List of (Advanced) JavaScript Questions

---

1. What's the output?

function sayHi() {
  console.log(name);
  console.log(age);
  var name = "Lydia";
  let age = 21;
}

sayHi();

• A: Lydia and undefined
• B: Lydia and ReferenceError
• C: ReferenceError and 21
• D: undefined and ReferenceError

---

2. What's the output?

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1);
}

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1);
}

• A: 0 1 2 and 0 1 2
• B: 0 1 2 and 3 3 3
• C: 3 3 3 and 0 1 2

---

3. What's the output?

const shape = {
  radius: 10,
  diameter() {
    return this.radius * 2;
  },
  perimeter: () => 2 * Math.PI * this.radius
};

console.log(shape.diameter());
console.log(shape.perimeter());

• A: 20 and 62.83185307179586
• B: 20 and NaN
• C: 20 and 63
• D: NaN and 63

---

4. What's the output?

+true;
!"Lydia";

• A: 1 and false
• B: false and NaN
• C: false and false

---

5. Which one is true?

const bird = {
  size: "small"
};

const mouse = {
  name: "Mickey",
  small: true
};

• A: mouse.bird.size is not valid
• B: mouse[bird.size] is not valid
• C: mouse[bird["size"]] is not valid
• D: All of them are valid

---

6. What's the output?

let c = { greeting: "Hey!" };
let d;

d = c;
c.greeting = "Hello";
console.log(d.greeting);

• A: Hello
• B: Hey!
• C: undefined
• D: ReferenceError
• E: TypeError

---

7. What's the output?

let a = 3;
let b = new Number(3);
let c = 3;

console.log(a == b);
console.log(a === b);
console.log(b === c);

• A: true false true
• B: false false true
• C: true false false
• D: false true true

---

8. What's the output?

class Chameleon {
  static colorChange(newColor) {
    this.newColor = newColor;
    return this.newColor;
  }

  constructor({ newColor = "green" } = {}) {
    this.newColor = newColor;
  }
}

const freddie = new Chameleon({ newColor: "purple" });
console.log(freddie.colorChange("orange"));

• A: orange
• B: purple
• C: green
• D: TypeError

---

9. What's the output?

let greeting;
greetign = {}; // Typo!
console.log(greetign);

• A: {}
• B: ReferenceError: greetign is not defined
• C: undefined

---

10. What happens when we do this?

function bark() {
  console.log("Woof!");
}

bark.animal = "dog";

• A: Nothing, this is totally fine!
• B: SyntaxError. You cannot add properties to a function this way.
• C: "Woof" gets logged.
• D: ReferenceError

---

11. What's the output?

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

const member = new Person("Lydia", "Hallie");
Person.getFullName = function() {
  return `${this.firstName} ${this.lastName}`;
};

console.log(member.getFullName());

• A: TypeError
• B: SyntaxError
• C: Lydia Hallie
• D: undefined undefined

---

12. What's the output?

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

const lydia = new Person("Lydia", "Hallie");
const sarah = Person("Sarah", "Smith");

console.log(lydia);
console.log(sarah);

• A: Person {firstName: "Lydia", lastName: "Hallie"} and undefined
• B: Person {firstName: "Lydia", lastName: "Hallie"} and Person {firstName: "Sarah", lastName: "Smith"}
• C: Person {firstName: "Lydia", lastName: "Hallie"} and {}
• D:Person {firstName: "Lydia", lastName: "Hallie"} and ReferenceError

---

13. What are the three phases of event propagation?

• A: Target > Capturing > Bubbling
• B: Bubbling > Target > Capturing
• C: Target > Bubbling > Capturing
• D: Capturing > Target > Bubbling

---

14. All object have prototypes.

• A: true
• B: false

---

15. What's the output?

function sum(a, b) {
  return a + b;
}

sum(1, "2");

• A: NaN
• B: TypeError
• C: "12"
• D: 3

---

16. What's the output?

let number = 0;
console.log(number++);
console.log(++number);
console.log(number);

• A: 1 1 2
• B: 1 2 2
• C: 0 2 2
• D: 0 1 2

---

17. What's the output?

function getPersonInfo(one, two, three) {
  console.log(one);
  console.log(two);
  console.log(three);
}

const person = "Lydia";
const age = 21;

getPersonInfo`${person} is ${age} years old`;

• A: "Lydia" 21 ["", " is ", " years old"]
• B: ["", " is ", " years old"] "Lydia" 21
• C: "Lydia" ["", " is ", " years old"] 21

---

18. What's the output?

function checkAge(data) {
  if (data === { age: 18 }) {
    console.log("You are an adult!");
  } else if (data == { age: 18 }) {
    console.log("You are still an adult.");
  } else {
    console.log(`Hmm.. You don't have an age I guess`);
  }
}

checkAge({ age: 18 });

• A: You are an adult!
• B: You are still an adult.
• C: Hmm.. You don't have an age I guess

---

19. What's the output?

function getAge(...args) {
  console.log(typeof args);
}

getAge(21);

• A: "number"
• B: "array"
• C: "object"
• D: "NaN"

---

20. What's the output?

function getAge() {
  "use strict";
  age = 21;
  console.log(age);
}

getAge();

• A: 21
• B: undefined
• C: ReferenceError
• D: TypeError

---

21. What's value of sum?

const sum = eval("10*10+5");

• A: 105
• B: "105"
• C: TypeError
• D: "10*10+5"

---

22. How long is cool_secret accessible?

sessionStorage.setItem("cool_secret", 123);

• A: Forever, the data doesn't get lost.
• B: When the user closes the tab.
• C: When the user closes the entire browser, not only the tab.
• D: When the user shuts off their computer.

---

23. What's the output?

var num = 8;
var num = 10;

console.log(num);

• A: 8
• B: 10
• C: SyntaxError
• D: ReferenceError

---

24. What's the output?

const obj = { 1: "a", 2: "b", 3: "c" };
const set = new Set([1, 2, 3, 4, 5]);

obj.hasOwnProperty("1");
obj.hasOwnProperty(1);
set.has("1");
set.has(1);

• A: false true false true
• B: false true true true
• C: true true false true
• D: true true true true

---

25. What's the output?

const obj = { a: "one", b: "two", a: "three" };
console.log(obj);

• A: { a: "one", b: "two" }
• B: { b: "two", a: "three" }
• C: { a: "three", b: "two" }
• D: SyntaxError

---

26. The JavaScript global execution context creates two things for you: the global object, and the "this" keyword.

• A: true
• B: false
• C: it depends

---

27. What's the output?

for (let i = 1; i < 5; i++) {
  if (i === 3) continue;
  console.log(i);
}

• A: 1 2
• B: 1 2 3
• C: 1 2 4
• D: 1 3 4

---

28. What's the output?

String.prototype.giveLydiaPizza = () => {
  return "Just give Lydia pizza already!";
};

const name = "Lydia";

name.giveLydiaPizza();

• A: "Just give Lydia pizza already!"
• B: TypeError: not a function
• C: SyntaxError
• D: undefined

---

29. What's the output?

const a = {};
const b = { key: "b" };
const c = { key: "c" };

a[b] = 123;
a[c] = 456;

console.log(a[b]);

• A: 123
• B: 456
• C: undefined
• D: ReferenceError

---

30. What's the output?

const foo = () => console.log("First");
const bar = () => setTimeout(() => console.log("Second"));
const baz = () => console.log("Third");

bar();
foo();
baz();

• A: First Second Third
• B: First Third Second
• C: Second First Third
• D: Second Third First

---

31. What is the event.target when clicking the button?

<div onclick="console.log('first div')">
  <div onclick="console.log('second div')">
    <button onclick="console.log('button')">
      Click!
    </button>
  </div>
</div>

• A: Outer div
• B: Inner div
• C: button
• D: An array of all nested elements.

---

32. When you click the paragraph, what's the logged output?

<div onclick="console.log('div')">
  <p onclick="console.log('p')">
    Click here!
  </p>
</div>

• A: p div
• B: div p
• C: p
• D: div

---

33. What's the output?

const person = { name: "Lydia" };

function sayHi(age) {
  console.log(`${this.name} is ${age}`);
}

sayHi.call(person, 21);
sayHi.bind(person, 21);

• A: undefined is 21 Lydia is 21
• B: function function
• C: Lydia is 21 Lydia is 21
• D: Lydia is 21 function

---

34. What's the output?

function sayHi() {
  return (() => 0)();
}

console.log(typeof sayHi());

• A: "object"
• B: "number"
• C: "function"
• D: "undefined"

---

35. Which of these values are falsy?

0;
new Number(0);
("");
(" ");
new Boolean(false);
undefined;

• A: 0, '', undefined
• B: 0, new Number(0), '', new Boolean(false), undefined
• C: 0, '', new Boolean(false), undefined
• D: All of them are falsy

---

36. What's the output?

console.log(typeof typeof 1);

• A: "number"
• B: "string"
• C: "object"
• D: "undefined"

---

37. What's the output?

const numbers = [1, 2, 3];
numbers[10] = 11;
console.log(numbers);

• A: [1, 2, 3, 7 x null, 11]
• B: [1, 2, 3, 11]
• C: [1, 2, 3, 7 x empty, 11]
• D: SyntaxError

---

38. What's the output?

(() => {
  let x, y;
  try {
    throw new Error();
  } catch (x) {
    (x = 1), (y = 2);
    console.log(x);
  }
  console.log(x);
  console.log(y);
})();

• A: 1 undefined 2
• B: undefined undefined undefined
• C: 1 1 2
• D: 1 undefined undefined

---

39. Everything in JavaScript is either a...

• A: primitive or object
• B: function or object
• C: trick question! only objects
• D: number or object

---

40. What's the output?

[[0, 1], [2, 3]].reduce(
  (acc, cur) => {
    return acc.concat(cur);
  },
  [1, 2]
);

• A: [0, 1, 2, 3, 1, 2]
• B: [6, 1, 2]
• C: [1, 2, 0, 1, 2, 3]
• D: [1, 2, 6]

---

41. What's the output?

!!null;
!!"";
!!1;

• A: false true false
• B: false false true
• C: false true true
• D: true true false

---

42. What does the setInterval method return in the browser?

setInterval(() => console.log("Hi"), 1000);

• A: a unique id
• B: the amount of milliseconds specified
• C: the passed function
• D: undefined

---

43. What does this return?

[..."Lydia"];

• A: ["L", "y", "d", "i", "a"]
• B: ["Lydia"]
• C: [[], "Lydia"]
• D: [["L", "y", "d", "i", "a"]]

---

44. What's the output?

function* generator(i) {
  yield i;
  yield i * 2;
}

const gen = generator(10);

console.log(gen.next().value);
console.log(gen.next().value);

• A: [0, 10], [10, 20]
• B: 20, 20
• C: 10, 20
• D: 0, 10 and 10, 20

---

45. What does this return?

const firstPromise = new Promise((res, rej) => {
  setTimeout(res, 500, "one");
});

const secondPromise = new Promise((res, rej) => {
  setTimeout(res, 100, "two");
});

Promise.race([firstPromise, secondPromise]).then(res => console.log(res));

• A: "one"
• B: "two"
• C: "two" "one"
• D: "one" "two"

---

46. What's the output?

let person = { name: "Lydia" };
const members = [person];
person = null;

console.log(members);

• A: null
• B: [null]
• C: [{}]
• D: [{ name: "Lydia" }]

---

47. What's the output?

const person = {
  name: "Lydia",
  age: 21
};

for (const item in person) {
  console.log(item);
}

• A: { name: "Lydia" }, { age: 21 }
• B: "name", "age"
• C: "Lydia", 21
• D: ["name", "Lydia"], ["age", 21]

---

48. What's the output?

console.log(3 + 4 + "5");

• A: "345"
• B: "75"
• C: 12
• D: "12"

---

49. What's the value of num?

const num = parseInt("7*6", 10);

• A: 42
• B: "42"
• C: 7
• D: NaN

---

50. What's the output`?

[1, 2, 3].map(num => {
  if (typeof num === "number") return;
  return num * 2;
});

• A: []
• B: [null, null, null]
• C: [undefined, undefined, undefined]
• D: [ 3 x empty ]

---

51. What's the output?

function getInfo(member, year) {
  member.name = "Lydia";
  year = "1998";
}

const person = { name: "Sarah" };
const birthYear = "1997";

getInfo(person, birthYear);

console.log(person, birthYear);

• A: { name: "Lydia" }, "1997"
• B: { name: "Sarah" }, "1998"
• C: { name: "Lydia" }, "1998"
• D: { name: "Sarah" }, "1997"

---

52. What's the output?

function greeting() {
  throw "Hello world!";
}

function sayHi() {
  try {
    const data = greeting();
    console.log("It worked!", data);
  } catch (e) {
    console.log("Oh no an error!", e);
  }
}

sayHi();

• A: "It worked! Hello world!"
• B: "Oh no an error: undefined
• C: SyntaxError: can only throw Error objects
• D: "Oh no an error: Hello world!

---

53. What's the output?

function Car() {
  this.make = "Lamborghini";
  return { make: "Maserati" };
}

const myCar = new Car();
console.log(myCar.make);

• A: "Lamborghini"
• B: "Maserati"
• C: ReferenceError
• D: TypeError

---

54. What's the output?

(() => {
  let x = (y = 10);
})();

console.log(typeof x);
console.log(typeof y);

• A: "undefined", "number"
• B: "number", "number"
• C: "object", "number"
• D: "number", "undefined"

---

55. What's the output?

class Dog {
  constructor(name) {
    this.name = name;
  }
}

Dog.prototype.bark = function() {
  console.log(`Woof I am ${this.name}`);
};

const pet = new Dog("Mara");

pet.bark();

delete Dog.prototype.bark;

pet.bark();

• A: "Woof I am Mara", TypeError
• B: "Woof I am Mara","Woof I am Mara"
• C: "Woof I am Mara", undefined
• D: TypeError, TypeError

---

56. What's the output?

const set = new Set([1, 1, 2, 3, 4]);

console.log(set);

• A: [1, 1, 2, 3, 4]
• B: [1, 2, 3, 4]
• C: {1, 1, 2, 3, 4}
• D: {1, 2, 3, 4}

---

57. What's the output?

// counter.js
let counter = 10;
export default counter;

// index.js
import myCounter from "./counter";

myCounter += 1;

console.log(myCounter);

• A: 10
• B: 11
• C: Error
• D: NaN

---

58. What's the output?

const name = "Lydia";
age = 21;

console.log(delete name);
console.log(delete age);

• A: false, true
• B: "Lydia", 21
• C: true, true
• D: undefined, undefined

---

59. What's the output?

const numbers = [1, 2, 3, 4, 5];
const [y] = numbers;

console.log(y);

• A: [[1, 2, 3, 4, 5]]
• B: [1, 2, 3, 4, 5]
• C: 1
• D: [1]

---

60. What's the output?

const user = { name: "Lydia", age: 21 };
const admin = { admin: true, ...user };

console.log(admin);

• A: { admin: true, user: { name: "Lydia", age: 21 } }
• B: { admin: true, name: "Lydia", age: 21 }
• C: { admin: true, user: ["Lydia", 21] }
• D: { admin: true }

---

61. What's the output?

const person = { name: "Lydia" };

Object.defineProperty(person, "age", { value: 21 });

console.log(person);
console.log(Object.keys(person));

• A: { name: "Lydia", age: 21 }, ["name", "age"]
• B: { name: "Lydia", age: 21 }, ["name"]
• C: { name: "Lydia"}, ["name", "age"]
• D: { name: "Lydia"}, ["age"]

---

62. What's the output?

const settings = {
  username: "lydiahallie",
  level: 19,
  health: 90
};

const data = JSON.stringify(settings, ["level", "health"]);
console.log(data);

• A: "{"level":19, "health":90}"
• B: "{"username": "lydiahallie"}"
• C: "["level", "health"]"
• D: "{"username": "lydiahallie", "level":19, "health":90}"

---

63. What's the output?

let num = 10;

const increaseNumber = () => num++;
const increasePassedNumber = number => number++;

const num1 = increaseNumber();
const num2 = increasePassedNumber(num1);

console.log(num1);
console.log(num2);

• A: 10, 10
• B: 10, 11
• C: 11, 11
• D: 11, 12

---

64. What's the output?

const value = { number: 10 };

const multiply = (x = { ...value }) => {
  console.log((x.number * 2));
};

multiply();
multiply();
multiply(value);
multiply(value);

• A: 20, 40, 80, 160
• B: 20, 40, 20, 40
• C: 20, 20, 20, 40
• D: NaN, NaN, 20, 40

---

65. What's the output?

[1, 2, 3, 4].reduce((x, y) => console.log(x, y));

• A: 1 2 and 3 3 and 6 4
• B: 1 2 and 2 3 and 3 4
• C: 1 undefined and 2 undefined and 3 undefined and 4 undefined
• D: 1 2 and undefined 3 and undefined 4

---

66. With which constructor can we successfully extend the Dog class?

class Dog {
  constructor(name) {
    this.name = name;
  }
};

class Labrador extends Dog {
  // 1 
  constructor(name, size) {
    this.size = size;
  }
  // 2
  constructor(name, size) {
    super(name);
    this.size = size;
  }
  // 3
  constructor(size) {
    super(name);
    this.size = size;
  }
  // 4 
  constructor(name, size) {
    this.name = name;
    this.size = size;
  }

};

• A: 1
• B: 2
• C: 3
• D: 4

---

67. With which constructor can we successfully extend the Dog class?

// index.js
console.log('running index.js);
import { sum } from './sum.js';
console.log(sum(1, 2));

// sum.js
console.log('running sum.js');
export const sum = (a, b) => a + b;

• A: running index.js, running sum.js, 3
• B: running sum.js, running index.js, 3
• C: running sum.js, 3, running index.js
• D: running index.js, undefined, running sum.js

---

68. What's the output?

console.log(Number(2) === Number(2))
console.log(Boolean(false) === Boolean(false))
console.log(Symbol('foo') === Symbol('foo'))

• A: true, true, false
• B: false, true, false
• C: true, false, true
• D: true, true, true

---

69. What's the output?

const name = "Lydia Hallie"
console.log(name.padStart(13))
console.log(name.padStart(2))

• A: "Lydia Hallie", "Lydia Hallie"
• B: " Lydia Hallie", " Lydia Hallie" ("[13x whitespace]Lydia Hallie", "[2x whitespace]Lydia Hallie")
• C: " Lydia Hallie", "Lydia Hallie" ("[1x whitespace]Lydia Hallie", "Lydia Hallie")
• D: "Lydia Hallie", "Lyd",