[expect] Improve mock "return" matchers to properly handle calls that throw (#6172)

* Improve mock "return" matchers to properly handle calls that throw

* Fix unit test that was broken because expect().toThrow() does not
proeprly detect a thrown value of undefined.

* update CHANGELOG

* Fix comment typo

* Simplify structure of how mock function results are maintained.

Author: UselessPickles

CHANGELOG.md

@@ -92,6 +92,8 @@
   ([#6181](https://github.com/facebook/jest/pull/6181))
 * `[expect]` Include custom mock names in error messages
   ([#6199](https://github.com/facebook/jest/pull/6199))
+* `[expect]` Improve return matchers
+  ([#6172](https://github.com/facebook/jest/pull/6172))
 
 ### Fixes
 

docs/ExpectAPI.md

@@ -683,8 +683,9 @@ Note: the nth argument must be positive integer starting from 1.
 Also under the alias: `.toReturn()`
 
 If you have a mock function, you can use `.toHaveReturned` to test that the mock
-function returned a value. For example, let's say you have a mock `drink` that
-returns `true`. You can write:
+function successfully returned (i.e., did not throw an error) at least one time.
+For example, let's say you have a mock `drink` that returns `true`. You can
+write:
 
 ```js
 test('drinks returns', () => {
@@ -700,9 +701,13 @@ test('drinks returns', () => {
 
 Also under the alias: `.toReturnTimes(number)`
 
-Use `.toHaveReturnedTimes` to ensure that a mock function returned an exact
-number of times. For example, let's say you have a mock `drink` that returns
-`true`. You can write:
+Use `.toHaveReturnedTimes` to ensure that a mock function returned successfully
+(i.e., did not throw an error) an exact number of times. Any calls to the mock
+function that throw an error are not counted toward the number of times the
+function returned.
+
+For example, let's say you have a mock `drink` that returns `true`. You can
+write:
 
 ```js
 test('drink returns twice', () => {
@@ -741,7 +746,9 @@ test('drink returns La Croix', () => {
 Also under the alias: `.lastReturnedWith(value)`
 
 Use `.toHaveLastReturnedWith` to test the specific value that a mock function
-last returned.
+last returned. If the last call to the mock function threw an error, then this
+matcher will fail no matter what value you provided as the expected return
+value.
 
 For example, let's say you have a mock `drink` that returns the name of the
 beverage that was consumed. You can write:
@@ -764,7 +771,9 @@ test('drink returns La Croix (Orange) last', () => {
 Also under the alias: `.nthReturnedWith(nthCall, value)`
 
 Use `.toHaveNthReturnedWith` to test the specific value that a mock function
-returned for the nth call.
+returned for the nth call. If the nth call to the mock function threw an error,
+then this matcher will fail no matter what value you provided as the expected
+return value.
 
 For example, let's say you have a mock `drink` that returns the name of the
 beverage that was consumed. You can write:

docs/MockFunctionAPI.md

@@ -22,9 +22,9 @@ Returns the mock name string set by calling `mockFn.mockName(value)`.
 
 ### `mockFn.mock.calls`
 
-An array that represents all calls that have been made into this mock function.
-Each call is represented by an array of arguments that were passed during the
-call.
+An array containing the call arguments of all calls that have been made to this
+mock function. Each item in the array is an array of arguments that were passed
+during the call.
 
 For example: A mock function `f` that has been called twice, with the arguments
 `f('arg1', 'arg2')`, and then with the arguments `f('arg3', 'arg4')`, would have
@@ -34,33 +34,35 @@ a `mock.calls` array that looks like this:
 [['arg1', 'arg2'], ['arg3', 'arg4']];
 ```
 
-### `mockFn.mock.returnValues`
+### `mockFn.mock.results`
 
-An array containing values that have been returned by all calls to this mock
-function. For any call to the mock that throws an error, a value of `undefined`
-will be stored in `mock.returnValues`.
+An array containing the results of all calls that have been made to this mock
+function. Each entry in this array is an object containing a boolean `isThrow`
+property, and a `value` property. `isThrow` is true if the call terminated due
+to a `throw`, or false if the the call returned normally. The `value` property
+contains the value that was thrown or returned.
 
 For example: A mock function `f` that has been called three times, returning
 `result1`, throwing an error, and then returning `result2`, would have a
-`mock.returnValues` array that looks like this:
+`mock.results` array that looks like this:
 
 ```js
-['result1', undefined, 'result2'];
-```
-
-### `mockFn.mock.thrownErrors`
-
-An array containing errors that have been thrown by all calls to this mock
-function.
-
-For example: A mock function `f` that has been called twice, throwing an
-`Error`, and then executing successfully without an error, would have the
-following `mock.thrownErrors` array:
-
-```js
-f.mock.thrownErrors.length === 2; // true
-f.mock.thrownErrors[0] instanceof Error; // true
-f.mock.thrownErrors[1] === undefined; // true
+[
+  {
+    isThrow: false,
+    value: 'result1',
+  },
+  {
+    isThrow: true,
+    value: {
+      /* Error instance */
+    },
+  },
+  {
+    isThrow: false,
+    value: 'result2',
+  },
+];
 ```
 
 ### `mockFn.mock.instances`