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Rework the docs for "Testing Asynchronous Code" (#12555)
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adi611 authored Apr 11, 2022
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1 change: 0 additions & 1 deletion .eslintrc.cjs
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Original file line number Diff line number Diff line change
Expand Up @@ -375,7 +375,6 @@ module.exports = {
'no-bitwise': 'warn',
'no-caller': 'error',
'no-case-declarations': 'off',
'no-catch-shadow': 'error',
'no-class-assign': 'warn',
'no-cond-assign': 'off',
'no-confusing-arrow': 'off',
Expand Down
137 changes: 73 additions & 64 deletions docs/TestingAsyncCode.md
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Original file line number Diff line number Diff line change
Expand Up @@ -5,18 +5,82 @@ title: Testing Asynchronous Code

It's common in JavaScript for code to run asynchronously. When you have code that runs asynchronously, Jest needs to know when the code it is testing has completed, before it can move on to another test. Jest has several ways to handle this.

## Callbacks
## Promises

Return a promise from your test, and Jest will wait for that promise to resolve. If the promise is rejected, the test will fail.

For example, let's say that `fetchData` returns a promise that is supposed to resolve to the string `'peanut butter'`. We could test it with:

```js
test('the data is peanut butter', () => {
return fetchData().then(data => {
expect(data).toBe('peanut butter');
});
});
```

## Async/Await

Alternatively, you can use `async` and `await` in your tests. To write an async test, use the `async` keyword in front of the function passed to `test`. For example, the same `fetchData` scenario can be tested with:

```js
test('the data is peanut butter', async () => {
const data = await fetchData();
expect(data).toBe('peanut butter');
});

The most common asynchronous pattern is callbacks.
test('the fetch fails with an error', async () => {
expect.assertions(1);
try {
await fetchData();
} catch (e) {
expect(e).toMatch('error');
}
});
```

You can combine `async` and `await` with `.resolves` or `.rejects`.

```js
test('the data is peanut butter', async () => {
await expect(fetchData()).resolves.toBe('peanut butter');
});

test('the fetch fails with an error', async () => {
await expect(fetchData()).rejects.toMatch('error');
});
```

In these cases, `async` and `await` are effectively syntactic sugar for the same logic as the promises example uses.

:::caution

Be sure to return (or `await`) the promise - if you omit the `return`/`await` statement, your test will complete before the promise returned from `fetchData` resolves or rejects.

:::

If you expect a promise to be rejected, use the `.catch` method. Make sure to add `expect.assertions` to verify that a certain number of assertions are called. Otherwise, a fulfilled promise would not fail the test.

```js
test('the fetch fails with an error', () => {
expect.assertions(1);
return fetchData().catch(e => expect(e).toMatch('error'));
});
```

## Callbacks

For example, let's say that you have a `fetchData(callback)` function that fetches some data and calls `callback(data)` when it is complete. You want to test that this returned data is the string `'peanut butter'`.
If you don't use promises, you can use callbacks. For example, let's say that `fetchData`, instead of returning a promise, expects a callback, i.e. fetches some data and calls `callback(null, data)` when it is complete. You want to test that this returned data is the string `'peanut butter'`.

By default, Jest tests complete once they reach the end of their execution. That means this test will _not_ work as intended:

```js
// Don't do this!
test('the data is peanut butter', () => {
function callback(data) {
function callback(error, data) {
if (error) {
throw error;
}
expect(data).toBe('peanut butter');
}

Expand All @@ -30,7 +94,11 @@ There is an alternate form of `test` that fixes this. Instead of putting the tes

```js
test('the data is peanut butter', done => {
function callback(data) {
function callback(error, data) {
if (error) {
done(error);
return;
}
try {
expect(data).toBe('peanut butter');
done();
Expand All @@ -49,31 +117,6 @@ If the `expect` statement fails, it throws an error and `done()` is not called.

_Note: `done()` should not be mixed with Promises as this tends to lead to memory leaks in your tests._

## Promises

If your code uses promises, there is a more straightforward way to handle asynchronous tests. Return a promise from your test, and Jest will wait for that promise to resolve. If the promise is rejected, the test will automatically fail.

For example, let's say that `fetchData`, instead of using a callback, returns a promise that is supposed to resolve to the string `'peanut butter'`. We could test it with:

```js
test('the data is peanut butter', () => {
return fetchData().then(data => {
expect(data).toBe('peanut butter');
});
});
```

Be sure to return the promise - if you omit this `return` statement, your test will complete before the promise returned from `fetchData` resolves and then() has a chance to execute the callback.

If you expect a promise to be rejected, use the `.catch` method. Make sure to add `expect.assertions` to verify that a certain number of assertions are called. Otherwise, a fulfilled promise would not fail the test.

```js
test('the fetch fails with an error', () => {
expect.assertions(1);
return fetchData().catch(e => expect(e).toMatch('error'));
});
```

## `.resolves` / `.rejects`

You can also use the `.resolves` matcher in your expect statement, and Jest will wait for that promise to resolve. If the promise is rejected, the test will automatically fail.
Expand All @@ -94,38 +137,4 @@ test('the fetch fails with an error', () => {
});
```

## Async/Await

Alternatively, you can use `async` and `await` in your tests. To write an async test, use the `async` keyword in front of the function passed to `test`. For example, the same `fetchData` scenario can be tested with:

```js
test('the data is peanut butter', async () => {
const data = await fetchData();
expect(data).toBe('peanut butter');
});

test('the fetch fails with an error', async () => {
expect.assertions(1);
try {
await fetchData();
} catch (e) {
expect(e).toMatch('error');
}
});
```

You can combine `async` and `await` with `.resolves` or `.rejects`.

```js
test('the data is peanut butter', async () => {
await expect(fetchData()).resolves.toBe('peanut butter');
});

test('the fetch fails with an error', async () => {
await expect(fetchData()).rejects.toMatch('error');
});
```

In these cases, `async` and `await` are effectively syntactic sugar for the same logic as the promises example uses.

None of these forms is particularly superior to the others, and you can mix and match them across a codebase or even in a single file. It just depends on which style you feel makes your tests simpler.
137 changes: 73 additions & 64 deletions website/versioned_docs/version-25.x/TestingAsyncCode.md
View file
Original file line number Diff line number Diff line change
Expand Up @@ -5,18 +5,82 @@ title: Testing Asynchronous Code

It's common in JavaScript for code to run asynchronously. When you have code that runs asynchronously, Jest needs to know when the code it is testing has completed, before it can move on to another test. Jest has several ways to handle this.

## Callbacks
## Promises

Return a promise from your test, and Jest will wait for that promise to resolve. If the promise is rejected, the test will fail.

For example, let's say that `fetchData` returns a promise that is supposed to resolve to the string `'peanut butter'`. We could test it with:

```js
test('the data is peanut butter', () => {
return fetchData().then(data => {
expect(data).toBe('peanut butter');
});
});
```

## Async/Await

Alternatively, you can use `async` and `await` in your tests. To write an async test, use the `async` keyword in front of the function passed to `test`. For example, the same `fetchData` scenario can be tested with:

```js
test('the data is peanut butter', async () => {
const data = await fetchData();
expect(data).toBe('peanut butter');
});

The most common asynchronous pattern is callbacks.
test('the fetch fails with an error', async () => {
expect.assertions(1);
try {
await fetchData();
} catch (e) {
expect(e).toMatch('error');
}
});
```

You can combine `async` and `await` with `.resolves` or `.rejects`.

```js
test('the data is peanut butter', async () => {
await expect(fetchData()).resolves.toBe('peanut butter');
});

test('the fetch fails with an error', async () => {
await expect(fetchData()).rejects.toMatch('error');
});
```

In these cases, `async` and `await` are effectively syntactic sugar for the same logic as the promises example uses.

:::caution

Be sure to return (or `await`) the promise - if you omit the `return`/`await` statement, your test will complete before the promise returned from `fetchData` resolves or rejects.

:::

If you expect a promise to be rejected, use the `.catch` method. Make sure to add `expect.assertions` to verify that a certain number of assertions are called. Otherwise, a fulfilled promise would not fail the test.

```js
test('the fetch fails with an error', () => {
expect.assertions(1);
return fetchData().catch(e => expect(e).toMatch('error'));
});
```

## Callbacks

For example, let's say that you have a `fetchData(callback)` function that fetches some data and calls `callback(data)` when it is complete. You want to test that this returned data is the string `'peanut butter'`.
If you don't use promises, you can use callbacks. For example, let's say that `fetchData`, instead of returning a promise, expects a callback, i.e. fetches some data and calls `callback(null, data)` when it is complete. You want to test that this returned data is the string `'peanut butter'`.

By default, Jest tests complete once they reach the end of their execution. That means this test will _not_ work as intended:

```js
// Don't do this!
test('the data is peanut butter', () => {
function callback(data) {
function callback(error, data) {
if (error) {
throw error;
}
expect(data).toBe('peanut butter');
}

Expand All @@ -30,7 +94,11 @@ There is an alternate form of `test` that fixes this. Instead of putting the tes

```js
test('the data is peanut butter', done => {
function callback(data) {
function callback(error, data) {
if (error) {
done(error);
return;
}
try {
expect(data).toBe('peanut butter');
done();
Expand All @@ -49,31 +117,6 @@ If the `expect` statement fails, it throws an error and `done()` is not called.

_Note: `done()` should not be mixed with Promises as this tends to lead to memory leaks in your tests._

## Promises

If your code uses promises, there is a more straightforward way to handle asynchronous tests. Return a promise from your test, and Jest will wait for that promise to resolve. If the promise is rejected, the test will automatically fail.

For example, let's say that `fetchData`, instead of using a callback, returns a promise that is supposed to resolve to the string `'peanut butter'`. We could test it with:

```js
test('the data is peanut butter', () => {
return fetchData().then(data => {
expect(data).toBe('peanut butter');
});
});
```

Be sure to return the promise - if you omit this `return` statement, your test will complete before the promise returned from `fetchData` resolves and then() has a chance to execute the callback.

If you expect a promise to be rejected, use the `.catch` method. Make sure to add `expect.assertions` to verify that a certain number of assertions are called. Otherwise, a fulfilled promise would not fail the test.

```js
test('the fetch fails with an error', () => {
expect.assertions(1);
return fetchData().catch(e => expect(e).toMatch('error'));
});
```

## `.resolves` / `.rejects`

You can also use the `.resolves` matcher in your expect statement, and Jest will wait for that promise to resolve. If the promise is rejected, the test will automatically fail.
Expand All @@ -94,38 +137,4 @@ test('the fetch fails with an error', () => {
});
```

## Async/Await

Alternatively, you can use `async` and `await` in your tests. To write an async test, use the `async` keyword in front of the function passed to `test`. For example, the same `fetchData` scenario can be tested with:

```js
test('the data is peanut butter', async () => {
const data = await fetchData();
expect(data).toBe('peanut butter');
});

test('the fetch fails with an error', async () => {
expect.assertions(1);
try {
await fetchData();
} catch (e) {
expect(e).toMatch('error');
}
});
```

You can combine `async` and `await` with `.resolves` or `.rejects`.

```js
test('the data is peanut butter', async () => {
await expect(fetchData()).resolves.toBe('peanut butter');
});

test('the fetch fails with an error', async () => {
await expect(fetchData()).rejects.toMatch('error');
});
```

In these cases, `async` and `await` are effectively syntactic sugar for the same logic as the promises example uses.

None of these forms is particularly superior to the others, and you can mix and match them across a codebase or even in a single file. It just depends on which style you feel makes your tests simpler.
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