如何使用Jest测试异步组件？

Question

有人能告诉我如何在挂载调用componendDidMount()的组件时等待被嘲弄的承诺解决吗？

class Something extends React.Component {
    state = {
      res: null,
    };

    componentDidMount() {
        API.get().then(res => this.setState({ res }));
    }

    render() {
      if (!!this.state.res) return
      return <span>user: ${this.state.res.user}</span>;
    }
}

API.get()在我的玩笑测试中被嘲笑了

data = [
  'user': 1,
  'name': 'bob'
];

function mockPromiseResolution(response) {
  return new Promise((resolve, reject) => {
    process.nextTick(
      resolve(response)
    );
  });
}

const API = {
    get: () => mockPromiseResolution(data),
};

然后我的测试文件：

import { API } from 'api';
import { API as mockAPI } from '__mocks/api';

API.get = jest.fn().mockImplementation(mockAPI.get);

describe('Something Component', () => {
  it('renders after data loads', () => {
    const wrapper = mount(<Something />);
    expect(mountToJson(wrapper)).toMatchSnapshot();
    // here is where I dont know how to wait to do the expect until the mock promise does its nextTick and resolves
  });
});

问题是，我的expect(mountToJson(wrapper))正在返回null，因为<Something />的模拟api调用和生命周期方法还没有完成。

Answer 1

Jest有仿冒的时间旅行，要在您的情况下使用它，我想您可以用以下方式修改代码：

import { API } from 'api';
import { API as mockAPI } from '__mocks/api';

API.get = jest.fn().mockImplementation(mockAPI.get);

jest.useFakeTimers(); // this statement makes sure you use fake timers

describe('Something Component', () => {
  it('renders after data loads', () => {
    const wrapper = mount(<Something />);

    // skip forward to a certain time
    jest.runTimersToTime(1);

    expect(mountToJson(wrapper)).toMatchSnapshot();
  });
});

对于jest.runTimersToTime()，您也可以使用jest.runAllTimers()

Answer 2

作为解决办法，将其从异步转换为同步。

jest.spyOn(Api, 'get').mockReturnValue({
  then: fn => fn('hello');
});