reactjs - How do test async components with Jest?

Question

can anyone tell me how to wait in jest for a mocked promise to resolve when mounting a component that calls componendDidMount()?

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>;
    }
}

the API.get() is mocked in my jest test

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

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

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

Then my testing file:

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
  });
});

The issue is that I the expect(mountToJson(wrapper)) is returning null because the mocked api call and lifecycle methods of <Something /> haven't gone through yet.

Answer 1

Jest has mocks to fake time travelling, to use it in your case, I guess you can change your code in the following style:

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();
  });
});

Alternatively to jest.runTimersToTime() you could also use jest.runAllTimers()