An Async Example

First, enable Babel support in Jest as documented in the Getting Started guide.

Let's implement a module that fetches user data from an API and returns the user name.

// user.js
import request from './request';
 
export function getUserName(userID) {
 return request('/users/' + userID).then(user => user.name);
}

In the above implementation, we expect the request.js module to return a promise. We chain a call to then to receive the user name.

Now imagine an implementation of request.js that goes to the network and fetches some user data:

// request.js
const http = require('http');
 
export default function request(url) {
 return new Promise(resolve => {
 // This is an example of an http request, for example to fetch
 // user data from an API.
 // This module is being mocked in __mocks__/request.js
 http.get({path: url}, response => {
 let data = '';
 response.on('data', _data => (data += _data));
 response.on('end', () => resolve(data));
 });
 });
}

Because we don't want to go to the network in our test, we are going to create a manual mock for our request.js module in the __mocks__ folder (the folder is case-sensitive, __MOCKS__ will not work). It could look something like this:

// __mocks__/request.js
const users = {
 4: {name: 'Mark'},
 5: {name: 'Paul'},
};
 
export default function request(url) {
 return new Promise((resolve, reject) => {
 const userID = parseInt(url.substr('/users/'.length), 10);
 process.nextTick(() =>
 users[userID]
 ? resolve(users[userID])
 : reject({
 error: 'User with ' + userID + ' not found.',
 }),
 );
 });
}

Now let's write a test for our async functionality.

// __tests__/user-test.js
jest.mock('../request');
 
import * as user from '../user';
 
// The assertion for a promise must be returned.
it('works with promises', () => {
 expect.assertions(1);
 return user.getUserName(4).then(data => expect(data).toEqual('Mark'));
});

We call jest.mock('../request') to tell Jest to use our manual mock. it expects the return value to be a Promise that is going to be resolved. You can chain as many Promises as you like and call expect at any time, as long as you return a Promise at the end.

`.resolves`

There is a less verbose way using resolves to unwrap the value of a fulfilled promise together with any other matcher. If the promise is rejected, the assertion will fail.

it('works with resolves', () => {
 expect.assertions(1);
 return expect(user.getUserName(5)).resolves.toEqual('Paul');
});

`async`/`await`

Writing tests using the async/await syntax is also possible. Here is how you'd write the same examples from before:

// async/await can be used.
it('works with async/await', async () => {
 expect.assertions(1);
 const data = await user.getUserName(4);
 expect(data).toEqual('Mark');
});
 
// async/await can also be used with `.resolves`.
it('works with async/await and resolves', async () => {
 expect.assertions(1);
 await expect(user.getUserName(5)).resolves.toEqual('Paul');
});

To enable async/await in your project, install @babel/preset-env and enable the feature in your babel.config.js file.

Error handling

Errors can be handled using 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:

// Testing for async errors using Promise.catch.
it('tests error with promises', () => {
 expect.assertions(1);
 return user.getUserName(2).catch(e =>
 expect(e).toEqual({
 error: 'User with 2 not found.',
 }),
 );
});
 
// Or using async/await.
it('tests error with async/await', async () => {
 expect.assertions(1);
 try {
 await user.getUserName(1);
 } catch (e) {
 expect(e).toEqual({
 error: 'User with 1 not found.',
 });
 }
});

`.rejects`

The.rejects helper works like the .resolves helper. If the promise is fulfilled, the test will automatically fail. expect.assertions(number) is not required but recommended to verify that a certain number of assertions are called during a test. It is otherwise easy to forget to return/await the .resolves assertions.

// Testing for async errors using `.rejects`.
it('tests error with rejects', () => {
 expect.assertions(1);
 return expect(user.getUserName(3)).rejects.toEqual({
 error: 'User with 3 not found.',
 });
});
 
// Or using async/await with `.rejects`.
it('tests error with async/await and rejects', async () => {
 expect.assertions(1);
 await expect(user.getUserName(3)).rejects.toEqual({
 error: 'User with 3 not found.',
 });
});

The code for this example is available at examples/async.

If you'd like to test timers, like setTimeout, take a look at the Timer mocks documentation.