首先, 像 Getting Started 里面所说的那样, 启用babel的支持

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

  1. // user.js
  2. import request from './request';
  3. export function getUserName(userID) {
  4. return request('/users/' + userID).then(user => user.name);
  5. }

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:

  1. // request.js
  2. const http = require('http');
  3. export default function request(url) {
  4. return new Promise(resolve => {
  5. // 这是一个HTTP请求的例子, 用来从API获取用户信息
  6. // This module is being mocked in __mocks__/request.js
  7. http.get({path: url}, response => {
  8. let data = '';
  9. response.on('data', _data => (data += _data));
  10. response.on('end', () => resolve(data));
  11. });
  12. });
  13. }

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). 就像是这样:

  1. // __mocks__/request.js
  2. const users = {
  3. 4: {name: 'Mark'},
  4. 5: {name: 'Paul'},
  5. };
  6. export default function request(url) {
  7. return new Promise((resolve, reject) => {
  8. const userID = parseInt(url.substr('/users/'.length), 10);
  9. process.nextTick(() =>
  10. users[userID]
  11. ? resolve(users[userID])
  12. : reject({
  13. error: 'User with ' + userID + ' not found.',
  14. }),
  15. );
  16. });
  17. }

现在我们就来编写我们的异步函数的测试

  1. // __tests__/user-test.js
  2. jest.mock('../request');
  3. import * as user from '../user';
  4. //断言必须返回一个primose
  5. it('works with promises', () => {
  6. expect.assertions(1);
  7. return user.getUserName(4).then(data => expect(data).toEqual('Mark'));
  8. });

我们调用 jest.mock('../request ') 告诉jest 使用我们手动的创建的模拟数据。 it 断言的是将会返回一个Promise对象. 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.

  1. it('works with resolves', () => {
  2. expect.assertions(1);
  3. return expect(user.getUserName(5)).resolves.toEqual('Paul');
  4. });

async/await

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

  1. // 使用async/await
  2. it('works with async/await', async () => {
  3. expect.assertions(1);
  4. const data = await user.getUserName(4);
  5. expect(data).toEqual('Mark');
  6. });
  7. // async/await 也可以和 `.resolves` 一起使用.
  8. it('works with async/await and resolves', async () => {
  9. expect.assertions(1);
  10. await expect(user.getUserName(5)).resolves.toEqual('Paul');
  11. });

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

错误处理

可以使用 .catch 方法处理错误。 请确保添加 expect.assertions 来验证一定数量的断言被调用。 否则一个fulfilled态的Promise 不会让测试失败︰

  1. //用 Promise.catch 测试一个异步的错误
  2. it('tests error with promises', () => {
  3. expect.assertions(1);
  4. return user.getUserName(2).catch(e =>
  5. expect(e).toEqual({
  6. error: 'User with 2 not found.',
  7. }),
  8. );
  9. });
  10. // Or using async/await.
  11. it('tests error with async/await', async () => {
  12. expect.assertions(1);
  13. try {
  14. await user.getUserName(1);
  15. } catch (e) {
  16. expect(e).toEqual({
  17. error: 'User with 1 not found.',
  18. });
  19. }
  20. });

.rejects

The.rejects helper works like the .resolves helper. 如果 Promise 被拒绝,则测试将自动失败。 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.

  1. // 用`.rejects`.来测试一个异步的错误
  2. it('tests error with rejects', () => {
  3. expect.assertions(1);
  4. return expect(user.getUserName(3)).rejects.toEqual({
  5. error: 'User with 3 not found.',
  6. });
  7. });
  8. // 或者与async/await 一起使用 `.rejects`.
  9. it('tests error with async/await and rejects', async () => {
  10. expect.assertions(1);
  11. await expect(user.getUserName(3)).rejects.toEqual({
  12. error: 'User with 3 not found.',
  13. });
  14. });

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.