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

In the above implementation we expect the 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');
  4. export default function request(url) {
  5.   return new Promise(resolve => {
  6.     // 这是一个HTTP请求的例子, 用来从API获取用户信息
  7.     // This module is being mocked in __mocks__/request.js
  8.     http.get({path: url}, response => {
  9.       let data = '';
  10.       response.on('data', _data => (data += _data));
  11.       response.on('end', () => resolve(data));
  12.     });
  13.   });
  14. }

现在我们就来编写我们的异步函数的测试

  1. jest.mock('../request');
  5. //断言必须返回一个primose
  6. it('works with promises', () => {
  7.   expect.assertions(1);
  8.   return user.getUserName(4).then(data => expect(data).toEqual('Mark'));
  9. });

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.

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. // 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. });
  8. // async/await 也可以和 `.resolves` 一起使用.
  9.   expect.assertions(1);
  11. });

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

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︰

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

  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. });
  9. // 或者与async/await 一起使用 `.rejects`.
  10. it('tests error with async/await and rejects', async () => {
  11.   expect.assertions(1);
  12.   await expect(user.getUserName(3)).rejects.toEqual({
  13.     error: 'User with 3 not found.',
  14.   });

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