this kind of wireless network traffic and turns on a light in her lair whenever there is activity outside, so she knows when to keep quiet.
+
+{{index "Date class", "Date.now function", timestamp}}
+
+She's also been logging the times at which the camera is tripped for a while and wants to use this information to visualize which times, in an average week, tend to be quiet and which tend to be busy. The log is stored in files holding one time stamp number (as returned by `Date.now()`) per line.
+
+```{lang: null}
+1695709940692
+1695701068331
+1695701189163
+```
+
+The `"camera_logs.txt"` file holds a list of logfiles. Write an asynchronous function `activityTable(day)` that for a given day of the week returns an array of 24 numbers, one for each hour of the day, that hold the number of camera network traffic observations seen in that hour of the day. Days are identified by number using the system used by `Date.getDay`, where Sunday is 0 and Saturday is 6.
+
+The `activityGraph` function, provided by the sandbox, summarizes such a table into a string.
+
+{{index "textFile function"}}
+
+To read the files, use the `textFile` function defined earlier—given a filename, it returns a promise that resolves to the file's content. Remember that `new Date(timestamp)` creates a `Date` object for that time, which has `getDay` and `getHours` methods returning the day of the week and the hour of the day.
+
+Both types of files—the list of logfiles and the logfiles themselves—have each piece of data on its own line, separated by newline (`"\n"`) characters.
+
+{{if interactive
+
+```{test: no}
+async function activityTable(day) {
+ let logFileList = await textFile("camera_logs.txt");
+ // Your code here
+}
+
+activityTable(1)
+ .then(table => console.log(activityGraph(table)));
+```
+
+if}}
+
+{{hint
+
+{{index "quiet times (exercise)", "split method", "textFile function", "Date class"}}
+
+You will need to convert the content of these files to an array. The easiest way to do that is to use the `split` method on the string produced by `textFile`. Note that for the logfiles, that will still give you an array of strings, which you have to convert to numbers before passing them to `new Date`.
+
+Summarizing all the time points into a table of hours can be done by creating a table (array) that holds a number for each hour in the day. You can then loop over all the timestamps (over the logfiles and the numbers in every logfile) and for each one, if it happened on the correct day, take the hour it occurred in, and add one to the corresponding number in the table.
+
+{{index "async function", "await keyword", "Promise class"}}
+
+Make sure you use `await` on the result of asynchronous functions before doing anything with it, or you'll end up with a `Promise` where you expected a string.
+
+hint}}
+
+
+### Real Promises
+
+{{index "real promises (exercise)", "Promise class"}}
+
+Rewrite the function from the previous exercise without `async`/`await`, using plain `Promise` methods.
+
+{{if interactive
+
+```{test: no}
+function activityTable(day) {
+ // Your code here
+}
+
+activityTable(6)
+ .then(table => console.log(activityGraph(table)));
+```
+
+if}}
+
+{{index "async function", "await keyword", performance}}
+
+In this style, using `Promise.all` will be more convenient than trying to model a loop over the logfiles. In the `async` function, just using `await` in a loop is simpler. If reading a file takes some time, which of these two approaches will take the least time to run?
+
+{{index "rejecting (a promise)"}}
+
+If one of the files listed in the file list has a typo, and reading it fails, how does that failure end up in the `Promise` object that your function returns?
+
+{{hint
+
+{{index "real promises (exercise)", "then method", "textFile function", "Promise.all function"}}
+
+The most straightforward approach to writing this function is to use a chain of `then` calls. The first promise is produced by reading the list of logfiles. The first callback can split this list and map `textFile` over it to get an array of promises to pass to `Promise.all`. It can return the object returned by `Promise.all`, so that whatever that returns becomes the result of the return value of this first `then`.
+
+{{index "asynchronous programming"}}
+
+We now have a promise that returns an array of logfiles. We can call `then` again on that, and put the timestamp-counting logic in there. Something like this:
+
+```{test: no}
+function activityTable(day) {
+ return textFile("camera_logs.txt").then(files => {
+ return Promise.all(files.split("\n").map(textFile));
+ }).then(logs => {
+ // analyze...
+ });
+}
+```
+
+Or you could, for even better work scheduling, put the analysis of each file inside of the `Promise.all`, so that that work can be started for the first file that comes back from disk, even before the other files come back.
+
+```{test: no}
+function activityTable(day) {
+ let table = []; // init...
+ return textFile("camera_logs.txt").then(files => {
+ return Promise.all(files.split("\n").map(name => {
+ return textFile(name).then(log => {
+ // analyze...
+ });
+ }));
+ }).then(() => table);
+}
+```
+
+{{index "await keyword", scheduling}}
+
+This shows that the way you structure your promises can have a real effect on the way the work is scheduled. A simple loop with `await` in it will make the process completely linear—it waits for each file to load before proceeding. `Promise.all` makes it possible for multiple tasks to conceptually be worked on at the same time, allowing them to make progress while files are still being loaded. This can be faster, but it also makes the order in which things will happen less predictable. In this case, we're only going to be incrementing numbers in a table, which isn't hard to do in a safe way. For other kinds of problems, it may be a lot more difficult.
+
+{{index "rejecting (a promise)", "then method"}}
+
+When a file in the list doesn't exist, the promise returned by `textFile` will be rejected. Because `Promise.all` rejects if any of the promises given to it fail, the return value of the callback given to the first `then` will also be a rejected promise. That makes the promise returned by `then` fail, so the callback given to the second `then` isn't even called, and a rejected promise is returned from the function.
+
+hint}}
+
+### Building Promise.all
+
+{{index "Promise class", "Promise.all function", "building Promise.all (exercise)"}}
+
+As we saw, given an array of ((promise))s, `Promise.all` returns a promise that waits for all of the promises in the array to finish. It then succeeds, yielding an array of result values. If a promise in the array fails, the promise returned by `all` fails too, passing on the failure reason from the failing promise.
+
+Implement something like this yourself as a regular function called `Promise_all`.
+
+Remember that after a promise has succeeded or failed, it can't succeed or fail again, and further calls to the functions that resolve it are ignored. This can simplify the way you handle a failure of your promise.
+
+{{if interactive
+
+```{test: no}
+function Promise_all(promises) {
+ return new Promise((resolve, reject) => {
+ // Your code here.
+ });
+}
+
+// Test code.
+Promise_all([]).then(array => {
+ console.log("This should be []:", array);
+});
+function soon(val) {
+ return new Promise(resolve => {
+ setTimeout(() => resolve(val), Math.random() * 500);
+ });
+}
+Promise_all([soon(1), soon(2), soon(3)]).then(array => {
+ console.log("This should be [1, 2, 3]:", array);
+});
+Promise_all([soon(1), Promise.reject("X"), soon(3)])
+ .then(array => {
+ console.log("We should not get here");
+ })
+ .catch(error => {
+ if (error != "X") {
+ console.log("Unexpected failure:", error);
+ }
+ });
+```
+
+if}}
+
+{{hint
+
+{{index "Promise.all function", "Promise class", "then method", "building Promise.all (exercise)"}}
+
+The function passed to the `Promise` constructor will have to call `then` on each of the promises in the given array. When one of them succeeds, two things need to happen. The resulting value needs to be stored in the correct position of a result array, and we must check whether this was the last pending ((promise)) and finish our own promise if it was.
+
+{{index "counter variable"}}
+
+The latter can be done with a counter that is initialized to the length of the input array and from which we subtract 1 every time a promise succeeds. When it reaches 0, we are done. Make sure you take into account the situation where the input array is empty (and thus no promise will ever resolve).
+
+Handling failure requires some thought but turns out to be extremely simple. Just pass the `reject` function of the wrapping promise to each of the promises in the array as a `catch` handler or as a second argument to `then` so that a failure in one of them triggers the rejection of the whole wrapper promise.
+
+hint}}
diff --git a/11_language.txt b/11_language.txt
deleted file mode 100644
index e1159a668..000000000
--- a/11_language.txt
+++ /dev/null
@@ -1,878 +0,0 @@
-:chap_num: 11
-:prev_link: 10_modules
-:next_link: 12_browser
-:load_files: ["code/chapter/11_language.js"]
-
-= Project: A Programming Language =
-
-[chapterquote="true"]
-[quote, Hal Abelson and Gerald Sussman, Structure and Interpretation of Computer Programs]
-____
-The evaluator, which determines the meaning of expressions in a
-programming language, is just another program.
-____
-
-ifdef::html_target[]
-
-[chapterquote="true"]
-[quote, Master Yuan-Ma, The Book of Programming]
-____
-When a student asked the master about the nature of the cycle of Data
-and Control, Yuan-Ma replied ‘Think of a compiler, compiling itself.’
-____
-
-endif::html_target[]
-
-(((Abelson+++,+++ Hal)))(((Sussman+++,+++
-Gerald)))(((SICP)))(((project chapter)))Building your own
-((programming language)) is surprisingly easy (as long as you do not
-aim too high) and very enlightening.
-
-The main thing I want to show in this chapter is that there is no
-((magic)) involved in building your own language. I've often felt that
-some human inventions were so immensely clever and complicated that
-I'd never be able to understand them. But with a little reading and
-tinkering, such things often turn out to be quite mundane.
-
-(((Egg language)))We will build a programming language called Egg. It
-will be a tiny, simple language, but one that is powerful enough to
-express any computation you can think of. It will also allow simple
-((abstraction)) based on ((function))s.
-
-[[parsing]]
-== Parsing ==
-
-(((parsing)))(((validation)))The most immediately visible part of a
-programming language is its _((syntax))_, or notation. A parser is a
-program that reads a piece of text and produces a data structure that
-reflects the structure of the program contained in that text. If the
-text does not form a valid program, the parser should complain and
-point out the error.
-
-(((special form)))Our language will have a very simple and uniform
-syntax. Everything in Egg is an ((expression)). An expression can be a
-variable, a number, a string, or an _application_. Applications are
-used for function calls, but also for constructs like `if` or `while`.
-
-(((double-quote character)))(((parsing)))(((escaping,in strings)))To
-keep the parser simple, strings in Egg do not support anything like
-backslash escapes. A string is simply a sequence of characters that
-are not double quotes, wrapped in double quotes. A number is a
-sequence of digits. Variable names can consist of any character that
-is not ((whitespace)) and does not have a special meaning in the
-syntax.
-
-(((comma character)))Applications are written the way they are in
-JavaScript, by putting ((parentheses)) after an expression, and having
-any number of ((argument))s separated by commas between those
-parentheses.
-
-----
-do(define(x, 10),
- if(>(x, 5)),
- print("large"),
- print("small"))
-----
-
-(((block)))The ((uniformity)) of the ((Egg language)) means that
-things which are ((operator))s in JavaScript (such as `>`) are normal
-variables in this language, applied just like other ((function))s. And
-since the ((syntax)) has no concept of a block, we need a `do`
-construct (like above) to represent doing multiple things in sequence.
-
-(((type property)))(((parsing)))The ((data structure)) the parser will
-use to describe a program will consist of ((expression)) objects, each
-of which has a `type` property indicating the kind of expression it is
-and other properties to describe its content.
-
-(((identifier)))Expresions of type `"value"` represent literal strings
-or numbers. Their `value` property contains the string or number value
-that they represent. Expressions of type `"word"` are used for
-identifiers (names). Such objects have a `name` property that holds
-the identifier's name, as a string. Finally, `"apply"` expressions
-represent applications. They have an `operator` property that refers
-to the expression that is being applied, and an `args` property that
-refers to an array of argument expressions.
-
-The `>(x, 5)` part of the program above would be represented like this:
-
-[source,application/json]
-----
-{
- type: "apply",
- operator: {type: "word", name: ">"},
- args: [
- {type: "word", name: "x"},
- {type: "value", value: 5}
- ]
-}
-----
-
-indexsee:[abstract syntax tree,syntax tree]
-
-Such a ((data structure)) is called a _((syntax tree))_. If you
-imagine the objects as dots, and the links between them as lines
-between those dots, it has a ((tree))-like shape. The fact that
-expressions contain other expressions, which in turn might contain
-more expression, is similar to the way branches split and split again.
-
-image::img/syntax_tree.svg[alt="The structure of a syntax tree",width="5cm"]
-
-(((parsing)))Contrast this to the parser we wrote for the
-configuration file format in link:09_regexp.html#ini[Chapter 9], which
-had a very simple structure: it split the input into lines, and
-handled those lines one at a time. There were only a few simple forms
-that a line was allowed to have.
-
-(((recursion)))(((nesting,of expressions)))Here we must find a
-different approach. Expressions are not separated into lines, and they
-have a recursive structure. Application expressions _contain_ other
-expressions.
-
-(((elegance)))Fortunately, this problem can be solved elegantly by
-writing a parser function that is recursive in a way that reflects the
-recursive nature of the language.
-
-(((parseExpression function)))(((syntax tree)))We define a function
-`parseExpression`, which takes a string as input, and returns an
-object containing the data structure for the expression at the start
-of the string, along with the part of the string left after parsing
-this expression. When parsing sub-expressions (the argument to an
-application, for example), this function can be called again, yielding
-the argument expression as well as the text that remains. This text
-may in turn contain more arguments, or may be the closing parenthesis
-that ends the list of arguments.
-
-This is the first part of the parser:
-
-// include_code
-
-[source,javascript]
-----
-function parseExpression(program) {
- program = skipSpace(program);
- var match, expr;
- if (match = /^"([^"]*)"/.exec(program))
- expr = {type: "value", value: match[1]};
- else if (match = /^\d+\b/.exec(program))
- expr = {type: "value", value: Number(match[0])};
- else if (match = /^[^\s(),"]+/.exec(program))
- expr = {type: "word", name: match[0]};
- else
- throw new SyntaxError("Unexpected syntax: " + program);
-
- return parseApply(expr, program.slice(match[0].length));
-}
-
-function skipSpace(string) {
- var first = string.search(/\S/);
- if (first == -1) return "";
- return string.slice(first);
-}
-----
-
-(((skipSpace function)))Because Egg allows any amount of
-((whitespace)) between its elements, we have to repeatedly cut the
-whitespace off the start of the program string. This is what the
-`skipSpace` function helps with.
-
-(((literal expression)))(((SyntaxError type)))After skipping any
-leading space, `parseExpression` uses three ((regular expression))s to
-spot the three simple (atomic) elements that Egg supports: strings,
-numbers, and words. The parser constructs a different kind of data
-structure depending on which one matches. If none match, the input is
-not a valid expression, and it throws an error. `SyntaxError` is a
-standard error object type, which is raised when an attempt is made to
-run an invalid JavaScript program.
-
-(((parseApply function)))We can then cut off the part that we matched
-from the program string and pass that, along with the object for the
-expression, to `parseApply`, which checks whether the expression is an
-application. If so, it parses a parenthesized list of arguments.
-
-// include_code
-
-[source,javascript]
-----
-function parseApply(expr, program) {
- program = skipSpace(program);
- if (program[0] != "(")
- return {expr: expr, rest: program};
-
- program = skipSpace(program.slice(1));
- expr = {type: "apply", operator: expr, args: []};
- while (program[0] != ")") {
- var arg = parseExpression(program);
- expr.args.push(arg.expr);
- program = skipSpace(arg.rest);
- if (program[0] == ",")
- program = skipSpace(program.slice(1));
- else if (program[0] != ")")
- throw new SyntaxError("Expected ',' or ')'");
- }
- return parseApply(expr, program.slice(1));
-}
-----
-
-(((parsing)))If the next character in the program is not an opening
-parenthesis, this is not an application, and `parseApply` simply
-returns the expression it was given.
-
-(((recursion)))Otherwise, it skips the opening parenthesis, and
-creates the ((syntax tree)) object for this application expression. It
-then recusively calls `parseExpression` to parse each argument until a
-closing parenthesis is found. The recursion is indirect, through
-`parseApply` and `parseExpression` calling each other.
-
-Because an application expression can itself be applied (such as in
-`multiplier(2)(1)`), `parseApply` must, after it has parsed an
-application, call itself again to check whether another pair of
-parentheses follow.
-
-(((syntax tree)))(((Egg language)))(((parse function)))This is all we
-need to parse Egg. We wrap it in a convenient `parse` function which
-verifies that it has reached the end of the input string after parsing
-the program, and which gives us the program's data structure.
-
-// include_code strip_log
-// test: join
-
-[source,javascript]
-----
-function parse(program) {
- var result = parseExpression(program);
- if (skipSpace(result.rest).length > 0)
- throw new SyntaxError("Unexpected text after program");
- return result.expr;
-}
-
-console.log(parse("+(a, 10)"));
-// → {type: "apply",
-// operator: {type: "word", name: "+"},
-// args: [{type: "word", name: "a"},
-// {type: "value", value: 10}]}
-----
-
-(((error message))It works! It doesn't give us very helpful
-information when it fails, and doesn't store the line and column on
-which each expression starts, which might be helpful when reporting
-errors later on, but it's good enough for our purposes.
-
-== The evaluator ==
-
-(((evaluate function)))(((evaluation)))(((interpretation)))(((syntax
-tree)))(((Egg language)))What can we do with the syntax tree for a
-program? Run it, of course! And that is what the evaluator does. You
-give it a syntax tree and an environment object that associates names
-with values, and it will evaluate the expression that the tree
-represents and return the value that this produces.
-
-// include_code
-
-[source,javascript]
-----
-function evaluate(expr, env) {
- switch(expr.type) {
- case "value":
- return expr.value;
-
- case "word":
- if (expr.name in env)
- return env[expr.name];
- else
- throw new ReferenceError("Undefined variable: " +
- expr.name);
- case "apply":
- if (expr.operator.type == "word" &&
- expr.operator.name in specialForms)
- return specialForms[expr.operator.name](expr.args,
- env);
- var op = evaluate(expr.operator, env);
- if (typeof op != "function")
- throw new TypeError("Applying a non-function.");
- return op.apply(null, expr.args.map(function(arg) {
- return evaluate(arg, env);
- }));
- }
-}
-
-var specialForms = Object.create(null);
-----
-
-(((literal expression)))(((environment)))The evaluator has code for
-each of the ((expression)) types. A literal value expression simply
-produces its value. (For example, the expression `100` just evaluates
-to the number 100.) For a variable, we must check whether it is
-actually defined in the environment, and if it is, fetch the
-variable's value.
-
-(((function,application)))Applications are more involved. If they are
-a ((special form)), like `if`, we do not evaluate anything, and simply
-pass the argument expressions, along with the environment, to the
-function that handles this form. If it is a normal call, we evaluate
-the operator, verify that it is a function, and call it with the
-result of evaluating the arguments.
-
-We will use plain JavaScript function values to represent Egg's
-function values. We will come back to this
-link:11_language.html#egg_fun[later], when the special form called
-`fun` is defined.
-
-(((readability)))(((evaluate
-function)))(((recursion)))(((parsing)))The recursive structure of
-`evaluate` resembles the similar structure of the parser. Both mirror
-the structure of the language itself. It would also be possible
-integrate the parser with the evaluator, and evaluate during parsing,
-but splitting them up this way makes the program more readable.
-
-(((Egg language)))(((interpretation)))This is really all that is
-needed to interpret Egg. It is that simple. But without defining a few
-special forms and adding some useful values to the ((environment)),
-you can't do anything with this language yet.
-
-== Special forms ==
-
-(((special form)))(((specialForms object)))The `specialForms` object
-is used to define special syntax in Egg. It associates words with
-functions that evaluate such special forms. It is currently empty.
-Let's add some forms.
-
-// include_code
-
-[source,javascript]
-----
-specialForms["if"] = function(args, env) {
- if (args.length != 3)
- throw new SyntaxError("Bad number of args to if");
-
- if (evaluate(args[0], env) !== false)
- return evaluate(args[1], env);
- else
- return evaluate(args[2], env);
-};
-----
-
-(((conditional execution)))Egg's `if` construct expects exactly three
-arguments. It will evaluate the first, and if the result isn't the
-value `false`, it will evaluate the second. Otherwise, the third gets
-evaluated. Because this `if` form is an expression, not a statement as
-it is in JavaScript, it has a value—namely, the result of the second
-or third argument.
-
-(((Boolean)))Egg differs from JavaScript in how it handles the
-condition value to `if`. It will not treat things like zero or the
-empty string as false, but only the precise value `false`.
-
-(((short-circuit evaluation)))The reason we need to represent `if` as
-a special form, rather than a regular function, is that all arguments
-to functional are evaluated before the function is called, whereas
-`if` should only evaluate _either_ its second or its third argument,
-depending on the value of the first.
-
-The `while` form is similar:
-
-// include_code
-
-[source,javascript]
-----
-specialForms["while"] = function(args, env) {
- if (args.length != 2)
- throw new SyntaxError("Bad number of args to while");
-
- while (evaluate(args[0], env) !== false)
- evaluate(args[1], env);
-
- // Since undefined does not exist in Egg, we return false,
- // for lack of a meaningful result.
- return false;
-};
-----
-
-Another basic building block is `do`, which executes all its arguments
-from top to bottom. Its value is the value produced by the last
-argument.
-
-// include_code
-
-[source,javascript]
-----
-specialForms["do"] = function(args, env) {
- var value = false;
- args.forEach(function(arg) {
- value = evaluate(arg, env);
- });
- return value;
-};
-----
-
-(((= operator)))To be able to create ((variable))s and give them new
-values, we also create a form called `define`. It expects a word as
-its first argument, and an expression producing the value to assign to
-that word as its second argument. Since `define`, like everything, is
-an expression, it must return a value. We'll make it return the value
-that was assigned (just like JavaScript's “`=`” operator).
-
-// include_code
-
-[source,javascript]
-----
-specialForms["define"] = function(args, env) {
- if (args.length != 2 || args[0].type != "word")
- throw new SyntaxError("Bad use of define");
- var value = evaluate(args[1], env);
- env[args[0].name] = value;
- return value;
-};
-----
-
-== The environment ==
-
-(((Egg language)))(((evaluate function)))The ((environment)) accepted
-by `evaluate` is an object with properties whose names correspond to
-variable names, and whose values correspond to the values those
-((variable))s are bound to. Let's define an environment object to
-represent the ((global scope)).
-
-To be able to use the `if` construct we just defined, we must
-((Boolean)) values in this global scope. Since there are only two
-boolean values, we do not need special syntax for them. We simply bind
-two variables to the values `true` and `false`, and use those.
-
-// include_code
-
-[source,javascript]
-----
-var topEnv = Object.create(null);
-
-topEnv["true"] = true;
-topEnv["false"] = false;
-----
-
-We can now evaluate a simple expression that negates a Boolean value.
-
-[source,javascript]
-----
-var prog = parse("if(true, false, true)");
-console.log(evaluate(prog, topEnv));
-// → false
-----
-
-(((arithmetic)))(((Function constructor)))To supply basic
-((arithmetic)) and ((comparison)) ((operator))s, we will also add some
-function values to the ((environment)). In the interest of keeping the
-code short, we'll use `new Function` to synthesize a bunch of operator
-functions in a loop, rather than defining them all individually.
-
-// include_code
-
-[source,javascript]
-----
-["+", "-", "*", "/", "==", "<", ">"].forEach(function(op) {
- topEnv[op] = new Function("a, b", "return a " + op + " b;");
-});
-----
-
-A way to ((output)) values is also very useful, so we'll wrap
-`console.log` in a function and call it `print`.
-
-// include_code
-
-[source,javascript]
-----
-topEnv["print"] = function(value) {
- console.log(value);
- return value;
-};
-----
-
-(((parsing)))(((run function)))That gives us enough elementary tools
-to write simple programs. The following `run` function provides a
-convenient way to write and run them. It creates a fresh environment,
-and parses and evaluates the strings we give it as a single program.
-
-// include_code
-
-[source,javascript]
-----
-function run() {
- var env = Object.create(topEnv);
- var program = Array.prototype.slice
- .call(arguments, 0).join("\n");
- return evaluate(parse(program), env);
-}
-----
-
-(((join method)))(((call method)))The use of
-`Array.prototype.slice.call` above is a trick to turn an ((array-like
-object)), such as `arguments`, into a real array, so that we can call
-`join` on it. It takes all the arguments given to `run` and treats
-them as the lines of a program.
-
-[source,javascript]
-----
-run("do(define(total, 0),",
- " define(count, 1),",
- " while(<(count, 11),",
- " do(define(total, +(total, count)),",
- " define(count, +(count, 1)))),",
- " print(total))");
-// → 55
-----
-
-(((summing example)))(((Egg language)))This is the program we've seen
-several times before, which computes the sum of the numbers 1 to 10,
-expressed in Egg. It is clearly uglier than the equivalent JavaScript
-program, but not bad for a language implemented in less than 150
-((lines of code)).
-
-[[egg_fun]]
-== Functions ==
-
-(((function)))(((Egg language)))A programming language without
-functions is a poor programming language indeed.
-
-Fortunately, it is not hard to add a `fun` construct, which treats its
-last argument as the function's body, and all arguments before that as
-the names of the function's arguments.
-
-// include_code
-
-[source,javascript]
-----
-specialForms["fun"] = function(args, env) {
- if (!args.length)
- throw new SyntaxError("Functions need a body");
- function name(expr) {
- if (expr.type != "word")
- throw new SyntaxError("Arg names must be words");
- return expr.name;
- }
- var argNames = args.slice(0, args.length - 1).map(name);
- var body = args[args.length - 1];
-
- return function() {
- if (arguments.length != argNames.length)
- throw new TypeError("Wrong number of arguments");
- var localEnv = Object.create(env);
- for (var i = 0; i < arguments.length; i++)
- localEnv[argNames[i]] = arguments[i];
- return evaluate(body, localEnv);
- };
-};
-----
-
-(((local scope)))(((Object.create function)))(((prototype)))Functions
-in Egg have their own local environment, just like in JavaScript. We
-use `Object.create` to make a new object that has access to the
-variables in the outer environment (its prototype), but can also
-contain new variables without modifying that outer scope.
-
-(((power example)))(((evaluation)))(((interpretation)))The function
-created by the `fun` form creates this local environment, and adds the
-argument variables to it. It then evaluates the function body in this
-environment, and returns the result.
-
-// start_code
-
-[source,javascript]
-----
-run("do(define(plusOne, fun(a, +(a, 1))),",
- " print(plusOne(10)))");
-// → 11
-
-run("do(define(pow, fun(base, exp,",
- " if(==(exp, 0),",
- " 1,",
- " *(base, pow(base, -(exp, 1)))))),",
- " print(pow(2, 10)))");
-// → 1024
-----
-
-== Compilation ==
-
-(((interpretation)))(((compilation)))What we have built is an
-interpreter. During evaluation, it acts directly on the representation
-of the program produced by the parser.
-
-(((efficiency)))(((performance)))_Compilation_ is the process of
-adding another step between the parsing and the running of a program,
-which transforms the program into something that can be evaluated more
-efficiently by doing as much work as possible in advance. For example,
-in well-designed languages it is obvious, for each use of a
-((variable)), which variable is being referred to, without actually
-running the program. This can be used to avoid looking up the variable
-by name every time it is accessed, and directly fetch it from some
-predetermined ((memory)) location.
-
-Traditionally, ((compilation)) involves converting the program to
-((machine code)), the raw format that a computer's processor can
-execute. But any process that converts a program to a different
-representation can be thought of as compilation.
-
-(((simplicity)))(((Function constructor)))(((transpilation)))It would
-be possible to write an alternative ((evaluation)) strategy for Egg,
-one that first converts the program to a JavaScript program, uses `new
-Function` to invoke the JavaScript compiler on it, and then runs the
-result. When done right, this would make Egg run very fast, while
-still being quite simple to implement.
-
-If you are interested in this topic and willing to spend some time on
-it, I encourage you to try to implement such a compiler as an
-exercise.
-
-== Cheating ==
-
-(((Egg language)))When we defined `if` and `while`, you probably
-noticed that they were more or less trivial wrappers around
-JavaScript's own `if` and `while`. Similarly, the values in Egg are
-just regular old JavaScript values.
-
-If you compare the implementation of Egg, built on top of JavaScript,
-with the amount of work and complexity required to build a programming
-language directly on the raw functionality provided by a machine, the
-difference is huge. Regardless, this example hopefully gave you an
-impression of the way ((programming language))s work.
-
-And when it comes to getting something done, cheating is more
-effective than doing everything yourself. Though the toy language in
-this chapter doesn't do anything that couldn't be done better in
-JavaScript, there _are_ situations where writing small languages helps
-get real work done.
-
-Such a language does not have to resemble a typical programming
-language. If JavaScript didn't come equipped with regular expressions,
-you could write your own parser and evaluator for such a sublanguage.
-
-(((artificial intelligence)))Or imagine you are building a giant
-robotic ((dinosaur)), and need to program its ((behavior)). JavaScript
-might not be the most effective way to do this. You might instead opt
-for a language that looks like this:
-
-----
-behavior walk
- perform when
- destination ahead
- actions
- move left-foot
- move right-foot
-
-behavior attack
- perform when
- Godzilla in-view
- actions
- fire laser-eyes
- launch arm-rockets
-----
-
-(((expressivity)))This is what is usually called a _((domain-specific
-language))_, a language tailored to express a narrow domain of
-knowledge. Such a language can be more expressive than a
-general-purpose language because it is designed to express exactly the
-things that need expressing in its domain, and nothing else.
-
-== Exercises ==
-
-=== Arrays ===
-
-(((Egg language)))Add support for ((array))s to Egg by adding the
-following three functions to the top scope: `array(...)` which
-constructs an array containing the argument values, `length(array)` to
-get an array's length, and `element(array, n)` to fetch the n^th^
-element from an array.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// Modify these definitions...
-
-topEnv["array"] = "...";
-
-topEnv["length"] = "...";
-
-topEnv["element"] = "...";
-
-run("do(define(sum, fun(array,",
- " do(define(i, 0),",
- " define(sum, 0),",
- " while(<(i, length(array)),",
- " do(define(sum, +(sum, element(array, i))),",
- " define(i, +(i, 1)))),",
- " sum))),",
- " print(sum(array(1, 2, 3))))");
-// → 6
-----
-endif::html_target[]
-
-!!solution!!
-
-The easiest way to do this is to represent Egg arrays
-with JavaScript arrays.
-
-(((slice method)))The values added to the top environment must be
-functions. `Array.prototype.slice` can be used to convert an
-`arguments` array-like object into a regular array.
-
-!!solution!!
-
-=== Closure ===
-
-(((closure)))(((function,scope)))The way we have defined `fun` allows
-functions in Egg to “close over” the surrounding environment, allowing
-the function's body to use local values that were visible at the time
-the function was defined, just like JavaScript functions do.
-
-The program below illustrates this: function `f` returns a function
-that adds its argument to `f`'s argument, meaning that it needs access
-to the local ((scope)) inside `f` to be able to use variable `a`.
-
-[source,javascript]
-----
-run("do(define(f, fun(a, fun(b, +(a, b)))),",
- " print(f(4)(5)))");
-// → 9
-----
-
-Go back to the definition of the `fun` form and explain which
-mechanism causes this to work.
-
-!!solution!!
-
-(((closure)))Again, we are riding along on a JavaScript mechanism to
-get the equivalent feature in Egg. Special forms are passed the local
-environment in which they are evaluated, so that they can evaluate
-their sub-forms in that environment. The function returned by `fun`
-closes over the `env` argument given to its enclosing function, and
-uses that to create the function's local ((environment)) when it is
-called.
-
-(((compilation)))This means that the ((prototype)) of the local
-environment will be the environment in which the function was created,
-which makes it possible to access variables in that environment from
-the function. This is all there is to implementing closure (though to
-compile it in a way that is actually efficient, you'd need to do some
-more work).
-
-!!solution!!
-
-=== Comments ===
-
-(((hash character)))(((Egg language)))It would be nice if we could
-write ((comment))s in Egg. For example, whenever we find a hash sign
-(“#”), we could treat the rest of the line as a comment and ignore it,
-similar to “`//`” in JavaScript.
-
-(((skipSpace function)))We do not have to make any big changes to the
-parser to support this. We can simply change `skipSpace` to skip
-comments as if they are ((whitespace)), so that all the points where
-`skipSpace` is called will now also skip comments. Make this change.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-// This is the old skipSpace. Modify it...
-function skipSpace(string) {
- var first = string.search(/\S/);
- if (first == -1) return "";
- return string.slice(first);
-}
-
-console.log(parse("# hello\nx"));
-// → {type: "word", name: "x"}
-
-console.log(parse("a # one\n # two\n()"));
-// → {type: "apply",
-// operator: {type: "word", name: "x"},
-// args: []}
-----
-endif::html_target[]
-
-!!solution!!
-
-(((comment)))Make sure your solution handles multiple comments in a
-row, with potentially ((whitespace)) between or after them.
-
-A ((regular expression)) is probably the easiest way to solve this.
-Write something that matches “whitespace or a comment, zero or more
-times”. Use the `exec` or `match` method, and look at the length of
-the first element in the returned array (the whole match) to find out
-how many characters to slice off.
-
-!!solution!!
-
-=== Fixing scope ===
-
-(((variable,definition)))(((assignment)))(((Currently, the only way to
-assign a ((variable)) a value is `define`. This construct acts both as
-a way to define new variables and to give existing ones a new value.
-
-(((local variable)))This ((ambiguity)) causes a problem. When you try
-to give a non-local variable a new value, you will end up defining a
-local one with the same name instead. (Some languages work like this
-by design, but I've always found it a silly way to handle ((scope)).)
-
-(((ReferenceError type)))Add a special form `set`, similar to
-`define`, which gives a variable a new value, updating the variable in
-an outer scope if it doesn't already exist in the inner scope. If the
-variable is not defined at all, throw a `ReferenceError` (which is
-another standard error type).
-
-(((hasOwnProperty method)))(((prototype)))(((getPrototypeOf
-function)))The technique of representing scopes as simple objects,
-which has made things very convenient so far, will get in your way a
-little at this point. You might want to use the
-`Object.getPrototypeOf` function, which returns the prototype of an
-object. Also remember that scopes do not derive from
-`Object.prototype`, so if you want to call `hasOwnProperty` on them,
-you have to use this clumsy expression:
-
-// test: no
-
-[source,javascript]
-----
-Object.prototype.hasOwnProperty.call(scope, name);
-----
-
-This fetches the `hasOwnProperty` method from the `Object` prototype,
-and then calls it on a scope object.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,javascript]
-----
-specialForms["set"] = function(args, env) {
- // Your code here.
-};
-
-run("do(define(x, 4),",
- " define(setx, fun(val, set(x, val))),",
- " setx(50),",
- " print(x))");
-// → 50
-run("set(quux, true)");
-// → Some kind of ReferenceError
-----
-endif::html_target[]
-
-!!solution!!
-
-(((variable,definition)))(((assignment)))(((getPrototypeOf
-function)))(((hasOwnProperty method)))You will have to loop through
-one ((scope)) at a time, using `Object.getPrototypeOf` to go the next
-outer scope. For each scope, use `hasOwnProperty` to find out if the
-variable, indicated by the `name` property of the first argument to
-`set`, exists in that scope. If it does, set it to the result of
-evaluating the second argument to `set`, and return that value.
-
-(((global scope)))(((run-time error)))If the outermost scope is
-reached (`Object.getPrototypeOf` returns null) and we haven't found
-the variable yet, it doesn't exist, and an error should be thrown.
-
-!!solution!!
diff --git a/12_browser.txt b/12_browser.txt
deleted file mode 100644
index 3237f37d1..000000000
--- a/12_browser.txt
+++ /dev/null
@@ -1,400 +0,0 @@
-:chap_num: 12
-:prev_link: 11_language
-:next_link: 13_dom
-
-= JavaScript and the Browser =
-
-[chapterquote="true"]
-[quote,Douglas Crockford,The JavaScript Programming Language (video lecture)]
-____
-The browser is a really hostile programming environment.
-____
-
-(((Crockford+++,+++ Douglas)))(((JavaScript,history of)))(((World Wide
-Web)))The next part of this book will talk about web browsers. Without
-web ((browser))s, there would be no JavaScript. And even if there
-were, no one would ever have paid any attention to it.
-
-(((decentralization)))(((compatibility)))Web technology has, from the
-very start, been decentralized, not just technically, but also in the
-way it has evolved. Various browser vendors have added new
-functionality in ad-hoc and sometimes poorly thought out ways, which
-then sometimes ended up being adopted by others, and finally set down
-as a ((standard)).
-
-This is both a blessing and a curse. On the one hand, it is empowering
-to not have a central party control a system, but have it be improved
-by various parties working in loose ((collaboration)) (or,
-occasionally, open hostility). On the other hand, the haphazard way in
-which the Web was developed means that the resulting system is not
-exactly a shining example of internal ((consistency)). In fact, some
-parts of it are downright messy and confusing.
-
-== Networks and the Internet ==
-
-Computer ((network))s have been around since the 1950s. If you put
-cables between two or more computers, and allow them to send data back
-and forth through these cables, you can do all kinds of wonderful
-things.
-
-If connecting two machines in the same building allows us to do
-wonderful things, connecting machines all over the planet should be
-even better. The technology to start implementing this vision was
-developed in the 1980s, and the resulting network is called the
-_((Internet))_. It has lived up to its promise.
-
-A computer can use this network to spew bits at another computer. For
-any effective ((communication)) to arise out of this bit-spewing, the
-computers at both ends must know what the bits are supposed to
-represent. The meaning of any given sequence of bits depends entirely
-on the kind of thing that it is trying to express, and the
-((encoding)) mechanism used.
-
-A _network ((protocol))_ describes a style of communication over a
-((network)). There are protocols for sending email, fetching email,
-sharing files, or even for controlling computers that happen to be
-infected by malicious software.
-
-For example, a simple ((chat)) protocol might consist of one computer
-sending the bits that represent the text “CHAT?” to another machine,
-and the other responding with “OK!” to confirm that it understands the
-protocol. They can then proceed to send each other strings of text,
-read the text sent by the other from the network, and display whatever
-they receive on their screens.
-
-(((layering)))(((stream)))(((ordering)))Most protocols are built on
-top of other protocols. Our example chat protocol treats the network
-as a stream-like device into which you can put bits, and have them
-arrive at the correct destination in the correct order. Ensuring those
-things is already a rather difficult technical problem.
-
-indexsee:[Transmission Control Protocol,TCP]
-
-(((TCP)))TCP (_Transmission Control Protocol_) is a ((protocol)) that
-solves this problem. All internet-connected devices “speak” it, and
-most communication on the ((Internet)) is built on top of it.
-
-(((listening (TCP))))A TCP ((connection)) works as follows: one
-computer must be waiting, or _listening_, for other computers to start
-talking to it. To be able to listen for different kinds of
-communication at the same time on a single machine, each listener has
-a number (called a _((port))_) associated with it. Most ((protocol))s
-specify which port should be used by default. For example, when we
-want to send an email using the ((SMTP)) protocol, the machine through
-which we send it is expected to be listening on port 25.
-
-Another computer can then establish a ((connection)) by connecting to
-the target machine using the correct port number. If the target
-machine can be reached, and is listening on that port, the connection
-is successfully created. The listening computer is called the
-_((server))_, and the connecting computer the _((client))_.
-
-Such a connection acts as a two-way ((pipe)) through which bits can
-flow—the machines on both ends can put data into it. Once the bits are
-successfully transmitted, they can be read out again by the machine on
-the other side. This is a very convenient model. You could say that
-((TCP)) provides an ((abstraction)) of the network.
-
-[[web]]
-== The Web ==
-
-The _((World Wide Web))_ (not to be confused with the ((Internet)) as
-a whole) is a set of ((protocol))s and formats that allow us to visit
-web pages in a browser. The “Web” part in the name refers to the fact
-that such pages can easily link to each other, thus connecting into a
-huge ((mesh)) that users can move through.
-
-indexsee:[Hyper-Text Transfer Prototol,HTTP]
-
-To add content to the Web, all you need to do is connect a machine to
-the ((Internet)), and have it listen on port 80, using ((HTTP)) (the
-_Hyper-Text Transfer Protocol_). This protocol allows other computers
-to request documents over the ((network)).
-
-indexsee:[Universal Resource Locator,URL]
-
-Each ((document)) on the Web is named by a ((URL)) (_Universal Resource
-Locator_), which looks something like this:
-
-----
- http://eloquentjavascript.net/12_browser.html
- | | | |
- protocol server path
-----
-
-(((HTTPS)))The first part tells us that this URL uses the HTTP
-((protocol)) (as opposed to, for example, encrypted HTTP, which would
-be _https://_). Then comes the part that identifies which ((server))
-we are requesting the document from. Last is a path string that
-identifies the specific document (or _((resource))_) we are interested
-in.
-
-Each machine connected to the Internet gets a unique _((IP address))_,
-which looks something like `37.187.37.82`. You can use these directly
-as the server part of a ((URL)). But lists of more or less random
-numbers are hard to remember and awkward to type, so you can instead
-register a _((domain)) name_ to point towards a specific machine or
-set of machines. I registered _eloquentjavascript.net_ to point at the
-IP address of a machine I control, and can thus use that domain name
-to serve web pages.
-
-(((browser)))If you type the URL above into your browser's ((address
-bar)), it will try to retrieve and display the ((document)) at that
-URL. First, your browser has to find out what address
-_eloquentjavascript.net_ refers to. Then, using the ((HTTP)) protocol,
-it makes a connection to the server at that address, and asks for the
-resource _/12_browser.html_.
-
-We will take a closer look at the HTTP protocol in
-link:17_http.html#http[Chapter 17].
-
-== HTML ==
-
-indexsee:[Hyper-Text Markup Language,HTML]
-
-(((HTML)))HTML, which stands for _Hyper-Text Markup Language_, is the
-document format used for web pages. An HTML document contains
-((text)), as well as _((tag))s_ that gives structure to the text,
-describing things like links, paragraphs, and headings.
-
-A simple HTML document looks like this:
-
-[source,text/html]
-----
-
-
-
- My home page
-
-
-
-
-
-----
-
-ifdef::tex_target[]
-
-This is what such a document would look like in the browser:
-
-image::img/home-page.png[alt="My home page",width="6.3cm"]
-
-endif::tex_target[]
-
-(((angular brackets)))The tags, wrapped in angular brackets (“++<++”
-and “++>++”), provide information about the ((structure)) of the
-document. The other ((text)) is just plain text.
-
-(((doctype)))(((version)))The document starts with ``,
-which tells the browser to interpret it as _modern_ HTML, as opposed
-to various dialects that were in use in the past.
-
-(((head (HTML tag))))(((body (HTML tag))))(((title (HTML tag))))(((h1
-(HTML tag))))(((p (HTML tag))))HTML documents have a head and a body.
-The head contains information _about_ the document, and the body
-contains the document itself. In this case, we first declared that the
-title of this document is “My home page”, and then gave a document
-containing a heading (`
`, meaning “heading 1”—++
++ to `
`
-produce more minor headings) and two ((paragraph))s (`
`).
-
-(((href attribute)))(((a (HTML tag))))Tags come in several forms. An
-((element)), such as the body, a paragraph, or a link, is started by
-an _((opening tag))_ like `
I also wrote a book! Read it
- here.
-----
-
-(((title (HTML tag))))(((head (HTML tag))))(((body (HTML
-tag))))(((html (HTML tag))))The ``, `
` and `` tags
-are gone completely. The browser knows that `` belongs in a
-head, and `<h1>` in a body. Furthermore, I am no longer explicitly
-closing the paragraphs, since opening a new paragraph or ending the
-document will close them implicitly. The quotes around the link target
-are also gone.
-
-This book will usually omit the `<html>`, `<head>`, and `<body>` tags
-from examples, to keep them short and free of clutter. But I _will_
-close tags and include quotes around attributes.
-
-(((browser)))I will also usually omit the ((doctype)). This is not to
-be taken as an encouragement to omit doctype declarations. Browsers
-will often do ridiculous things when you forget them. You should
-consider doctypes implicitly present in examples, even when they are
-not actually shown in the text.
-
-[[script_tag]]
-== HTML and JavaScript ==
-
-(((JavaScript,in HTML)))(((script (HTML tag))))In the context of this
-book, the most important ((HTML)) tag is `<script>`. This tag allows
-us to include a piece of JavaScript in a document.
-
-[source,text/html]
-----
-<h1>Testing alert</h1>
-<script>alert("hello!");</script>
-----
-
-(((alert function)))(((timeline)))Such a script will run as soon as
-its `<script>` tag is encountered as the browser reads the HTML. The
-page above will pop up an `alert` dialog when opened.
-
-(((src attribute)))Including large programs directly in HTML documents
-is often impractical. The `<script>` tag can be given an `src`
-attribute in order to fetch a script file (a text file containing a
-JavaScript program) from a URL.
-
-[source,text/html]
-----
-<h1>Testing alert</h1>
-<script src="code/hello.js"></script>
-----
-
-The _code/hello.js_ file included here contains the same simple program,
-`alert("hello!")`. When an HTML page references other URLs as part of
-itself, for example an image file or a script, web browsers will
-retrieve them immediately, and include them in the page.
-
-(((script (HTML tag))))(((closing tag)))A script tag must always be
-closed with `</script>`, even if it refers to a script file and
-doesn't contain any code. If you forget this, the rest of the page
-will be interpreted as part of the script.
-
-(((button (HTML tag))))(((onclick attribute)))Some attributes can also
-contain a JavaScript program. The `<button>` tag below (which shows up
-as a button) has an `onclick` attribute, whose content will be run
-whenever the button is pressed.
-
-[source,text/html]
-----
-<button onclick="alert('Boom!')">DO NOT PRESS</button>
-----
-
-(((single-quote character)))(((escaping,in HTML)))Note that I had to
-use single quotes for the string in the `onclick` attribute, because
-double quotes are already used to quote the whole attribute. I could
-also have used `"`, but that'd make the program harder to read.
-
-== In the sandbox ==
-
-(((malicious script)))(((World Wide
-Web)))(((browser)))(((website)))(((security)))Running programs
-downloaded from the ((Internet)) is potentially dangerous. You do not
-know much about the people behind most sites you visit, and they do
-not necessarily mean well. Running programs by people who do not mean
-well is how you get your computer infected by ((virus))es, your data
-stolen, and your accounts hacked.
-
-Yet the attraction of the Web is that you can surf it without
-necessarily ((trust))ing all the pages you visit. This is why browsers
-severely limit the things a JavaScript program may do: it can't look
-at the files on your computer, or modify anything not related to the
-web page it was embedded in.
-
-(((isolation)))Isolating a programming environment in this way is
-called _((sandbox))ing_, the idea being that the program is harmlessly
-playing in a sandbox. But you should imagine this particular kind of
-sandbox as having a cage of thick steel bars over it, which makes it
-somewhat different from your typical playground sandbox.
-
-The hard part of sandboxing is allowing the programs enough room to be
-useful, yet at the same time restricting them from doing anything
-dangerous. Lots of useful functionality, such as communicating with
-other servers, or reading the content of the copy-paste ((clipboard)),
-can also be used to do problematic, ((privacy))-invading things.
-
-(((leak)))(((exploit)))(((security)))Every now and then, someone comes
-up with a new way to circumvent the limitations of a ((browser)) and
-do something harmful, ranging from leaking minor private information
-to taking over the whole machine that the browser runs on. The browser
-developers respond by fixing the hole, and all is well again. That is,
-until the next problem is discovered—and hopefully publicized, rather
-than secretly exploited by some government or ((mafia)).
-
-== Compatibility and the browser wars ==
-
-(((Microsoft)))(((World Wide Web)))In the very early stages of the
-Web, a browser called ((Mosaic)) dominated the market. After a few
-years, the balance had shifted to ((Netscape)), which was then, in
-turn, largely supplanted by Microsoft's ((Internet Explorer)). At any
-point where a single ((browser)) was dominant, that browser's vendor
-would feel entitled to unilaterally invent new features for the Web.
-Since most users used the same browser, ((website))s would simply
-start using those features—never mind the other browsers.
-
-This was the dark age of ((compatibility)), often called the
-“((browser wars))”. Web developers were left with not one unified Web,
-but two or three incompatible platforms. To make things worse, the
-browsers in use around 2003 were all full of ((bug))s, and of course
-the bugs were different for each ((browser)). Life was hard for people
-writing web pages.
-
-(((Apple)))(((Internet Explorer)))(((Mozilla)))Mozilla ((Firefox)), a
-not-for-profit offshoot of ((Netscape)), challenged Internet
-Explorer's hegemony in the late 2000s. Because ((Microsoft)) was not
-particularly interested in staying competitive at the time, Firefox
-took quite a chunk of market share away from them. Around the same
-time, ((Google)) introduced its ((Chrome)) browser, and Apple's
-((Safari)) browser gained popularity, leading to a situation where
-there were four major players, rather than one.
-
-(((compatibility)))The new players had a more serious attitude towards
-((standards)) and better ((engineering)) practices, leading to less
-incompatibility and fewer ((bug))s. Microsoft, seeing its market share
-crumble, came around and adopted these attitudes. If you are starting
-to learn web development today, consider yourself lucky. The latest
-versions of the major browsers behave quite uniformly, and have
-relatively few bugs.
-
-(((World Wide Web)))Which is not to say that the situation is perfect
-just yet. Some of the people using the Web are, for reasons of inertia
-or corporate policy, stuck with very old ((browser))s. Until those
-browsers die out entirely, writing websites that work for them will
-require a lot of arcane knowledge about their shortcomings and quirks.
-This book is not about those ((quirks)). Rather, it aims to present
-the modern, sane style of ((web programming)).
diff --git a/12_language.md b/12_language.md
new file mode 100644
index 000000000..04a287336
--- /dev/null
+++ b/12_language.md
@@ -0,0 +1,654 @@
+{{meta {load_files: ["code/chapter/12_language.js"], zip: "node/html"}}}
+
+# Project: A Programming Language
+
+{{quote {author: "Hal Abelson and Gerald Sussman", title: "Structure and Interpretation of Computer Programs", chapter: true}
+
+The evaluator, which determines the meaning of expressions in a programming language, is just another program.
+
+quote}}
+
+{{index "Abelson, Hal", "Sussman, Gerald", SICP, "project chapter"}}
+
+{{figure {url: "img/chapter_picture_12.jpg", alt: "Illustration showing an egg with holes in it, showing smaller eggs inside, which in turn have even smaller eggs in them, and so on", chapter: "framed"}}}
+
+Building your own ((programming language)) is surprisingly easy (as long as you do not aim too high) and very enlightening.
+
+The main thing I want to show in this chapter is that there's no ((magic)) involved in building a programming language. I've often felt that some human inventions were so immensely clever and complicated that I'd never be able to understand them. But with a little reading and experimenting, they often turn out to be quite mundane.
+
+{{index "Egg language", [abstraction, "in Egg"]}}
+
+We will build a programming language called Egg. It will be a tiny, simple language—but one that is powerful enough to express any computation you can think of. It will allow simple ((abstraction)) based on ((function))s.
+
+{{id parsing}}
+
+## Parsing
+
+{{index parsing, validation, [syntax, "of Egg"]}}
+
+The most immediately visible part of a programming language is its _syntax_, or notation. A _parser_ is a program that reads a piece of text and produces a data structure that reflects the structure of the program contained in that text. If the text does not form a valid program, the parser should point out the error.
+
+{{index "special form", [function, application]}}
+
+Our language will have a simple and uniform syntax. Everything in Egg is an ((expression)). An expression can be the name of a binding, a number, a string, or an _application_. Applications are used for function calls but also for constructs such as `if` or `while`.
+
+{{index "double-quote character", parsing, [escaping, "in strings"], [whitespace, syntax]}}
+
+To keep the parser simple, strings in Egg do not support anything like backslash escapes. A string is simply a sequence of characters that are not double quotes, wrapped in double quotes. A number is a sequence of digits. Binding names can consist of any character that is not whitespace and that does not have a special meaning in the syntax.
+
+{{index "comma character", [parentheses, arguments]}}
+
+Applications are written the way they are in JavaScript, by putting parentheses after an expression and having any number of ((argument))s between those parentheses, separated by commas.
+
+```{lang: null}
+do(define(x, 10),
+ if(>(x, 5),
+ print("large"),
+ print("small")))
+```
+
+{{index block, [syntax, "of Egg"]}}
+
+The ((uniformity)) of the ((Egg language)) means that things that are ((operator))s in JavaScript (such as `>`) are normal bindings in this language, applied just like other ((function))s. Since the syntax has no concept of a block, we need a `do` construct to represent doing multiple things in sequence.
+
+{{index "type property", parsing, ["data structure", tree]}}
+
+The data structure that the parser will use to describe a program consists of ((expression)) objects, each of which has a `type` property indicating the kind of expression it is and other properties to describe its content.
+
+{{index identifier}}
+
+Expressions of type `"value"` represent literal strings or numbers. Their `value` property contains the string or number value that they represent. Expressions of type `"word"` are used for identifiers (names). Such objects have a `name` property that holds the identifier's name as a string. Finally, `"apply"` expressions represent applications. They have an `operator` property that refers to the expression that is being applied, as well as an `args` property that holds an array of argument expressions.
+
+The `>(x, 5)` part of the previous program would be represented like this:
+
+```{lang: "json"}
+{
+ type: "apply",
+ operator: {type: "word", name: ">"},
+ args: [
+ {type: "word", name: "x"},
+ {type: "value", value: 5}
+ ]
+}
+```
+
+{{indexsee "abstract syntax tree", "syntax tree", ["data structure", tree]}}
+
+Such a data structure is called a _((syntax tree))_. If you imagine the objects as dots and the links between them as lines between those dots, as shown in the following diagram, the structure has a ((tree))like shape. The fact that expressions contain other expressions, which in turn might contain more expressions, is similar to the way tree branches split and split again.
+
+{{figure {url: "img/syntax_tree.svg", alt: "A diagram showing the structure of the syntax tree for the example program. The root is labeled 'do' and has two children, one labeled 'define' and one labeled 'if'. Those in turn have more children, describing their content.", width: "5cm"}}}
+
+{{index parsing}}
+
+Contrast this to the parser we wrote for the configuration file format in [Chapter ?](regexp#ini), which had a simple structure: it split the input into lines and handled those lines one at a time. There were only a few simple forms that a line was allowed to have.
+
+{{index recursion, [nesting, "of expressions"]}}
+
+Here we must find a different approach. Expressions are not separated into lines, and they have a recursive structure. Application expressions _contain_ other expressions.
+
+{{index elegance}}
+
+Fortunately, this problem can be solved very well by writing a parser function that is recursive in a way that reflects the recursive nature of the language.
+
+{{index "parseExpression function", "syntax tree"}}
+
+We define a function `parseExpression` that takes a string as input. It returns an object containing the data structure for the expression at the start of the string, along with the part of the string left after parsing this expression. When parsing subexpressions (the argument to an application, for example), this function can be called again, yielding the argument expression as well as the text that remains. This text may in turn contain more arguments or may be the closing parenthesis that ends the list of arguments.
+
+This is the first part of the parser:
+
+```{includeCode: true}
+function parseExpression(program) {
+ program = skipSpace(program);
+ let match, expr;
+ if (match = /^"([^"]*)"/.exec(program)) {
+ expr = {type: "value", value: match[1]};
+ } else if (match = /^\d+\b/.exec(program)) {
+ expr = {type: "value", value: Number(match[0])};
+ } else if (match = /^[^\s(),#"]+/.exec(program)) {
+ expr = {type: "word", name: match[0]};
+ } else {
+ throw new SyntaxError("Unexpected syntax: " + program);
+ }
+
+ return parseApply(expr, program.slice(match[0].length));
+}
+
+function skipSpace(string) {
+ let first = string.search(/\S/);
+ if (first == -1) return "";
+ return string.slice(first);
+}
+```
+
+{{index "skipSpace function", [whitespace, syntax]}}
+
+Because Egg, like JavaScript, allows any amount of whitespace between its elements, we have to repeatedly cut the whitespace off the start of the program string. The `skipSpace` function helps with this.
+
+{{index "literal expression", "SyntaxError type"}}
+
+After skipping any leading space, `parseExpression` uses three ((regular expression))s to spot the three atomic elements that Egg supports: strings, numbers, and words. The parser constructs a different kind of data structure depending on which expression matches. If the input does not match one of these three forms, it is not a valid expression, and the parser throws an error. We use the `SyntaxError` constructor here. This is an exception class defined by the standard, like `Error`, but more specific.
+
+{{index "parseApply function"}}
+
+We then cut off the part that was matched from the program string and pass that, along with the object for the expression, to `parseApply`, which checks whether the expression is an application. If so, it parses a parenthesized list of arguments.
+
+```{includeCode: true}
+function parseApply(expr, program) {
+ program = skipSpace(program);
+ if (program[0] != "(") {
+ return {expr: expr, rest: program};
+ }
+
+ program = skipSpace(program.slice(1));
+ expr = {type: "apply", operator: expr, args: []};
+ while (program[0] != ")") {
+ let arg = parseExpression(program);
+ expr.args.push(arg.expr);
+ program = skipSpace(arg.rest);
+ if (program[0] == ",") {
+ program = skipSpace(program.slice(1));
+ } else if (program[0] != ")") {
+ throw new SyntaxError("Expected ',' or ')'");
+ }
+ }
+ return parseApply(expr, program.slice(1));
+}
+```
+
+{{index parsing, recursion}}
+
+If the next character in the program is not an opening parenthesis, this is not an application, and `parseApply` returns the expression it was given. Otherwise, it skips the opening parenthesis and creates the ((syntax tree)) object for this application expression. It then recursively calls `parseExpression` to parse each argument until a closing parenthesis is found. The recursion is indirect, through `parseApply` and `parseExpression` calling each other.
+
+Because an application expression can itself be applied (such as in `multiplier(2)(1)`), `parseApply` must, after it has parsed an application, call itself again to check whether another pair of parentheses follows.
+
+{{index "syntax tree", "Egg language", "parse function"}}
+
+This is all we need to parse Egg. We wrap it in a convenient `parse` function that verifies that it has reached the end of the input string after parsing the expression (an Egg program is a single expression), and that gives us the program's data structure.
+
+```{includeCode: strip_log, test: join}
+function parse(program) {
+ let {expr, rest} = parseExpression(program);
+ if (skipSpace(rest).length > 0) {
+ throw new SyntaxError("Unexpected text after program");
+ }
+ return expr;
+}
+
+console.log(parse("+(a, 10)"));
+// → {type: "apply",
+// operator: {type: "word", name: "+"},
+// args: [{type: "word", name: "a"},
+// {type: "value", value: 10}]}
+```
+
+{{index "error message"}}
+
+It works! It doesn't give us very helpful information when it fails and doesn't store the line and column on which each expression starts, which might be helpful when reporting errors later, but it's good enough for our purposes.
+
+## The evaluator
+
+{{index "evaluate function", evaluation, interpretation, "syntax tree", "Egg language"}}
+
+What can we do with the syntax tree for a program? Run it, of course! And that is what the evaluator does. You give it a syntax tree and a scope object that associates names with values, and it will evaluate the expression that the tree represents and return the value that this produces.
+
+```{includeCode: true}
+const specialForms = Object.create(null);
+
+function evaluate(expr, scope) {
+ if (expr.type == "value") {
+ return expr.value;
+ } else if (expr.type == "word") {
+ if (expr.name in scope) {
+ return scope[expr.name];
+ } else {
+ throw new ReferenceError(
+ `Undefined binding: ${expr.name}`);
+ }
+ } else if (expr.type == "apply") {
+ let {operator, args} = expr;
+ if (operator.type == "word" &&
+ operator.name in specialForms) {
+ return specialForms[operator.name](expr.args, scope);
+ } else {
+ let op = evaluate(operator, scope);
+ if (typeof op == "function") {
+ return op(...args.map(arg => evaluate(arg, scope)));
+ } else {
+ throw new TypeError("Applying a non-function.");
+ }
+ }
+ }
+}
+```
+
+{{index "literal expression", scope}}
+
+The evaluator has code for each of the ((expression)) types. A literal value expression produces its value. (For example, the expression `100` evaluates to the number 100.) For a binding, we must check whether it is actually defined in the scope and, if it is, fetch the binding's value.
+
+{{index [function, application]}}
+
+Applications are more involved. If they are a ((special form)), like `if`, we do not evaluate anything—we just and pass the argument expressions, along with the scope, to the function that handles this form. If it is a normal call, we evaluate the operator, verify that it is a function, and call it with the evaluated arguments.
+
+We use plain JavaScript function values to represent Egg's function values. We will come back to this [later](language#egg_fun), when the special form `fun` is defined.
+
+{{index readability, "evaluate function", recursion, parsing}}
+
+The recursive structure of `evaluate` resembles the structure of the parser, and both mirror the structure of the language itself. It would also be possible to combine the parser and the evaluator into one function and evaluate during parsing, but splitting them up this way makes the program clearer and more flexible.
+
+{{index "Egg language", interpretation}}
+
+This is really all that's needed to interpret Egg. It's that simple. But without defining a few special forms and adding some useful values to the ((environment)), you can't do much with this language yet.
+
+## Special forms
+
+{{index "special form", "specialForms object"}}
+
+The `specialForms` object is used to define special syntax in Egg. It associates words with functions that evaluate such forms. It is currently empty. Let's add `if`.
+
+```{includeCode: true}
+specialForms.if = (args, scope) => {
+ if (args.length != 3) {
+ throw new SyntaxError("Wrong number of args to if");
+ } else if (evaluate(args[0], scope) !== false) {
+ return evaluate(args[1], scope);
+ } else {
+ return evaluate(args[2], scope);
+ }
+};
+```
+
+{{index "conditional execution", "ternary operator", "?: operator", "conditional operator"}}
+
+Egg's `if` construct expects exactly three arguments. It will evaluate the first, and if the result isn't the value `false`, it will evaluate the second. Otherwise, the third gets evaluated. This `if` form is more similar to JavaScript's ternary `?:` operator than to JavaScript's `if`. It is an expression, not a statement, and it produces a value—namely, the result of the second or third argument.
+
+{{index Boolean}}
+
+Egg also differs from JavaScript in how it handles the condition value to `if`. It will treat only the value `false` as false, not things like zero or the empty string.
+
+{{index "short-circuit evaluation"}}
+
+The reason we need to represent `if` as a special form rather than a regular function is that all arguments to functions are evaluated before the function is called, whereas `if` should evaluate only _either_ its second or its third argument, depending on the value of the first.
+
+The `while` form is similar.
+
+```{includeCode: true}
+specialForms.while = (args, scope) => {
+ if (args.length != 2) {
+ throw new SyntaxError("Wrong number of args to while");
+ }
+ while (evaluate(args[0], scope) !== false) {
+ evaluate(args[1], scope);
+ }
+
+ // Since undefined does not exist in Egg, we return false,
+ // for lack of a meaningful result
+ return false;
+};
+```
+
+Another basic building block is `do`, which executes all its arguments from top to bottom. Its value is the value produced by the last argument.
+
+```{includeCode: true}
+specialForms.do = (args, scope) => {
+ let value = false;
+ for (let arg of args) {
+ value = evaluate(arg, scope);
+ }
+ return value;
+};
+```
+
+{{index ["= operator", "in Egg"], [binding, "in Egg"]}}
+
+To be able to create bindings and give them new values, we also create a form called `define`. It expects a word as its first argument and an expression producing the value to assign to that word as its second argument. Since `define`, like everything, is an expression, it must return a value. We'll make it return the value that was assigned (just like JavaScript's `=` operator).
+
+```{includeCode: true}
+specialForms.define = (args, scope) => {
+ if (args.length != 2 || args[0].type != "word") {
+ throw new SyntaxError("Incorrect use of define");
+ }
+ let value = evaluate(args[1], scope);
+ scope[args[0].name] = value;
+ return value;
+};
+```
+
+## The environment
+
+{{index "Egg language", "evaluate function", [binding, "in Egg"]}}
+
+The ((scope)) accepted by `evaluate` is an object with properties whose names correspond to binding names and whose values correspond to the values those bindings are bound to. Let's define an object to represent the ((global scope)).
+
+To be able to use the `if` construct we just defined, we must have access to ((Boolean)) values. Since there are only two Boolean values, we do not need special syntax for them. We simply bind two names to the values `true` and `false` and use them.
+
+```{includeCode: true}
+const topScope = Object.create(null);
+
+topScope.true = true;
+topScope.false = false;
+```
+
+We can now evaluate a simple expression that negates a Boolean value.
+
+```
+let prog = parse(`if(true, false, true)`);
+console.log(evaluate(prog, topScope));
+// → false
+```
+
+{{index arithmetic, "Function constructor"}}
+
+To supply basic ((arithmetic)) and ((comparison)) ((operator))s, we will also add some function values to the ((scope)). In the interest of keeping the code short, we'll use `Function` to synthesize a bunch of operator functions in a loop instead of defining them individually.
+
+```{includeCode: true}
+for (let op of ["+", "-", "*", "/", "==", "<", ">"]) {
+ topScope[op] = Function("a, b", `return a ${op} b;`);
+}
+```
+
+It is also useful to have a way to ((output)) values, so we'll wrap `console.log` in a function and call it `print`.
+
+```{includeCode: true}
+topScope.print = value => {
+ console.log(value);
+ return value;
+};
+```
+
+{{index parsing, "run function"}}
+
+That gives us enough elementary tools to write simple programs. The following function provides a convenient way to parse a program and run it in a fresh scope:
+
+```{includeCode: true}
+function run(program) {
+ return evaluate(parse(program), Object.create(topScope));
+}
+```
+
+{{index "Object.create function", prototype}}
+
+We'll use object prototype chains to represent nested scopes so that the program can add bindings to its local scope without changing the top-level scope.
+
+```
+run(`
+do(define(total, 0),
+ define(count, 1),
+ while(<(count, 11),
+ do(define(total, +(total, count)),
+ define(count, +(count, 1)))),
+ print(total))
+`);
+// → 55
+```
+
+{{index "summing example", "Egg language"}}
+
+This is the program we've seen several times before that computes the sum of the numbers 1 to 10, expressed in Egg. It is clearly uglier than the equivalent JavaScript program—but not bad for a language implemented in fewer than 150 ((lines of code)).
+
+{{id egg_fun}}
+
+## Functions
+
+{{index function, "Egg language"}}
+
+A programming language without functions is a poor programming language indeed. Fortunately, it isn't hard to add a `fun` construct, which treats its last argument as the function's body and uses all arguments before that as the names of the function's parameters.
+
+```{includeCode: true}
+specialForms.fun = (args, scope) => {
+ if (!args.length) {
+ throw new SyntaxError("Functions need a body");
+ }
+ let body = args[args.length - 1];
+ let params = args.slice(0, args.length - 1).map(expr => {
+ if (expr.type != "word") {
+ throw new SyntaxError("Parameter names must be words");
+ }
+ return expr.name;
+ });
+
+ return function(...args) {
+ if (args.length != params.length) {
+ throw new TypeError("Wrong number of arguments");
+ }
+ let localScope = Object.create(scope);
+ for (let i = 0; i < args.length; i++) {
+ localScope[params[i]] = args[i];
+ }
+ return evaluate(body, localScope);
+ };
+};
+```
+
+{{index "local scope"}}
+
+Functions in Egg get their own local scope. The function produced by the `fun` form creates this local scope and adds the argument bindings to it. It then evaluates the function body in this scope and returns the result.
+
+```{startCode: true}
+run(`
+do(define(plusOne, fun(a, +(a, 1))),
+ print(plusOne(10)))
+`);
+// → 11
+
+run(`
+do(define(pow, fun(base, exp,
+ if(==(exp, 0),
+ 1,
+ *(base, pow(base, -(exp, 1)))))),
+ print(pow(2, 10)))
+`);
+// → 1024
+```
+
+## Compilation
+
+{{index interpretation, compilation}}
+
+What we have built is an interpreter. During evaluation, it acts directly on the representation of the program produced by the parser.
+
+{{index efficiency, performance, [binding, definition], [memory, speed]}}
+
+_Compilation_ is the process of adding another step between the parsing and the running of a program, which transforms the program into something that can be evaluated more efficiently by doing as much work as possible in advance. For example, in well-designed languages it is obvious, for each use of a binding, which binding is being referred to, without actually running the program. This can be used to avoid looking up the binding by name every time it is accessed, instead directly fetching it from some predetermined memory location.
+
+Traditionally, ((compilation)) involves converting the program to ((machine code)), the raw format that a computer's processor can execute. But any process that converts a program to a different representation can be thought of as compilation.
+
+{{index simplicity, "Function constructor", transpilation}}
+
+It would be possible to write an alternative ((evaluation)) strategy for Egg, one that first converts the program to a JavaScript program, uses `Function` to invoke the JavaScript compiler on it, and runs the result. When done right, this would make Egg run very fast while still being quite simple to implement.
+
+If you are interested in this topic and willing to spend some time on it, I encourage you to try to implement such a compiler as an exercise.
+
+## Cheating
+
+{{index "Egg language"}}
+
+When we defined `if` and `while`, you probably noticed that they were more or less trivial wrappers around JavaScript's own `if` and `while`. Similarly, the values in Egg are just regular old JavaScript values. Bridging the gap to a more primitive system, such as the machine code the processor understands, takes more effort—but the way it works resembles what we are doing here.
+
+Though the toy language in this chapter doesn't do anything that couldn't be done better in JavaScript, there _are_ situations where writing small languages helps get real work done.
+
+Such a language does not have to resemble a typical programming language. If JavaScript didn't come equipped with regular expressions, for example, you could write your own parser and evaluator for regular expressions.
+
+{{index "parser generator"}}
+
+Or imagine you are building a program that makes it possible to quickly create parsers by providing a logical description of the language they need to parse. You could define a specific notation for that, and a compiler that compiles it to a parser program.
+
+```{lang: null}
+expr = number | string | name | application
+
+number = digit+
+
+name = letter+
+
+string = '"' (! '"')* '"'
+
+application = expr '(' (expr (',' expr)*)? ')'
+```
+
+{{index expressivity}}
+
+This is what is usually called a _((domain-specific language))_, a language tailored to express a narrow domain of knowledge. Such a language can be more expressive than a general-purpose language because it is designed to describe exactly the things that need to be described in its domain and nothing else.
+
+## Exercises
+
+### Arrays
+
+{{index "Egg language", "arrays in egg (exercise)", [array, "in Egg"]}}
+
+Add support for arrays to Egg by adding the following three functions to the top scope: `array(...values)` to construct an array containing the argument values, `length(array)` to get an array's length, and `element(array, n)` to fetch the *n*th element from an array.
+
+{{if interactive
+
+```{test: no}
+// Modify these definitions...
+
+topScope.array = "...";
+
+topScope.length = "...";
+
+topScope.element = "...";
+
+run(`
+do(define(sum, fun(array,
+ do(define(i, 0),
+ define(sum, 0),
+ while(<(i, length(array)),
+ do(define(sum, +(sum, element(array, i))),
+ define(i, +(i, 1)))),
+ sum))),
+ print(sum(array(1, 2, 3))))
+`);
+// → 6
+```
+
+if}}
+
+{{hint
+
+{{index "arrays in egg (exercise)"}}
+
+The easiest way to do this is to represent Egg arrays with JavaScript arrays.
+
+{{index "slice method"}}
+
+The values added to the top scope must be functions. By using a rest argument (with triple-dot notation), the definition of `array` can be _very_ simple.
+
+hint}}
+
+### Closure
+
+{{index closure, [function, scope], "closure in egg (exercise)"}}
+
+The way we have defined `fun` allows functions in Egg to reference the surrounding scope, allowing the function's body to use local values that were visible at the time the function was defined, just like JavaScript functions do.
+
+The following program illustrates this: function `f` returns a function that adds its argument to `f`'s argument, meaning that it needs access to the local ((scope)) inside `f` to be able to use binding `a`.
+
+```
+run(`
+do(define(f, fun(a, fun(b, +(a, b)))),
+ print(f(4)(5)))
+`);
+// → 9
+```
+
+Go back to the definition of the `fun` form and explain which mechanism causes this to work.
+
+{{hint
+
+{{index closure, "closure in egg (exercise)"}}
+
+Again, we are riding along on a JavaScript mechanism to get the equivalent feature in Egg. Special forms are passed the local scope in which they are evaluated so that they can evaluate their subforms in that scope. The function returned by `fun` has access to the `scope` argument given to its enclosing function and uses that to create the function's local ((scope)) when it is called.
+
+{{index compilation}}
+
+This means that the ((prototype)) of the local scope will be the scope in which the function was created, which makes it possible to access bindings in that scope from the function. This is all there is to implementing closure (though to compile it in a way that is actually efficient, you'd need to do some more work).
+
+hint}}
+
+### Comments
+
+{{index "hash character", "Egg language", "comments in egg (exercise)"}}
+
+It would be nice if we could write ((comment))s in Egg. For example, whenever we find a hash sign (`#`), we could treat the rest of the line as a comment and ignore it, similar to `//` in JavaScript.
+
+{{index "skipSpace function"}}
+
+We do not have to make any big changes to the parser to support this. We can simply change `skipSpace` to skip comments as if they are ((whitespace)) so that all the points where `skipSpace` is called will now also skip comments. Make this change.
+
+{{if interactive
+
+```{test: no}
+// This is the old skipSpace. Modify it...
+function skipSpace(string) {
+ let first = string.search(/\S/);
+ if (first == -1) return "";
+ return string.slice(first);
+}
+
+console.log(parse("# hello\nx"));
+// → {type: "word", name: "x"}
+
+console.log(parse("a # one\n # two\n()"));
+// → {type: "apply",
+// operator: {type: "word", name: "a"},
+// args: []}
+```
+if}}
+
+{{hint
+
+{{index "comments in egg (exercise)", [whitespace, syntax]}}
+
+Make sure your solution handles multiple comments in a row, with whitespace potentially between or after them.
+
+A ((regular expression)) is probably the easiest way to solve this. Write something that matches "whitespace or a comment, zero or more times". Use the `exec` or `match` method and look at the length of the first element in the returned array (the whole match) to find out how many characters to slice off.
+
+hint}}
+
+### Fixing scope
+
+{{index [binding, definition], assignment, "fixing scope (exercise)"}}
+
+Currently, the only way to assign a binding a value is `define`. This construct acts as a way both to define new bindings and to give existing ones a new value.
+
+{{index "local binding"}}
+
+This ((ambiguity)) causes a problem. When you try to give a nonlocal binding a new value, you will end up defining a local one with the same name instead. Some languages work like this by design, but I've always found it an awkward way to handle ((scope)).
+
+{{index "ReferenceError type"}}
+
+Add a special form `set`, similar to `define`, which gives a binding a new value, updating the binding in an outer scope if it doesn't already exist in the inner scope. If the binding is not defined at all, throw a `ReferenceError` (another standard error type).
+
+{{index "hasOwn function", prototype, "getPrototypeOf function"}}
+
+The technique of representing scopes as simple objects, which has made things convenient so far, will get in your way a little at this point. You might want to use the `Object.getPrototypeOf` function, which returns the prototype of an object. Also remember that you can use `Object.hasOwn` to find out if a given object has a property.
+
+{{if interactive
+
+```{test: no}
+specialForms.set = (args, scope) => {
+ // Your code here.
+};
+
+run(`
+do(define(x, 4),
+ define(setx, fun(val, set(x, val))),
+ setx(50),
+ print(x))
+`);
+// → 50
+run(`set(quux, true)`);
+// → Some kind of ReferenceError
+```
+if}}
+
+{{hint
+
+{{index [binding, "compilation of"], assignment, "getPrototypeOf function", "hasOwn function", "fixing scope (exercise)"}}
+
+You will have to loop through one ((scope)) at a time, using `Object.getPrototypeOf` to go to the next outer scope. For each scope, use `Object.hasOwn` to find out whether the binding, indicated by the `name` property of the first argument to `set`, exists in that scope. If it does, set it to the result of evaluating the second argument to `set` and then return that value.
+
+{{index "global scope", "run-time error"}}
+
+If the outermost scope is reached (`Object.getPrototypeOf` returns `null`) and we haven't found the binding yet, it doesn't exist, and an error should be thrown.
+
+hint}}
diff --git a/13_browser.md b/13_browser.md
new file mode 100644
index 000000000..7d040e243
--- /dev/null
+++ b/13_browser.md
@@ -0,0 +1,267 @@
+# JavaScript and the Browser
+
+{{quote {author: "Tim Berners-Lee", title: "The World Wide Web: A Very Short Personal Pistory", chapter: true}
+
+The dream behind the web is of a common information space in which we communicate by sharing information. Its universality is essential: the fact that a hypertext link can point to anything, be it personal, local or global, be it draft or highly polished.
+
+quote}}
+
+{{index "Berners-Lee, Tim", "World Wide Web", HTTP, [JavaScript, "history of"], "World Wide Web"}}
+
+{{figure {url: "img/chapter_picture_13.jpg", alt: "Illustration showing a telephone switchboard", chapter: "framed"}}}
+
+The next chapters of this book will discuss web browsers. Without ((browser))s, there would be no JavaScript—or if there were, no one would ever have paid any attention to it.
+
+{{index decentralization, compatibility}}
+
+Web technology has been decentralized from the start, not just technically but also in terms of the way it evolved. Various browser vendors have added new functionality in ad hoc and sometimes poorly thought-out ways, which were then—sometimes—adopted by others, and finally set down in ((standards)).
+
+This is both a blessing and a curse. On the one hand, it is empowering to not have a central party control a system but have it be improved by various parties working in loose ((collaboration)) (or occasionally, open hostility). On the other hand, the haphazard way in which the web was developed means that the resulting system is not exactly a shining example of internal ((consistency)). Some parts of it are downright confusing and badly designed.
+
+## Networks and the Internet
+
+Computer ((network))s have been around since the 1950s. If you put cables between two or more computers and allow them to send data back and forth through these cables, you can do all kinds of wonderful things.
+
+If connecting two machines in the same building allows us to do wonderful things, connecting machines all over the planet should be even better. The technology to start implementing this vision was developed in the 1980s, and the resulting network is called the _((internet))_. It has lived up to its promise.
+
+A computer can use this network to shoot bits at another computer. For any effective ((communication)) to arise out of this bit-shooting, the computers on both ends must know what the bits are supposed to represent. The meaning of any given sequence of bits depends entirely on the kind of thing that it is trying to express and on the ((encoding)) mechanism used.
+
+{{index [network, protocol]}}
+
+A _network ((protocol))_ describes a style of communication over a ((network)). There are protocols for sending email, for fetching email, for sharing files, and even for controlling computers that happen to be infected by malicious software.
+
+{{indexsee "HyperText Transfer Protocol", HTTP}}
+
+The _HyperText Transfer Protocol_ (((HTTP))) is a protocol for retrieving named ((resource))s (chunks of information, such as web pages or pictures). It specifies that the side making the request should start with a line like this, naming the resource and the version of the protocol that it is trying to use:
+
+```{lang: http}
+GET /index.html HTTP/1.1
+```
+
+There are many more rules about the way the requester can include more information in the ((request)) and the way the other side, which returns the resource, packages up its content. We'll look at HTTP in a little more detail in [Chapter ?](http).
+
+{{index layering, stream, ordering}}
+
+Most protocols are built on top of other protocols. HTTP treats the network as a streamlike device into which you can put bits and have them arrive at the correct destination in the correct order. Providing those guarantees on top of the primitive data-sending that the network gives you is already a rather tricky problem.
+
+{{index TCP}}
+
+{{indexsee "Transmission Control Protocol", TCP}}
+
+The _Transmission Control Protocol_ (TCP) is a ((protocol)) that addresses this problem. All internet-connected devices "speak" it, and most communication on the ((internet)) is built on top of it.
+
+{{index "listening (TCP)"}}
+
+A TCP ((connection)) works as follows: one computer must be waiting, or _listening_, for other computers to start talking to it. To be able to listen for different kinds of communication at the same time on a single machine, each listener has a number (called a _((port))_) associated with it. Most ((protocol))s specify which port should be used by default. For example, when we want to send an email using the ((SMTP)) protocol, the machine through which we send it is expected to be listening on port 25.
+
+Another computer can then establish a ((connection)) by connecting to the target machine using the correct port number. If the target machine can be reached and is listening on that port, the connection is successfully created. The listening computer is called the _((server))_, and the connecting computer is called the _((client))_.
+
+{{index [abstraction, "of the network"]}}
+
+Such a connection acts as a two-way ((pipe)) through which bits can flow—the machines on both ends can put data into it. Once the bits are successfully transmitted, they can be read out again by the machine on the other side. This is a convenient model. You could say that ((TCP)) provides an abstraction of the network.
+
+{{id web}}
+
+## The Web
+
+The _((World Wide Web))_ (not to be confused with the ((internet)) as a whole) is a set of ((protocol))s and formats that allow us to visit web pages in a browser. The word _Web_ refers to the fact that such pages can easily link to each other, thus connecting into a huge ((mesh)) that users can move through.
+
+To become part of the web, all you need to do is connect a machine to the ((internet)) and have it listen on port 80 with the ((HTTP)) protocol so that other computers can ask it for documents.
+
+{{index URL}}
+
+{{indexsee "uniform resource locator", URL}}
+
+Each ((document)) on the web is named by a _uniform resource locator_ (URL), which looks something like this:
+
+```{lang: null}
+ http://eloquentjavascript.net/13_browser.html
+ | | | |
+ protocol server path
+```
+
+{{index HTTPS}}
+
+The first part tells us that this URL uses the HTTP ((protocol)) (as opposed to, for example, encrypted HTTP, which would be _https://_). Then comes the part that identifies which ((server)) we are requesting the document from. Last is a path string that identifies the document (or _((resource))_) we are interested in.
+
+Machines connected to the internet get an _((IP address))_, a number that can be used to send messages to that machine, and looks something like `149.210.142.219` or `2001:4860:4860::8888`. Since lists of more or less random numbers are hard to remember and awkward to type, you can instead register a _((domain)) name_ for an address or set of addresses. I registered _eloquentjavascript.net_ to point at the IP address of a machine I control and can thus use that domain name to serve web pages.
+
+{{index browser}}
+
+If you type this URL into your browser's ((address bar)), the browser will try to retrieve and display the ((document)) at that URL. First, your browser has to find out what address _eloquentjavascript.net_ refers to. Then, using the ((HTTP)) protocol, it will make a connection to the server at that address and ask for the resource _/13_browser.html_. If all goes well, the server sends back a document, which your browser then displays on your screen.
+
+## HTML
+
+{{index HTML}}
+
+{{indexsee "HyperText Markup Language", HTML}}
+
+_HTML_, which stands for HyperText Markup Language, is the document format used for web pages. An HTML document contains ((text)), as well as _((tag))s_ that give structure to the text, describing things such as links, paragraphs, and headings.
+
+A short HTML document might look like this:
+
+```{lang: "html"}
+<!doctype html>
+<html>
+ <head>
+ <meta charset="utf-8">
+ <title>My home page
+
+
+
+
+
+```
+
+{{if book
+
+This is what such a document would look like in the browser:
+
+{{figure {url: "img/home-page.png", alt: "A rendered version of the home page example HTML",width: "6.3cm"}}}
+
+if}}
+
+{{index [HTML, notation]}}
+
+The tags, wrapped in ((angle brackets)) (`<` and `>`, the symbols for _less than_ and _greater than_), provide information about the ((structure)) of the document. The other ((text)) is just plain text.
+
+{{index doctype, version}}
+
+The document starts with ``, which tells the browser to interpret the page as _modern_ HTML, as opposed to obsolete styles used in the past.
+
+{{index "head (HTML tag)", "body (HTML tag)", "title (HTML tag)", "h1 (HTML tag)", "p (HTML tag)"}}
+
+HTML documents have a head and a body. The head contains information _about_ the document, and the body contains the document itself. In this case, the head declares that the title of this document is "My home page" and that it uses the UTF-8 encoding, which is a way to encode Unicode text as binary data. The document's body contains a heading (`
`, meaning "heading 1"—`
` to `
` produce subheadings) and two ((paragraph))s (`
`).
+
+{{index "href attribute", "a (HTML tag)"}}
+
+Tags come in several forms. An ((element)), such as the body, a paragraph, or a link, is started by an _((opening tag))_ like `
I also wrote a book! Read it
+ here.
+```
+
+{{index "title (HTML tag)", "head (HTML tag)", "body (HTML tag)", "html (HTML tag)"}}
+
+The ``, `
`, and `` tags are completely gone. The browser knows that `` and `` belong in the head and that `<h1>` means the body has started. Furthermore, I am no longer explicitly closing the paragraphs, since opening a new paragraph or ending the document will close them implicitly. The quotes around the attribute values are also gone.
+
+This book will usually omit the `<html>`, `<head>`, and `<body>` tags from examples to keep them short and free of clutter. I _will_ close tags and include quotes around attributes, though.
+
+{{index browser}}
+
+I will also usually omit the ((doctype)) and `charset` declaration. Don't take this as encouragement to drop these from HTML documents. Browsers will often do ridiculous things when you forget them. Consider the doctype and the `charset` metadata to be implicitly present in examples, even when they are not actually shown in the text.
+
+{{id script_tag}}
+
+## HTML and JavaScript
+
+{{index [JavaScript, "in HTML"], "script (HTML tag)"}}
+
+In the context of this book, the most important HTML tag is `<script>`, which allows us to include a piece of JavaScript in a document.
+
+```{lang: "html"}
+<h1>Testing alert</h1>
+<script>alert("hello!");</script>
+```
+
+{{index "alert function", timeline}}
+
+Such a script will run as soon as its `<script>` tag is encountered while the browser reads the HTML. This page will pop up a dialog when opened—the `alert` function resembles `prompt`, in that it pops up a little window, but only shows a message without asking for input.
+
+{{index "src attribute"}}
+
+Including large programs directly in HTML documents is often impractical. The `<script>` tag can be given an `src` attribute to fetch a script file (a text file containing a JavaScript program) from a URL.
+
+```{lang: "html"}
+<h1>Testing alert</h1>
+<script src="code/hello.js"></script>
+```
+
+The _code/hello.js_ file included here contains the same program—`alert("hello!")`. When an HTML page references other URLs as part of itself, such as an image file or a script, web browsers will retrieve them immediately and include them in the page.
+
+{{index "script (HTML tag)", "closing tag"}}
+
+A script tag must always be closed with `</script>`, even if it refers to a script file and doesn't contain any code. If you forget this, the rest of the page will be interpreted as part of the script.
+
+{{index "relative path", dependency}}
+
+You can load ((ES modules)) (see [Chapter ?](modules#es)) in the browser by giving your script tag a `type="module"` attribute. Such modules can depend on other modules by using ((URL))s relative to themselves as module names in `import` declarations.
+
+{{index "button (HTML tag)", "onclick attribute"}}
+
+Some attributes can also contain a JavaScript program. The `<button>` tag (which shows up as a button) supports an `onclick` attribute. The attribute's value will be run whenever the button is clicked.
+
+```{lang: "html"}
+<button onclick="alert('Boom!');">DO NOT PRESS</button>
+```
+
+{{index "single-quote character", [escaping, "in HTML"]}}
+
+Note that I had to use single quotes for the string in the `onclick` attribute because double quotes are already used to quote the whole attribute. I could also have used `"` to escape the inner quotes.
+
+## In the sandbox
+
+{{index "malicious script", "World Wide Web", browser, website, security}}
+
+Running programs downloaded from the ((internet)) is potentially dangerous. You don't know much about the people behind most sites you visit, and they do not necessarily mean well. Running programs by malicious actors is how you get your computer infected by ((virus))es, your data stolen, and your accounts hacked.
+
+Yet the attraction of the web is that you can browse it without necessarily ((trust))ing all the pages you visit. This is why browsers severely limit the things a JavaScript program may do: it can't look at the files on your computer or modify anything not related to the web page it was embedded in.
+
+{{index isolation}}
+
+Isolating a programming environment in this way is called _((sandbox))ing_, the idea being that the program is harmlessly playing in a sandbox. But you should imagine this particular kind of sandbox as having a cage of thick steel bars over it so that the programs playing in it can't actually get out.
+
+The hard part of sandboxing is allowing programs enough room to be useful while restricting them from doing anything dangerous. Lots of useful functionality, such as communicating with other servers or reading the content of the copy-paste ((clipboard)), can also be used for problematic, ((privacy))-invading purposes.
+
+{{index leak, exploit, security}}
+
+Every now and then, someone comes up with a new way to circumvent the limitations of a ((browser)) and do something harmful, ranging from leaking minor private information to taking over the whole machine on which the browser is running. The browser developers respond by fixing the hole, and all is well again—until the next problem is discovered, and hopefully publicized rather than secretly exploited by some government agency or criminal organization.
+
+## Compatibility and the browser wars
+
+{{index Microsoft, "World Wide Web"}}
+
+In the early stages of the web, a browser called ((Mosaic)) dominated the market. After a few years, the balance shifted to ((Netscape)), which was, in turn, largely supplanted by Microsoft's ((Internet Explorer)). At any point where a single ((browser)) was dominant, that browser's vendor would feel entitled to unilaterally invent new features for the web. Since most users used the most popular browser, ((website))s would simply start using those features—never mind the other browsers.
+
+This was the dark age of ((compatibility)), often called the _((browser wars))_. Web developers were left with not one unified web but two or three incompatible platforms. To make things worse, the browsers in use around 2003 were all full of ((bug))s, and of course the bugs were different for each ((browser)). Life was hard for people writing web pages.
+
+{{index Apple, "Internet Explorer", Mozilla}}
+
+Mozilla ((Firefox)), a not-for-profit offshoot of ((Netscape)), challenged Internet Explorer's position in the late 2000s. Because ((Microsoft)) was not particularly interested in staying competitive at the time, Firefox took a lot of market share away from it. Around the same time, ((Google)) introduced its ((Chrome)) browser and Apple's ((Safari)) browser gained popularity, leading to a situation where there were four major players, rather than one.
+
+{{index compatibility}}
+
+The new players had a more serious attitude toward ((standards)) and better ((engineering)) practices, giving us less incompatibility and fewer ((bug))s. Microsoft, seeing its market share crumble, came around and adopted these attitudes in its Edge browser, which replaced Internet Explorer. If you are starting to learn web development today, consider yourself lucky. The latest versions of the major browsers behave quite uniformly and have relatively few bugs.
+
+Unfortunately, with Firefox's market share getting ever smaller, and Edge becoming just a wrapper around Chrome's core in 2018, this uniformity might once again take the form of a single vendor—Google, this time—having enough control over the browser market to push its idea of what the web should look like onto the rest of the world.
+
+For what it is worth, this long chain of historical events and accidents has produced the web platform that we have today. In the next chapters, we are going to write programs for it.
\ No newline at end of file
diff --git a/13_dom.txt b/13_dom.txt
deleted file mode 100644
index 9660d86db..000000000
--- a/13_dom.txt
+++ /dev/null
@@ -1,1168 +0,0 @@
-:chap_num: 13
-:prev_link: 12_browser
-:next_link: 14_event
-:load_files: ["code/mountains.js", "code/chapter/13_dom.js"]
-
-= The Document Object Model =
-
-(((drawing)))(((parsing)))When you open a web page in your browser, it
-retrieves the page's ((HTML)) text and parses it, much like the way
-our parser from link:11_language.html#parsing[Chapter 11] parsed
-programs. The browser builds up a model of the document's
-((structure)), and then uses this model to draw the page on the
-screen.
-
-(((live data structure)))One of the toys that a JavaScript program has
-available in its ((sandbox)) is this representation of the
-((document)). You can read from it, and also change it. It acts as a
-_live_ data structure: when it is modified, the page on the screen is
-updated to reflect the changes.
-
-== Document structure ==
-
-You can imagine an ((HTML)) document as a nested set of ((box))es.
-Tags like `<body>` and `</body>` enclose other ((tag))s, which in turn
-contain other tags, or ((text)). Here's the example document from last
-chapter:
-
-[sandbox="homepage"]
-[source,text/html]
-----
-<!doctype html>
-<html>
- <head>
- <title>My home page
-
-
-
-
-
-----
-
-This page has the following structure:
-
-image::img/html-boxes.svg[alt="HTML document as nested boxes",width="7cm"]
-
-indexsee:[Document Object Model,DOM]
-
-The data structure the browser uses to represent the document follows
-this shape. For each box, there is an ((object)), which we can
-interact with to find out things like what HTML tag it represents, and
-which boxes and text it contains. This representation is called the
-_Document Object Model_, ((DOM)) for short.
-
-(((documentElement property)))(((head property)))(((body
-property)))(((html (HTML tag))))(((body (HTML tag))))(((head (HTML
-tag))))The global variable `document` gives us access to these
-objects. Its `documentElement` property refers to the object
-representing the `` tag. It also provides properties `head` and
-`body`, holding the objects for those elements.
-
-== Trees ==
-
-(((nesting,of objects)))Think back to the ((syntax tree))s from
-link:11_language.html#parsing[Chapter 11] for a moment. Their
-structure is strikingly similar to the structure of a browser's
-document. Each _((node))_ may refer to other nodes, _children_, which
-may have their own children. This shape is typical of nested
-structures where elements can contain sub-elements that are similar to
-themselves.
-
-(((documentElement property)))We call a data structure a _((tree))_
-when it has a branching structure, no ((cycle))s (a node may not
-contain itself, directly or indirectly), and has a single,
-well-defined “((root))”. In the case of the ((DOM)),
-`document.documentElement` serves as the root.
-
-(((sorting)))(((data structure)))(((syntax tree)))Trees come up a lot
-in computer science. Apart from representing recursive structures like
-HTML documents or programs, they are also often used to maintain
-sorted ((set))s of data, because elements can typically be found or
-inserted more efficiently in a sorted tree than in a sorted flat
-array.
-
-(((leaf node)))(((Egg language)))A typical tree has different kinds of
-((node))s. The syntax tree for link:11_language.html#language[the Egg
-language] had variables, values, and application nodes. Application
-nodes always had children, whereas variables and values were _leaves_,
-nodes without children.
-
-(((body property)))The same goes for the DOM. Nodes for regular
-_((element))s_, which represent ((HTML)) tags, determine the structure
-of the document. These can have ((child node))s. An example of such a
-node is `document.body`. Some of these children can be ((leaf node))s,
-such as pieces of ((text)) or ((comment))s (which are written between
-`` in HTML).
-
-(((text node)))(((ELEMENT_NODE code)))(((COMMENT_NODE
-code)))(((TEXT_NODE code)))(((nodeType property)))Each DOM node object
-has a `nodeType` property, which contains a numeric code that
-identifies the type of node. Regular elements have the value 1, which
-is also defined as the constant property `document.ELEMENT_NODE`. Text
-nodes, representing a section of text in the document, have the value
-3 (`document.TEXT_NODE`). Comments get the value 8
-(`document.COMMENT_NODE`).
-
-So another way to visualize our document ((tree)) is:
-
-image::img/html-tree.svg[alt="HTML document as a tree",width="8cm"]
-
-The leaves are text nodes, and the arrows indicate parent-child
-relationships between nodes.
-
-[[standard]]
-== The standard ==
-
-(((programming language)))(((interface,design)))Using cryptic numeric
-codes to represent node types is not a very JavaScript-like thing to
-do. Further on in this chapter, we'll see that other parts of the
-((DOM)) interface also feel cumbersome and alien. The reason for this
-is that the DOM wasn't designed for just JavaScript, but rather tries
-to define a language-neutral ((interface)) that can be used in other
-systems as well—not just HTML, but also ((XML)), which is a generic
-((data format)) with an HTML-like syntax.
-
-(((consistency)))(((integration)))This is unfortunate. Standards are
-often useful. But in this case, the advantage (cross-language
-consistency) isn't all that compelling. Having an interface that is
-properly integrated with the language you are using will save you more
-time than having a familiar interface across languages.
-
-(((array-like object)))(((NodeList type)))As an example of such poor
-integration, consider the `childNodes` property that element nodes in
-the DOM have. This property holds an array-like object, with a
-`length` property and properties labeled by numbers to access the
-child nodes. But it is an instance of the `NodeList` type, not a real
-array, so it does not have methods like `slice` and `forEach`.
-
-(((interface,design)))(((DOM,construction)))(((side effect)))Then
-there are issues that are simply poor design. For example, there is no
-way to create a new node and immediately add children or attributes to
-it. Instead, you have to first create it, then add the children one by
-one, and set the attributes one by one, using side effects. Code that
-interacts heavily with the DOM tends to get very long, repetitive, and
-ugly.
-
-(((library)))But none of these flaws are fatal, since JavaScript
-allows us to create our own ((abstraction))s. It is easy to write some
-((helper function))s that allow you to express the operations you are
-performing in a clearer and shorter way. In fact, many libraries
-intended for browser programming come with such tools.
-
-== Moving through the tree ==
-
-(((pointer)))DOM nodes contain a wealth of ((link))s to other nearby
-nodes. The following diagram tries to illustrate these.
-
-image::img/html-links.svg[alt="Links between DOM nodes",width="6cm"]
-
-(((child node)))(((parentNode property)))(((childNodes
-property)))Although the diagram only shows one link of each type,
-every node has a `parentNode` property that points to its containing
-node. Likewise, every element node (node type 1) has a `childNodes`
-property that points to an ((array-like object)) holding its children.
-
-(((firstChild property)))(((lastChild property)))(((previousSibling
-property)))(((nextSibling property)))In theory, you could move
-anywhere in the tree using just these parent and child links. But
-JavaScript also gives you access to a number of additional convenience
-links. The `firstChild` and `lastChild` properties point to the first
-and last child element, or have the value `null` for nodes without
-children. Similarly, `previousSibling` and `nextSibling` point to
-adjacent nodes, nodes with the same parent that appear immediately
-before or after the node itself. For a first child, `previousSibling`
-will be null, and for a last child, `nextSibling` is null.
-
-(((talksAbout function)))(((recursion)))(((nesting,of objects)))When
-dealing with a nested data structure like this, recursive functions
-are often useful. The one below scans a document for ((text node))s
-containing a given string, and returns `true` when it has found one.
-
-[[talksAbout]]
-[sandbox="homepage"]
-[source,javascript]
-----
-function talksAbout(node, string) {
- if (node.nodeType == document.ELEMENT_NODE) {
- for (var i = 0; i < node.childNodes.length; i++) {
- if (talksAbout(node.childNodes[i], string))
- return true;
- }
- return false;
- } else if (node.nodeType == document.TEXT_NODE) {
- return node.nodeValue.indexOf(string) > -1;
- }
-}
-
-console.log(talksAbout(document.body, "book"));
-// → true
-----
-
-(((nodeValue property)))The `nodeValue` property of a text node refers
-to the string of text that it represents.
-
-== Finding elements ==
-
-(((DOM)))(((body property)))(((hard-coding)))Navigating these
-((link))s among parents, children, and siblings is often useful, as in
-the function above, which runs through the whole document. But if we
-want to find a specific node in the document, reaching it by starting
-at `document.body` and blindly following a hard-coded path of links is
-a bad idea. Doing so bakes assumptions into our program about the
-precise structure of the document—a structure we might want to change
-later. Another complicating factor is that text nodes are created even
-for the ((whitespace)) between nodes. The example document's body tag
-does not have just three children (`
` and two `
`’s), but
-actually has seven: those three, plus the spaces before, after, and
-between them).
-
-(((searching)))(((href attribute)))(((getElementsByTagName method)))So
-if we want to get the `href` attribute of the link in that document,
-we don't want to say something like “get the second child of the sixth
-child of the document body”. It'd be better if we could say “get the
-first link in the document”. And we can.
-
-[sandbox="homepage"]
-[source,javascript]
-----
-var link = document.body.getElementsByTagName("a")[0];
-console.log(link.href);
-----
-
-(((child node)))All element nodes have a `getElementsByTagName`
-method, which collects all elements with the given tag name that are
-descendants (direct or indirect children) of the given node, and
-returns them as an array-like object.
-
-(((id attribute)))(((getElementById method)))To find a specific
-_single_ node, you can give it an `id` attribute, and use
-`document.getElementById` instead.
-
-[source,text/html]
-----
-
My ostrich Gertrude:
-
-
-
-----
-
-(((getElementsByClassName method)))(((class attribute)))A third,
-similar method is `getElementsByClassName`, which, like
-`getElementsByTagName`, searches through the contents of an element
-node, and retrieves all elements that have the given string in their
-`class` attribute.
-
-== Changing the document ==
-
-(((side effect)))(((removeChild method)))(((appendChild
-method)))(((insertBefore method)))(((DOM,construction)))Almost
-everything about the ((DOM)) data structure can be changed. Element
-nodes have a number of methods that can be used to change their
-content. The `removeChild` method removes the given child node from
-the document. To add a child, we can use `appendChild`, which puts it
-at the end of the list of children, or `insertBefore`, which inserts
-the node given as first argument before the node given as second
-argument.
-
-[source,text/html]
-----
-
One
-
Two
-
Three
-
-
-----
-
-A node can only exist in the document in one place. Thus, inserting
-paragraph “Three” in front of paragraph “One” will first remove it
-from the end of the document, and then insert it at the front,
-resulting in “Three/One/Two”. All operations that insert a node
-somewhere will, as a ((side effect)), cause it to be removed from its
-current position (if it has one).
-
-(((insertBefore method)))(((replaceChild method)))The `replaceChild`
-method is used to replace a child node with another one. It takes as
-arguments two nodes: a new node, and the node to be replaced. The
-replaced node must be a child of the element the method is called on.
-Note that both `replaceChild` and `insertBefore` expect the _new_ node
-as their first argument.
-
-== Creating nodes ==
-
-(((alt attribute)))(((img (HTML tag))))In the following example, we
-want to write a script that replaces all ((image))s (`` tags) in
-the document with the text held in their `alt` attribute, which
-specifies an alternative textual representation of the image.
-
-(((createTextNode method)))This involves not only removing the images,
-but adding a new text node to replace them. For this, we use the
-`document.createTextNode` method.
-
-[source,text/html]
-----
-
The in the
- .
-
-
-
-
-----
-
-(((text node)))Given a string, `createTextNode` gives us a type 3 DOM
-node (a text node), which we can insert into the document to make it
-show up on the screen.
-
-(((live data structure)))(((getElementsByTagName
-method)))(((childNodes property)))The loop that goes over the images
-starts at the end of the list of nodes. This is necessary because the
-node list returned by a method like `getElementsByTagName` (or a
-property like `childNodes`) is __live__—that is, it is updated as the
-document changes. If we started from the front, removing the first
-image would cause the list to lose its first element, so that the
-second time the loop repeats, where `i` is one, it would stop, because
-the length of the collection is now also one.
-
-(((slice method)))If you want a _solid_ collection of nodes, as
-opposed to a live one, you can convert the collection to a real array
-by calling the array `slice` method on it.
-
-[source,javascript]
-----
-var arrayish = {0: "one", 1: "two", length: 2};
-var real = Array.prototype.slice.call(arrayish, 0);
-real.forEach(function(elt) { console.log(elt); });
-// → one
-// two
-----
-
-(((createElement method)))To create regular ((element)) nodes (type
-1), you can use the `document.createElement` method. This method takes
-a tag name and returns a new empty node of the given type.
-
-[[elt]]
-(((Popper+++,+++ Karl)))(((DOM,construction)))(((elt function)))The
-following example defines a utility `elt`, which creates an element
-node, and treats the rest of its arguments as children to that node.
-This function is then used to add a simple attribution to a quote.
-
-[source,text/html]
-----
-
- No book can ever be finished. While working on it we learn
- just enough to find it immature the moment we turn away
- from it.
-
-
-
-----
-
-ifdef::tex_target[]
-
-image::img/blockquote.png[alt="A blockquote with attribution",width="8cm"]
-
-endif::tex_target[]
-
-== Attributes ==
-
-(((href attribute)))Some element ((attribute))s, such as `href` for
-links, can be accessed through a ((property)) of the same name on the
-element's ((DOM)) object. This is the case for a limited set of
-commonly used standard attributes.
-
-(((data attribute)))(((getAttribute method)))(((setAttribute
-method)))But HTML allows you to set any attribute you want on nodes.
-This can be useful, as it allows you to store extra information in a
-document. If you make up your own attribute names, though, such
-attributes will not be present as a property on the element's node.
-Instead, you'll have to use the `getAttribute` and `setAttribute`
-methods to work with them.
-
-[source,text/html]
-----
-
The launch code is 00000000.
-
I have two feet.
-
-
-----
-
-I recommended prefixing the names of such made-up attributes with
-`data-`, to ensure that they do not conflict with any other
-attributes.
-
-(((programming language)))(((syntax highlighing example)))As a simple
-example, we'll write a “syntax highlighter” that looks for `
`
-tags (“pre-formatted”, used for code and similar plain text) with a
-`data-language` attribute, and crudely tries to highlight the
-((keyword))s for that language.
-
-// include_code
-
-[sandbox="highlight"]
-[source,javascript]
-----
-function highlightCode(node, keywords) {
- var text = node.textContent;
- node.textContent = ""; // Clear the node
-
- var match, pos = 0;
- while (match = keywords.exec(text)) {
- var before = text.slice(pos, match.index);
- node.appendChild(document.createTextNode(before));
- var strong = document.createElement("strong");
- strong.appendChild(document.createTextNode(match[0]));
- node.appendChild(strong);
- pos = keywords.lastIndex;
- }
- var after = text.slice(pos);
- node.appendChild(document.createTextNode(after));
-}
-----
-
-(((pre (HTML tag))))(((syntax highlighing example)))(((highlightCode
-function)))The function `highlightCode` takes a `
` node and a
-((regular expression)) (with the “global” option turned on) that
-matches the keywords of the programming language that the element
-contains.
-
-(((strong (HTML tag))))(((clearing)))(((textContent property)))The
-`textContent` property is used to get all the ((text)) in the node,
-and is then set to an empty string, which has the effect of emptying
-the node. We loop over all matches of the keyword expression,
-appending the text _between_ them as regular text nodes, and the text
-matched as text nodes wrapped in `` (bold) elements.
-
-(((data attribute)))(((getElementsByTagName method)))We can
-automatically highlight all programs on the page by looping over all
-the `
` elements that have a `data-language` attribute, and
-calling `highlightCode` on each one with the correct regular
-expression for the language.
-
-// include_code
-
-[sandbox="highlight"]
-[source,javascript]
-----
-var languages = {
- javascript: /\b(function|return|var)\b/g /* … etc */
-};
-
-function highlightAllCode() {
- var pres = document.body.getElementsByTagName("pre");
- for (var i = 0; i < pres.length; i++) {
- var pre = pres[i];
- var lang = pre.getAttribute("data-language");
- if (languages.hasOwnProperty(lang))
- highlightCode(pre, languages[lang]);
- }
-}
-----
-
-(((syntax highlighing example)))For example:
-
-[sandbox="highlight"]
-[source,text/html]
-----
-
Here it is, the identity function:
-
-function id(x) { return x; }
-
-
-
-----
-
-ifdef::tex_target[]
-
-image::img/highlighted.png[alt="A highlighted piece of code",width="4.8cm"]
-
-endif::tex_target[]
-
-(((getAttribute method)))(((setAttribute method)))(((className
-property)))(((class attribute)))There is one commonly used attribute,
-`class`, which is a ((reserved word)) in the JavaScript language. For
-historical reasons—some old JavaScript implementations could not
-handle property names that matched keywords or reserved words—the
-property used to access this attribute is called `className`. You can
-also access it under its real name, `"class"`, by using the
-`getAttribute` and `setAttribute` methods.
-
-== Layout ==
-
-(((layout)))(((block element)))(((inline element)))(((p (HTML
-tag))))(((h1 (HTML tag))))(((a (HTML tag))))(((strong (HTML tag))))You
-might have noticed that different types of elements are laid out
-differently. Some, such as paragraphs (`
-----
-
-ifdef::tex_target[]
-
-image::img/colored-links.png[alt="A normal and a green link",width="2.2cm"]
-
-endif::tex_target[]
-
-(((border (CSS))))(((color (CSS))))(((CSS)))(((colon character)))A
-style attribute may contain one or more _((declaration))s_, which are
-a property (such as `color`) followed by a colon and a value (such as
-`green`). When there are more than one declaration, they must be
-separated by ((semicolon))s. For example, “`color: red; border:
-none`”.
-
-(((display (CSS))))(((layout)))There are a lot aspects that can be
-influenced with styling. For example, the `display` property controls
-whether an element is displayed as a block or inline element.
-
-[source,text/html]
-----
-This text is displayed inline,
-as a block, and
-not at all.
-----
-
-ifdef::tex_target[]
-
-image::img/display.png[alt="Different display styles",width="4cm"]
-
-endif::tex_target[]
-
-(((hidden element)))The `block` tag will end up on its own line, since
-((block element))s are not displayed inline with the text around them.
-The last tag is not displayed at all—`display: none` prevents an
-element from showing up on the screen. This is a way to hide elements,
-and it is often preferable to removing them from the document
-entirely—it makes it easy to reveal them again at a later time.
-
-(((color (CSS))))(((style attribute)))JavaScript code can directly
-manipulate the style of an element through the node's `style`
-property. This property holds an object that has properties for all
-possible style properties. The values of these properties are strings,
-which we can write to in order to change a particular aspect of the
-element's style.
-
-[source,text/html]
-----
-
- Pretty text
-
-
-
-----
-
-(((camel case)))(((capitalization)))(((dash character)))(((font-family
-(CSS))))Some style property names contain dashes, like `font-family`.
-Because such property names are awkward to work with in JavaScript
-(you'd have to say `style["font-family"]`), the property names in the
-`style` object for such properties have their dashes removed, and the
-letter that follows them capitalized (`style.fontFamily`).
-
-== Cascading styles ==
-
-indexsee:[Cascading Style Sheets,CSS]
-
-(((rule (CSS))))(((style (HTML tag))))The styling system for HTML is called ((CSS)),
-for _Cascading Style Sheets_. A _((style sheet))_ is a set of
-rules for how to style elements in the document. It can be given
-inside a `
-
Now strong text is italic and grey.
-----
-
-(((rule (CSS))))(((font-weight (CSS))))(((overlay)))The _((cascading))_ in the name
-refers to the fact that multiple such rules get combined to
-produce the final style for an element. In the example above, the
-default styling for `` tags, which gives them `font-weight:
-bold`, is overlaid by the rule in the `
-
-
-----
-endif::html_target[]
-
-!!solution!!
-
-(((createElement method)))(((table example)))(((appendChild
-method)))Use `document.createElement` to create new element nodes,
-`document.createTextNode` to create text nodes, and the `appendChild`
-method to put nodes into other nodes.
-
-You should loop over the key names once to fill in the top row, and
-then again for each object in the array to construct the data
-rows.
-
-Don't forget to return the enclosing `
` element at the end of
-the function.
-
-!!solution!!
-
-=== Elements by tag name ===
-
-(((getElementsByTagName method)))(((recursion)))The
-`getElementsByTagName` method returns all child elements with a given
-tag name. Implement your own version of it, as a regular non-method
-function, which takes a node and a string (the tag name) as arguments,
-and returns an array containing all descendant element nodes with the
-given tag name.
-
-(((tagName property)))(((capitalization)))(((toLowerCase
-method)))(((toUpperCase method)))To find the tag name of an element,
-use its `tagName` property. But note that this will return the tag
-name in all upper case. Use the `toLowerCase` or `toUpperCase` string
-method to compensate for this.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,text/html]
-----
-
Heading with a span element.
-
A paragraph with one, two
- spans.
-
-
-----
-endif::html_target[]
-
-!!solution!!
-
-(((getElementsByTagName method)))(((recursion)))The solution is most
-easily expressed with a recursive function, similar to the
-link:13_dom.html#talksAbout[`talksAbout` function] defined earlier in
-this chapter.
-
-(((concatenation)))(((concat method)))(((closure)))You could call
-`byTagname` itself recursively, concatenating the resulting arrays to
-produce the output. For a more efficient approach, define an inner
-function which calls itself recursively, and which has access to an
-array variable defined in the outer function to which it can add the
-matching elements it finds. Don't forget to call the ((inner
-function)) once from the outer function.
-
-(((nodeType property)))(((ELEMENT_NODE code)))The recursive function
-must check the node type. Here we are only interested in node type 1
-(`document.ELEMENT_NODE`). For such nodes, we must loop over their
-children, and for each child, see if it matches the query, but also do
-a recursive call on it to inspect its own children.
-
-!!solution!!
-
-=== The cat's hat ===
-
-(((cat's hat (exercise))))Extend the cat ((animation)) defined
-link:13_dom.html#animation[earlier] so that both the cat and his hat
-(``) orbit, at opposite sides of the ellipse.
-
-Or make the hat circle around the cat. Or alter the animation in some
-other interesting way.
-
-(((absolute positioning)))(((top (CSS))))(((left (CSS))))(((position
-(CSS))))To make positioning multiple objects easier, it is probably a
-good idea to switch to absolute positioning. This means that `top` and
-`left` are counted relative to the top left of the document. To avoid
-using negative coordinates, you can simply add a fixed number of
-pixels to the position values.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,text/html]
-----
-
-
-
-
-----
-
-endif::html_target[]
diff --git a/14_dom.md b/14_dom.md
new file mode 100644
index 000000000..aa7380826
--- /dev/null
+++ b/14_dom.md
@@ -0,0 +1,840 @@
+# The Document Object Model
+
+{{quote {author: "Friedrich Nietzsche", title: "Beyond Good and Evil", chapter: true}
+
+Too bad! Same old story! Once you've finished building your house you notice you've accidentally learned something that you really should have known—before you started.
+
+quote}}
+
+{{figure {url: "img/chapter_picture_14.jpg", alt: "Illustration showing a tree with letters, pictures, and gears hanging on its branches", chapter: "framed"}}}
+
+{{index drawing, parsing}}
+
+When you open a web page, your browser retrieves the page's ((HTML)) text and parses it, much like our parser from [Chapter ?](language#parsing) parsed programs. The browser builds up a model of the document's ((structure)) and uses this model to draw the page on the screen.
+
+{{index "live data structure"}}
+
+This representation of the ((document)) is one of the toys that a JavaScript program has available in its ((sandbox)). It is a ((data structure)) that you can read or modify. It acts as a _live_ data structure: when it's modified, the page on the screen is updated to reflect the changes.
+
+## Document structure
+
+{{index [HTML, structure]}}
+
+You can imagine an HTML document as a nested set of ((box))es. Tags such as `` and `` enclose other ((tag))s, which in turn contain other tags or ((text)). Here's the example document from the [previous chapter](browser):
+
+```{lang: html, sandbox: "homepage"}
+
+
+
+ My home page
+
+
+
+
+
+```
+
+This page has the following structure:
+
+{{figure {url: "img/html-boxes.svg", alt: "Diagram showing an HTML document as a set of nested boxes. The outer box is labeled 'html' and contains two boxes labeled 'head' and 'body'. Inside those are further boxes, with some of the innermost boxes containing the document's text.", width: "7cm"}}}
+
+{{indexsee "Document Object Model", DOM}}
+
+The data structure the browser uses to represent the document follows this shape. For each box, there is an object, which we can interact with to find out things such as what HTML tag it represents and which boxes and text it contains. This representation is called the _Document Object Model_, or _((DOM))_ for short.
+
+{{index "documentElement property", "head property", "body property", "html (HTML tag)", "body (HTML tag)", "head (HTML tag)"}}
+
+The global binding `document` gives us access to these objects. Its `documentElement` property refers to the object representing the `` tag. Since every HTML document has a head and a body, it also has `head` and `body` properties pointing at those elements.
+
+## Trees
+
+{{index [nesting, "of objects"]}}
+
+Think back to the ((syntax tree))s from [Chapter ?](language#parsing) for a moment. Their structures are strikingly similar to the structure of a browser's document. Each _((node))_ may refer to other nodes, _children_, which in turn may have their own children. This shape is typical of nested structures, where elements can contain subelements that are similar to themselves.
+
+{{index "documentElement property", [DOM, tree]}}
+
+We call a data structure a _((tree))_ when it has a branching structure, no ((cycle))s (a node may not contain itself, directly or indirectly), and a single, well-defined _((root))_. In the case of the DOM, `document.documentElement` serves as the root.
+
+{{index sorting, ["data structure", "tree"], "syntax tree"}}
+
+Trees come up a lot in computer science. In addition to representing recursive structures such as HTML documents or programs, they are often used to maintain sorted ((set))s of data because elements can usually be found or inserted more efficiently in a tree than in a flat array.
+
+{{index "leaf node", "Egg language"}}
+
+A typical tree has different kinds of ((node))s. The syntax tree for [the Egg language](language) had identifiers, values, and application nodes. Application nodes may have children, whereas identifiers and values are _leaves_, or nodes without children.
+
+{{index "body property", [HTML, structure]}}
+
+The same goes for the DOM. Nodes for _((element))s_, which represent HTML tags, determine the structure of the document. These can have ((child node))s. An example of such a node is `document.body`. Some of these children can be ((leaf node))s, such as pieces of ((text)) or ((comment)) nodes.
+
+{{index "text node", element, "ELEMENT_NODE code", "COMMENT_NODE code", "TEXT_NODE code", "nodeType property"}}
+
+Each DOM node object has a `nodeType` property, which contains a code (number) that identifies the type of node. Elements have code 1, which is also defined as the constant property `Node.ELEMENT_NODE`. Text nodes, representing a section of text in the document, get code 3 (`Node.TEXT_NODE`). Comments have code 8 (`Node.COMMENT_NODE`).
+
+Another way to visualize our document ((tree)) is as follows:
+
+{{figure {url: "img/html-tree.svg", alt: "Diagram showing the HTML document as a tree, with arrows from parent nodes to child nodes", width: "8cm"}}}
+
+The leaves are text nodes, and the arrows indicate parent-child relationships between nodes.
+
+{{id standard}}
+
+## The standard
+
+{{index "programming language", [interface, design], [DOM, interface]}}
+
+Using cryptic numeric codes to represent node types is not a very JavaScript-like thing to do. Later in this chapter, we'll see that other parts of the DOM interface also feel cumbersome and alien. This is because the DOM interface wasn't designed for JavaScript alone. Rather, it tries to be a language-neutral interface that can be used in other systems as well—not just for HTML but also for ((XML)), which is a generic ((data format)) with an HTML-like syntax.
+
+{{index consistency, integration}}
+
+This is unfortunate. Standards are often useful. But in this case, the advantage (cross-language consistency) isn't all that compelling. Having an interface that is properly integrated with the language you're using will save you more time than having a familiar interface across languages.
+
+{{index "array-like object", "NodeList type"}}
+
+As an example of this poor integration, consider the `childNodes` property that element nodes in the DOM have. This property holds an array-like object with a `length` property and properties labeled by numbers to access the child nodes. But it is an instance of the `NodeList` type, not a real array, so it does not have methods such as `slice` and `map`.
+
+{{index [interface, design], [DOM, construction], "side effect"}}
+
+Then there are issues that are simply caused by poor design. For example, there is no way to create a new node and immediately add children or ((attribute))s to it. Instead, you have to first create it and then add the children and attributes one by one, using side effects. Code that interacts heavily with the DOM tends to get long, repetitive, and ugly.
+
+{{index library}}
+
+But these flaws aren't fatal. Since JavaScript allows us to create our own ((abstraction))s, it is possible to design improved ways to express the operations we are performing. Many libraries intended for browser programming come with such tools.
+
+## Moving through the tree
+
+{{index pointer}}
+
+DOM nodes contain a wealth of ((link))s to other nearby nodes. The following diagram illustrates these:
+
+{{figure {url: "img/html-links.svg", alt: "Diagram that shows the links between DOM nodes. The 'body' node is shown as a box, with a 'firstChild' arrow pointing at the 'h1' node at its start, a 'lastChild' arrow pointing at the last paragraph node, and 'childNodes' arrow pointing at an array of links to all its children. The middle paragraph has a 'previousSibling' arrow pointing at the node before it, a 'nextSibling' arrow to the node after it, and a 'parentNode' arrow pointing at the 'body' node.", width: "6cm"}}}
+
+{{index "child node", "parentNode property", "childNodes property"}}
+
+Although the diagram shows only one link of each type, every node has a `parentNode` property that points to the node it is part of, if any. Likewise, every element node (node type 1) has a `childNodes` property that points to an ((array-like object)) holding its children.
+
+{{index "firstChild property", "lastChild property", "previousSibling property", "nextSibling property"}}
+
+In theory, you could move anywhere in the tree using just these parent and child links. But JavaScript also gives you access to a number of additional convenience links. The `firstChild` and `lastChild` properties point to the first and last child elements or have the value `null` for nodes without children. Similarly, `previousSibling` and `nextSibling` point to adjacent nodes, which are nodes with the same parent that appear immediately before or after the node itself. For a first child, `previousSibling` will be null, and for a last child, `nextSibling` will be null.
+
+{{index "children property", "text node", element}}
+
+There's also the `children` property, which is like `childNodes` but contains only element (type 1) children, not other types of child nodes. This can be useful when you aren't interested in text nodes.
+
+{{index "talksAbout function", recursion, [nesting, "of objects"]}}
+
+When dealing with a nested data structure like this one, recursive functions are often useful. The following function scans a document for ((text node))s containing a given string and returns `true` when it has found one:
+
+{{id talksAbout}}
+
+```{sandbox: "homepage"}
+function talksAbout(node, string) {
+ if (node.nodeType == Node.ELEMENT_NODE) {
+ for (let child of node.childNodes) {
+ if (talksAbout(child, string)) {
+ return true;
+ }
+ }
+ return false;
+ } else if (node.nodeType == Node.TEXT_NODE) {
+ return node.nodeValue.indexOf(string) > -1;
+ }
+}
+
+console.log(talksAbout(document.body, "book"));
+// → true
+```
+
+{{index "nodeValue property"}}
+
+The `nodeValue` property of a text node holds the string of text that it represents.
+
+## Finding elements
+
+{{index [DOM, querying], "body property", "hard-coding", [whitespace, "in HTML"]}}
+
+Navigating these ((link))s among parents, children, and siblings is often useful. But if we want to find a specific node in the document, reaching it by starting at `document.body` and following a fixed path of properties is a bad idea. Doing so bakes assumptions into our program about the precise structure of the document—a structure you might want to change later. Another complicating factor is that text nodes are created even for the whitespace between nodes. The example document's `` tag has not just three children (`
` and two `
` elements), but seven: those three, plus the spaces before, after, and between them.
+
+{{index "search problem", "href attribute", "getElementsByTagName method"}}
+
+If we want to get the `href` attribute of the link in that document, we don't want to say something like "Get the second child of the sixth child of the document body". It'd be better if we could say "Get the first link in the document". And we can.
+
+```{sandbox: "homepage"}
+let link = document.body.getElementsByTagName("a")[0];
+console.log(link.href);
+```
+
+{{index "child node"}}
+
+All element nodes have a `getElementsByTagName` method, which collects all elements with the given tag name that are descendants (direct or indirect children) of that node and returns them as an ((array-like object)).
+
+{{index "id attribute", "getElementById method"}}
+
+To find a specific _single_ node, you can give it an `id` attribute and use `document.getElementById` instead.
+
+```{lang: html}
+
My ostrich Gertrude:
+
+
+
+```
+
+{{index "getElementsByClassName method", "class attribute"}}
+
+A third, similar method is `getElementsByClassName`, which, like `getElementsByTagName`, searches through the contents of an element node and retrieves all elements that have the given string in their `class` attribute.
+
+## Changing the document
+
+{{index "side effect", "removeChild method", "appendChild method", "insertBefore method", [DOM, construction], [DOM, modification]}}
+
+Almost everything about the DOM data structure can be changed. The shape of the document tree can be modified by changing parent-child relationships. Nodes have a `remove` method to remove them from their current parent node. To add a child node to an element node, we can use `appendChild`, which puts it at the end of the list of children, or `insertBefore`, which inserts the node given as the first argument before the node given as the second argument.
+
+```{lang: html}
+
One
+
Two
+
Three
+
+
+```
+
+A node can exist in the document in only one place. Thus, inserting paragraph _Three_ in front of paragraph _One_ will first remove it from the end of the document and then insert it at the front, resulting in _Three_/_One_/_Two_. All operations that insert a node somewhere will, as a ((side effect)), cause it to be removed from its current position (if it has one).
+
+{{index "insertBefore method", "replaceChild method"}}
+
+The `replaceChild` method is used to replace a child node with another one. It takes as arguments two nodes: a new node and the node to be replaced. The replaced node must be a child of the element the method is called on. Note that both `replaceChild` and `insertBefore` expect the _new_ node as their first argument.
+
+## Creating nodes
+
+{{index "alt attribute", "img (HTML tag)", "createTextNode method"}}
+
+Say we want to write a script that replaces all ((image))s (`` tags) in the document with the text held in their `alt` attributes, which specifies an alternative textual representation of the image. This involves not only removing the images but also adding a new text node to replace them.
+
+```{lang: html}
+
The in the
+ .
+
+
+
+
+```
+
+{{index "text node"}}
+
+Given a string, `createTextNode` gives us a text node that we can insert into the document to make it show up on the screen.
+
+{{index "live data structure", "getElementsByTagName method", "childNodes property"}}
+
+The loop that goes over the images starts at the end of the list. This is necessary because the node list returned by a method like `getElementsByTagName` (or a property like `childNodes`) is _live_. That is, it is updated as the document changes. If we started from the front, removing the first image would cause the list to lose its first element so that the second time the loop repeats, where `i` is 1, it would stop because the length of the collection is now also 1.
+
+{{index "slice method"}}
+
+If you want a _solid_ collection of nodes, as opposed to a live one, you can convert the collection to a real array by calling `Array.from`.
+
+```
+let arrayish = {0: "one", 1: "two", length: 2};
+let array = Array.from(arrayish);
+console.log(array.map(s => s.toUpperCase()));
+// → ["ONE", "TWO"]
+```
+
+{{index "createElement method"}}
+
+To create ((element)) nodes, you can use the `document.createElement` method. This method takes a tag name and returns a new empty node of the given type.
+
+{{index "Popper, Karl", [DOM, construction], "elt function"}}
+
+{{id elt}}
+
+The following example defines a utility `elt`, which creates an element node and treats the rest of its arguments as children to that node. This function is then used to add an attribution to a quote.
+
+```{lang: html}
+
+ No book can ever be finished. While working on it we learn
+ just enough to find it immature the moment we turn away
+ from it.
+
+
+
+```
+
+{{if book
+
+This is what the resulting document looks like:
+
+{{figure {url: "img/blockquote.png", alt: "Rendered picture of the blockquote with attribution", width: "8cm"}}}
+
+if}}
+
+## Attributes
+
+{{index "href attribute", [DOM, attributes]}}
+
+Some element ((attribute))s, such as `href` for links, can be accessed through a property of the same name on the element's ((DOM)) object. This is the case for most commonly used standard attributes.
+
+{{index "data attribute", "getAttribute method", "setAttribute method", attribute}}
+
+HTML allows you to set any attribute you want on nodes. This can be useful because it allows you to store extra information in a document. To read or change custom attributes, which aren't available as regular object properties, you have to use the `getAttribute` and `setAttribute` methods.
+
+```{lang: html}
+
The launch code is 00000000.
+
I have two feet.
+
+
+```
+
+It is recommended to prefix the names of such made-up attributes with `data-` to ensure they do not conflict with any other attributes.
+
+{{index "getAttribute method", "setAttribute method", "className property", "class attribute"}}
+
+There is a commonly used attribute, `class`, which is a ((keyword)) in the JavaScript language. For historical reasons—some old JavaScript implementations could not handle property names that matched keywords—the property used to access this attribute is called `className`. You can also access it under its real name, `"class"`, with the `getAttribute` and `setAttribute` methods.
+
+## Layout
+
+{{index layout, "block element", "inline element", "p (HTML tag)", "h1 (HTML tag)", "a (HTML tag)", "strong (HTML tag)"}}
+
+You may have noticed that different types of elements are laid out differently. Some, such as paragraphs (`
+```
+
+{{if book
+
+The second link will be green instead of the default link color:
+
+{{figure {url: "img/colored-links.png", alt: "Rendered picture of a normal blue link and a styled green link", width: "2.2cm"}}}
+
+if}}
+
+{{index "border (CSS)", "color (CSS)", CSS, "colon character"}}
+
+A style attribute may contain one or more _((declaration))s_, which are a property (such as `color`) followed by a colon and a value (such as `green`). When there is more than one declaration, they must be separated by ((semicolon))s, as in `"color: red; border: none"`.
+
+{{index "display (CSS)", layout}}
+
+A lot of aspects of the document can be influenced by styling. For example, the `display` property controls whether an element is displayed as a block or an inline element.
+
+```{lang: html}
+This text is displayed inline,
+as a block, and
+not at all.
+```
+
+{{index "hidden element"}}
+
+The `block` tag will end up on its own line, since ((block element))s are not displayed inline with the text around them. The last tag is not displayed at all—`display: none` prevents an element from showing up on the screen. This is a way to hide elements. It is often preferable to removing them from the document entirely because it makes it easy to reveal them again later.
+
+{{if book
+
+{{figure {url: "img/display.png", alt: "Different display styles", width: "4cm"}}}
+
+if}}
+
+{{index "color (CSS)", "style attribute"}}
+
+JavaScript code can directly manipulate the style of an element through the element's `style` property. This property holds an object that has properties for all possible style properties. The values of these properties are strings, which we can write to in order to change a particular aspect of the element's style.
+
+```{lang: html}
+
+ Nice text
+
+
+
+```
+
+{{index "camel case", capitalization, "hyphen character", "font-family (CSS)"}}
+
+Some style property names contain hyphens, such as `font-family`. Because such property names are awkward to work with in JavaScript (you'd have to say `style["font-family"]`), the property names in the `style` object for such properties have their hyphens removed and the letters after them capitalized (`style.fontFamily`).
+
+## Cascading styles
+
+{{index "rule (CSS)", "style (HTML tag)"}}
+
+{{indexsee "Cascading Style Sheets", CSS}}
+{{indexsee "style sheet", CSS}}
+
+The styling system for HTML is called _((CSS))_, for _Cascading Style Sheets_. A _style sheet_ is a set of rules for how to style elements in a document. It can be given inside a `
+
Now strong text is italic and gray.
+```
+
+{{index "rule (CSS)", "font-weight (CSS)", overlay}}
+
+The _((cascading))_ in the name refers to the fact that multiple such rules are combined to produce the final style for an element. In the example, the default styling for `` tags, which gives them `font-weight: bold`, is overlaid by the rule in the `
+
+
+
+
+```
+
+if}}
+
+{{hint
+
+`Math.cos` and `Math.sin` measure angles in radians, where a full circle is 2π. For a given angle, you can get the opposite angle by adding half of this, which is `Math.PI`. This can be useful for putting the hat on the opposite side of the orbit.
+
+hint}}
diff --git a/14_event.txt b/14_event.txt
deleted file mode 100644
index eb45c9f8d..000000000
--- a/14_event.txt
+++ /dev/null
@@ -1,1113 +0,0 @@
-:chap_num: 14
-:prev_link: 13_dom
-:next_link: 15_game
-
-= Handling Events =
-
-[chapterquote="true"]
-[quote,Marcus Aurelius,Meditations]
-____
-You have power over your mind—not
-outside events. Realize this, and you will find strength.
-____
-
-(((stoicism)))(((Marcus Aurelius)))(((input)))(((timeline)))(((control
-flow)))Some of the things that a program works with, such as user
-input, happen at unpredictable times, in an unpredictable order. This
-requires a different approach to control than the one we have used so
-far.
-
-== Event handlers ==
-
-(((polling)))(((button)))(((real-time)))Imagine an interface where the
-only way to find out whether a button is pressed is to read the
-current state of that button. In order to be able to react to button
-presses, you would have to constantly read the button's state, so that
-you'd catch it before it was released again. It would be dangerous to
-perform other computations that took a while, since you might miss a
-button press.
-
-That is how things were done on _very_ primitive machines. A step up
-would be for the hardware or the ((operating system)) to notice the
-button press, and put it in a ((queue)) somewhere. Our program can
-then periodically check whether something has appeared in this queue,
-and react to what it finds there.
-
-(((responsiveness)))(((user experience)))Of course, it has to remember
-to look at the queue, and to do it often, because any time elapsed
-between the button being pressed and our program getting around to
-seeing if a new event came in will cause the software to feel
-unresponsive. This approach is called _((polling))_. Most programmers
-avoid it whenever possible.
-
-(((callback function)))(((event handling)))A better mechanism is for
-the underlying system to immediately give our code a chance to react
-to events as they occur. Browsers do this, by allowing us to register
-functions as _handlers_ for specific events.
-
-[source,text/html]
-----
-
Click on this document to activate the handler.
-
-----
-
-(((click event)))(((addEventListener method)))The `addEventListener`
-function registers its second argument to be called whenever the event
-described by its first argument occurs.
-
-== Events and DOM nodes ==
-
-(((addEventListener method)))(((event handling)))Each ((browser))
-event handler is registered in a context. When you call
-`addEventListener` as above, you are calling it as a method on the
-whole ((window)), because in the browser the ((global scope)) is
-equivalent to the `window` object. Every ((DOM)) element has an own
-`addEventListener` method, which allows you to listen specifically on
-that element.
-
-[source,text/html]
-----
-
-
No handler here.
-
-----
-
-(((click event)))(((button (HTML tag))))The example attaches a handler
-to the button node. Thus, clicks on the button cause that handler to
-run, whereas clicks on the rest of the document do not.
-
-(((onclick attribute)))(((encapsulation)))An `onclick` attribute put
-directly on a node has a similar effect. But a node only has one
-`onclick` attribute, so you can only register one handler per node
-that way. The `addEventListener` method allows any number of handlers
-to be added, so that you can't accidentally replace an already
-registered handler.
-
-(((removeEventListener method)))The `removeEventListener` method,
-called with the same type of arguments as `addEventListener` removes a
-handler again.
-
-[source,text/html]
-----
-
-
-----
-
-(((function,as value)))In order to be able to unregister it, we had to
-give our handler function a name (`once`), so that we can pass the
-exact same value that we passed to `addEventListener` to
-`removeEventListener`.
-
-== Event objects ==
-
-(((which property)))(((event handling)))(((event object)))Though we
-have ignored it in the examples above, event handler functions are
-passed an argument: the event object. This object gives us additional
-information about the event. For example, if we want to know _which_
-((mouse button)) was pressed, we can look at this object's `which`
-property.
-
-[source,text/html]
-----
-
-
-----
-
-(((event type)))(((type property)))The information stored in an event
-object differs per type of event. We'll discuss various types further
-on in this chapter. The object's `type` property always holds a string
-identifying the event (for example `"click"` or `"mousedown"`).
-
-== Propagation ==
-
-indexsee:[bubbling,event propagation]
-indexsee:[propagation,event propagation]
-
-(((event propagation)))(((parent node)))Event handlers registered on
-nodes with children will also receive some events that happen in the
-children. If a button inside a paragraph is clicked, event handlers on
-the paragraph will also receive the click event.
-
-(((event handling)))But if both the paragraph and the button have a
-handler, the more specific handler—the one on the button—gets to go
-first. The event _propagates_ outwards, from the node where it
-happened, to that node's parent node, and on to the root of the
-document. Finally, after all handlers registered on a specific node
-have had their turn, handlers registered on the whole ((window)) get a
-chance to respond to the event.
-
-(((stopPropagation method)))(((click event)))At any point, an event
-handler can call the `stopPropagation` method on the event object to
-prevent handlers “further up” from receiving the event. This can be
-useful when, for example, you have a button inside another clickable
-element, and you don't want clicks on the button to activate the outer
-element's click behavior.
-
-(((mousedown event)))The example below registers mousedown handler on
-both a button and the paragraph around it. When clicked with the right
-mouse button, the handler for the button calls `stopPropagation`,
-which will prevent the handler on the paragraph from running. When the
-button is clicked with another ((mouse button)), both handlers will
-run.
-
-[source,text/html]
-----
-
A paragraph with a .
-
-----
-
-(((event propagation)))(((target property)))Event objects have a
-`target` property that refers to the node where they originate. You
-can also use this to ensure that you are not accidentally handling
-something that propagated up from a node you do not want to handle. It
-is also possible to use the `target` property to cast a wide net for a
-specific type of event—for example, if you have a node containing a
-long list of buttons and you want to handle clicks on these buttons,
-it may be more convenient to register a single click handler on the
-outer node, and have it use the `target` property to figure out
-whether a button was clicked, than to register individual handlers on
-all of the buttons.
-
-[source,text/html]
-----
-
-
-
-
-----
-
-== Default actions ==
-
-(((scrolling)))(((default behavior)))(((event handling)))Many events
-have a default action associated with them by the browser. If you
-click a ((link)), you will be taken to the link's target. If you press
-the down arrow, the browser will scroll the page down. If you
-right-click, you get a context menu. And so on.
-
-(((preventDefault method)))For most types of events, the JavaScript
-event handlers are called _before_ the default behavior is performed.
-When the handler has taken care of handling the event, and does not
-want the normal behavior to happen in addition, it can call the
-`preventDefault` method on the event object.
-
-(((expectation)))This can be used to implement your own ((keyboard))
-shortcuts or ((context menu)). It can also be used to obnoxiously
-interfere with the behavior that users expect. For example, here is a
-link that can not be followed:
-
-[source,text/html]
-----
-MDN
-
-----
-
-Try not to do such things unless you have a really good reason to. For
-people using your page, it can be very unpleasant when expected
-behavior is broken.
-
-Depending on the browser, some events can not be intercepted. On
-Chrome, for example, ((keyboard)) shortcuts to close the current tab
-(Control-W or Command-W) can not be handled by JavaScript.
-
-== Key events ==
-
-(((keyboard)))(((keydown event)))(((keyup event)))(((event
-handling)))When a key on the keyboard is pressed down, your browser
-fires a `"keydown"` event. When it is released again, a `"keyup"`
-event is fired.
-
-[source,text/html]
-[focus="yes"]
-----
-
This page turns violet when you hold the V key.
-
-----
-
-(((repeating key)))Despite its name, `"keydown"` is not only fired
-when the key is physically pushed down. When a key is pressed and
-held, the event is fired again every time the key _repeats_.
-Sometimes, for example when you want to increase the acceleration of
-your ((game)) character when an arrow key is pressed, and decrease it
-again when the key is released, you have to be careful not to increase
-it again every time the key repeats, or you'd end up with
-unintentionally huge values.
-
-(((keyCode property)))(((key code)))The example above looked at the
-`keyCode` property of the event object. This is the way we can
-identify which key is being pressed or released. Unfortunately, it
-holds a number, and translating that number to an actual key is not
-always obvious.
-
-(((event object)))(((charCodeAt method)))For letter and number keys,
-the associated key code will be the ((Unicode)) character code
-associated with the (upper case) letter printed on the key. The
-`charCodeAt` method on ((string))s gives us a way to find this code:
-
-[source,javascript]
-----
-console.log("Violet".charCodeAt(0));
-// → 86
-console.log("1".charCodeAt(0));
-// → 49
-----
-
-Other keys have less predictable ((key code))s. The best way to find
-the codes you need is usually by ((experiment))—register a key event
-handler that logs the key codes it gets, and press the key you are
-interested in.
-
-(((modifier key)))(((shift key)))(((control key)))(((alt key)))(((meta
-key)))(((command key)))(((ctrlKey property)))(((shiftKey
-property)))(((altKey property)))(((metaKey property)))Modifier keys
-like shift, control, alt, and meta (“Command” on Mac) generate key
-events just like normal keys. But when looking for key combinations,
-you can also also find out whether these keys are held down by looking
-at the `shiftKey`, `ctrlKey`, `altKey`, and `metaKey` properties of
-keyboard (and mouse) events.
-
-[source,text/html]
-[focus="yes"]
-----
-
Press Control-Space to continue.
-
-----
-
-(((typing)))(((fromCharCode function)))(((charCode
-property)))(((keydown event)))(((keyup event)))(((keypress event)))The
-`"keydown"` and `"keyup"` events give you information about the
-physical key that is being hit. When you are interested in ((text))
-that is being typed, deriving that text from key codes is awkward.
-Instead, there exists another event, `"keypress"`, for this purpose.
-It is fired right after `"keydown"` (and repeated along with
-`"keydown"` when the key is held), but only for keys that produce
-character input. The `charCode` property in the event object contains
-a code that can be interpreted as a ((Unicode)) character code. The
-`String.fromCharCode` function can be used to turn this code into an
-actual (single-character) ((string)).
-
-[source,text/html]
-[focus="yes"]
-----
-
Focus this page and type something.
-
-----
-
-(((button (HTML tag))))(((tabindex attribute)))The ((DOM)) node where
-key events originate depends on the element that is currently
-((focus))ed. Normal nodes can not be focused (unless you give them a
-`tabindex` attribute), but things like ((link))s, buttons, and form
-fields can. We'll come back to form ((field))s in
-link:18_forms.html#forms[Chapter 18]. When nothing in particular is
-focused, `document.body` acts as the target node of key events.
-
-== Mouse clicks ==
-
-(((mousedown event)))(((mouseup event)))(((mouse cursor)))Clicking a
-((mouse button)) also causes a number of events to be fired. The
-`"mousedown"` and `"mouseup"` events are similar to `"keydown"` and
-`"keyup"`, fired when the button is pressed and released. These will
-happen on the DOM nodes that are immediately below the mouse pointer
-when the event occurs.
-
-(((click event)))After the `"mouseup"` event, a `"click"` event is
-fired on the most specific node that contained both the press and the
-release of the button. For example, if I press down the mouse button
-on one paragraph, and then move the pointer to another paragraph and
-release the button, the `"click"` event will happen on the element
-that contains both those paragraphs.
-
-(((dblclick event)))(((double click)))If two clicks quickly follow
-each other, a `"dblclick"` (double click) event is also fired, after
-the second click event.
-
-(((pixel)))(((pageX property)))(((pageY property)))(((event
-object)))To get precise information about the place where a mouse
-event happened, you can look at its `pageX` and `pageY` properties,
-which contain the event's ((coordinates)) (in pixels) relative to the
-top left corner of the document.
-
-[[mouse_drawing]]
-(((border-radius (CSS))))(((absolute positioning)))(((drawing program
-example)))The following implements a primitive drawing program. Every
-time you click on the document, it adds a dot under your mouse
-pointer. See link:19_paint.html#paint[Chapter 19] for a less primitive
-drawing program.
-
-[source,text/html]
-----
-
-
-----
-
-(((clientX property)))(((clientY property)))(((getBoundingClientRect
-method)))(((event object)))The `clientX` and `clientY` properties are
-similar to `pageX` and `pageY`, but relative to the part of the
-document that is currently scrolled into view. These can be useful
-when comparing mouse coordinates with the ((coordinates)) returned by
-`getBoundingClientRect`, which also returns ((viewport))-relative
-coordinates.
-
-== Mouse motion ==
-
-(((mousemove event)))Every time the mouse pointer moves, a
-`"mousemove"` event fires. This event can be used to track the
-position of the mouse. A common situation in which this is useful is
-when implementing some form of mouse-((dragging)) functionality.
-
-(((draggable bar example)))As an example, the program below displays a
-bar, and sets up event handlers so that dragging to the left or right
-on this bar makes it narrower or wider.
-
-[source,text/html]
-----
-
Drag the bar to change its width:
-
-
-
-----
-
-ifdef::tex_target[]
-
-image::img/drag-bar.png[alt="A draggable bar",width="5.3cm"]
-
-endif::tex_target[]
-
-(((mouseup event)))(((mousemove event)))Note that the `"mousemove"`
-handler is registered on the whole ((window)). Even if the mouse goes
-outside of the bar during resizing, we still want to update its size
-and stop dragging when the mouse is released.
-
-(((mouseover event)))(((mouseout event)))Whenever the mouse pointer
-enters or leaves a node, the `"mouseover"` or `"mouseout"` event is
-fired on this node. This can be used, among other things, to create
-((hover effect))s, showing or styling something when the mouse is over
-a given element.
-
-(((event propagation)))Unfortunately, that is not as simple as it
-would initially seem. When the mouse moves from a node onto one of its
-children, `"mouseout"` is also fired on the parent node. To make
-things worse, these events propagate just like other events, and thus
-you will also receive `"mouseout"` events when the mouse leaves one of
-the ((child node))s of the node on which the handler is registered.
-
-(((isInside function)))(((relatedTarget property)))(((target
-property)))To work around this problem, we can use the `relatedTarget`
-property of the event objects created for these events. It tells us,
-in the case of `"mouseover"`, what element the pointer was over
-before, and in the case of `"mouseout"`, what element it is going to.
-We only want to change our hover effect when the `relatedTarget` is
-outside of our target node. When that is the case, it means that this
-event actually represents a _crossing over_ from outside to inside the
-node (or the other way around).
-
-[source,text/html]
-----
-
Hover over this paragraph.
-
-----
-
-The `isInside` function follows the given node's parent links until it
-either reaches the top of the document (when `node` becomes null), or
-finds the parent we are looking for.
-
-I should add that a ((hover effect)) like this can be much more easily
-achieved using the ((CSS)) _((pseudo-selector))_ `:hover`, as the next
-example shows. But when your hover effect involves doing something
-more complicated than changing a style on the target node, the trick
-with `"mouseover"` and `"mouseout"` events must be used.
-
-[source,text/html]
-----
-
-
Hover over this paragraph.
-----
-
-== Scroll events ==
-
-(((scrolling)))(((scroll event)))(((event handling)))Whenever an
-element is scrolled, a `"scroll"` event is fired on it. This has
-various uses, such as knowing what the user is currently looking at
-(for disabling off-screen ((animation))s or sending ((spy)) reports to
-your evil headquarters) or showing some indication of progress (by
-highlighting a part of a table of contents, or showing a page number).
-
-The example below draws a ((progress bar)) in the top right corner of
-the document, and updates it to fill up as you scroll down.
-
-[source,text/html]
-----
-
-
-
Scroll me...
-
-----
-
-(((unit (CSS))))(((scrolling)))(((position (CSS))))(((fixed positioning)))(((absolute
-positioning)))(((percent)))Giving an element a `position` of `fixed`
-acts much like an `absolute` position, but also prevents it from
-scrolling along with the rest of the document. This is used to make
-our progress bar stay in its corner. Inside of it is another element,
-which is resized to indicate the current progress. We use `%`, rather
-than `px` as a unit when setting the width, so that the element is
-sized relative to the whole bar.
-
-(((innerHeight property)))(((innerWidth property)))(((pageYOffset
-property)))The global `innerHeight` variable gives us the height of
-the window, which we have to subtract from the total scrollable
-height, because you can't scroll down anymore when the bottom of the
-screen has reached the bottom of the document. (There is, of course,
-also `innerWidth`.) By dividing `pageYOffset` (the current scroll
-position) by the maximum scroll position, and multiplying that by a
-hundred, we get the percentage that we want to display.
-
-(((preventDefault method)))Calling `preventDefault` on a scroll event
-does not prevent the scrolling from happening. In fact, the event
-handler is called only _after_ the scrolling took place.
-
-== Focus events ==
-
-(((event handling)))(((focus event)))(((blur event)))When an element
-is ((focus))ed, the browser fires a `"focus"` event on it. When it
-loses focus, a `"blur"` event fires.
-
-(((event propagation)))Unlike to the events discussed earlier, these
-two events do not propagate. A handler on a parent element is not
-notified when a child element is focused or unfocused.
-
-(((input (HTML tag))))(((help text example)))The example below
-displays a help text for the ((text field)) that is currently focused.
-
-[source,text/html]
-----
-
Name:
-
Age:
-
-
-
-----
-
-ifdef::tex_target[]
-
-image::img/help-field.png[alt="Providing help when a field is focused",width="4.4cm"]
-
-endif::tex_target[]
-
-(((focus event)))(((blur event)))The ((window)) object will receive
-`"focus"` and `"blur"` events when the user moves from or to the tab
-or window in which the document is shown.
-
-== Load event ==
-
-(((script (HTML tag))))(((initialization)))(((load event)))When a page
-finishes loading, the `"load"` event is fired on the window and the
-document body. This is often used to schedule actions that initialize
-something, but require the whole ((document)) to have been built up.
-Remember that the content of `
-----
-
-(((clearTimeout function)))Sometimes you need to cancel a function you
-have scheduled. This is done by storing the value returned by
-`setTimeout`, and calling `clearTimeout` on it.
-
-[source,javascript]
-----
-var bombTimer = setTimeout(function() {
- console.log("BOOM!");
-}, 500);
-
-if (Math.random() < .5) { // 50% chance
- console.log("Defused.");
- clearTimeout(bombTimer);
-}
-----
-
-(((cancelAnimationFrame function)))(((requestAnimationFrame
-function)))The `cancelAnimationFrame` function works in the same way
-as ++clearTimeout++—calling it on a value returned by
-`requestAnimationFrame` will cancel that frame (assuming it hasn't
-already been called).
-
-(((setInterval function)))(((clearInterval
-function)))(((repetition)))A similar set of functions, `setInterval`
-and `clearInterval` are used to set timers that should repeat every X
-milliseconds.
-
-[source,javascript]
-----
-var ticks = 0;
-var clock = setInterval(function() {
- console.log("tick", ticks++);
- if (ticks == 10) {
- clearInterval(clock);
- console.log("stop.");
- }
-}, 200);
-----
-
-== Debouncing ==
-
-(((optimization)))(((mousemove event)))(((scroll
-event)))(((blocking)))Some types of events have the potential to fire
-rapidly, many times in a row. The `"mousemove"` and `"scroll"` events,
-for example. When handling such events, you must be careful not to do
-anything too time-consuming, or your handler will take up so much time
-that interaction with the document starts to feel slow and choppy.
-
-(((setTimeout function)))If you do need to do something non-trivial in
-such a handler, you can use `setTimeout` to make sure you are not
-doing it too often. This is usually called _((debouncing))_ the event.
-There are several slightly different approaches to this.
-
-(((textarea (HTML tag))))(((clearTimeout function)))(((keydown
-event)))In the first example, we want to do something when the user
-has typed something, but we don't want to do it immediately for every
-key event. When they are ((typing)) quickly, we just want to wait
-until a pause occurs. This is done by not immediately performing an
-action in the event handler, but setting a timeout instead. We also
-clear the previous timeout (if any), so that when events occur close
-together (closer than our timeout delay), the timeout from the
-previous event will be canceled.
-
-[source,text/html]
-----
-
-
-----
-
-(((sloppy programming)))Giving an undefined value to `clearTimeout` or
-calling it on a timeout that already fired has no effect. Thus, we
-don't have to be careful about when to call it, and simply do so for
-every event.
-
-(((mousemove event)))A slightly different pattern occurs when we want
-to space responses to an event at least a certain amount of ((time))
-apart, but do want to fire them _during_ a series of events, not just
-afterwards. For example, we want to respond to `"mousemove"` events by
-showing the current coordinates of the mouse, but only every 250
-milliseconds.
-
-[source,text/html]
-----
-
-----
-
-== Summary ==
-
-Event handlers make it possible to detect and react to events we have
-no direct control over. The `addEventListener` method is used to
-register such a handler.
-
-Each event has a name (`"keydown"`, `"focus"`, etc.) that identifies
-it. Most events are called on a specific DOM elements, and then
-_propagate_ to that element's ancestor elements, allowing handlers
-associated with those elements to handle them.
-
-When an event handler is called, it is passed an event object with
-additional information about the event. This object also has methods
-that allow us to stop further propagation (`stopPropagation`) and
-prevent the browser's default handling of the event
-(`preventDefault`).
-
-Pressing a key fires `"keydown"`, `"keypress"`, and `"keyup"` events.
-Pressing a mouse button fires `"mousedown"`, `"mouseup"`, and
-`"click"` events. Moving the mouse fires `"mousemove"` and possibly
-`"mouseenter"` and `"mouseout"` events.
-
-Scrolling can be detected with the `"scroll"` event, and focus changes
-with the `"focus"` and `"blur"` events. When the document finishes
-loading, a `"load"` event fires on the window.
-
-Only one piece of JavaScript program can run at a time. Thus, event
-handlers and other scheduled scripts have to wait until other scripts
-finish before they get their turn.
-
-== Exercises ==
-
-=== Censored keyboard ===
-
-(((Turkish)))(((Kurds)))(((censored keyboard (exercise))))Between 1928
-and 2013, Turkish law forbade the use of the letters “Q”, “W”, and “X”
-in official documents. This was part of a wider initiative to stifle
-Kurdish culture—those letters occur in the language used by Kurdish
-people, but not in Istanbul Turkish.
-
-(((typing)))(((input (HTML tag))))As an exercise in doing ridiculous
-things with technology, I'm asking you to program a ((text field)) (an
-`` tag) in such a way that these letters can not be
-typed into it.
-
-(((clipboard)))(Do not worry about copy-paste and other such
-loopholes.)
-
-ifdef::html_target[]
-
-// test: no
-
-[source,text/html]
-----
-
-
-----
-
-endif::html_target[]
-
-!!solution!!
-
-(((keypress event)))(((keydown event)))(((preventDefault
-method)))(((censored keyboard (exercise))))The solution to this exercise
-involves preventing the ((default behavior)) of key events. You can
-handle either `"keypress"` (the most obvious) or `"keydown"`. If
-either of them has `preventDefault` called on it, the letter will not
-appear.
-
-(((keyCode property)))(((charCode
-property)))(((capitalization)))Identifying the letter typed requires
-looking at the `keyCode` or `charCode` property, and comparing that
-with the codes for the letters we want to filter. In `"keydown"`, you
-do not have to worry about lower- and upper-case letters, since it
-only identifies the key pressed. If you decided to handle `"keypress"`
-instead, which identifies actual characters typed, you have to make
-sure you test both for both cases. One way to do that would be this:
-
-----
-/[qwx]/i.test(String.fromCharCode(event.charCode))
-----
-
-!!solution!!
-
-=== Mouse trail ===
-
-(((animation)))(((mouse trail (exercise))))In JavaScript's early days,
-which was the high time of ((gaudy homepages)) with lots of animated
-images, people came up with some truly inspiring ways to use the
-language.
-
-One of these was the “mouse trail”—a series of images following the
-mouse as you moved it across the page.
-
-(((absolute positioning)))(((background (CSS))))In this exercise, I
-want you to implement a mouse trail. Use absolutely positioned `
`
-elements with a fixed size and background color (refer back to the
-link:14_event.html#mouse_drawing[code] in the section on mouse click
-events for an example). Create twelve such elements, and when the
-mouse moves, display them in the wake of the mouse pointer, somehow.
-
-(((mousemove event)))There are various possible approaches here. You
-can make your solution as complex as you want. To start with, a simple
-solution is to keep a fixed number of trail elements, and cycle
-through them, moving the next one to the mouse's current position
-every time a `"mousemove"` event occurs.
-
-ifdef::html_target[]
-
-// test: no
-
-[source,text/html]
-----
-
-
-
-----
-
-endif::html_target[]
-
-!!solution!!
-
-(((mouse trail (exercise))))Creating the elements is best done in a
-loop. Append them to the document to make them show up. In order to be
-able to access them later, to change their position, store the trail
-elements in an array.
-
-(((mousemove event)))(((array,indexing)))(((remainder operator)))(((%
-operator)))Cycling through them can be done by keeping a ((counter
-variable)), and adding one to it every time the `"mousemove"` event
-fires. The remainder operator (`% 10`) can then be used to get a valid
-array index, to pick the element we want to position during a given
-event.
-
-(((simulation)))(((requestAnimationFrame function)))Another
-interesting effect can be gotten by modeling a simple ((physics))
-system. The `"mousemove"` event is then used only to update a pair of
-variables that track the mouse position. The animation is created by
-using `requestAnimationFrame` to simulate the trailing elements being
-attracted by the position of the mouse pointer. Every ((animation))
-step, their position is updated based on their relative position to
-the pointer (and optionally, a speed that is stored for each element).
-Figuring out a good way to do this is up to you.
-
-!!solution!!
-
-=== Tabs ===
-
-(((tabbed interface (exercise))))A tabbed interface is a commonly used
-pattern. It allows you to select from a number of panels by selecting
-little clips “sticking out” above an element.
-
-(((button (HTML tag))))(((display (CSS))))(((hidden element)))(((data
-attribute)))In this exercise we implement a crude tabbed interface.
-Write a function `asTabs` that, given a DOM node, creates a tabbed
-interface showing the children of that node. It should insert a list
-of `
in the
- 
.