前端测试工具Jest——使用

Snapshot 测试

一般用于UI测试，配合React Component可以直观地比较UI的变化。

import React from 'react';
import Link from '../Link.react';
import renderer from 'react-test-renderer';

it('renders correctly', () => {
  const tree = renderer
    .create(<Link page="http://www.facebook.com">Facebook</Link>)
    .toJSON();
  expect(tree).toMatchSnapshot();
});

第一次运行会生成一次snapshot，之后再运行，如果UI发生了变化，测试用例就会失败。这时，需要去检查是bug还是UI确实更新了，如果是bug则需要修复，如果是UI更新，可以通过命令jest --updateSnapshot来更新。该命令会更新所有的用例UI变化，所以应该在执行该命令前修复相应的bug，或者通过参数--testNamePattern来更新具体的用例。

最佳实践

视snapshots为代码利用相关工具进行代码美化:
- eslint-plugin-jest with its no-large-snapshots option
- snapshot-diff
测试应该是确定的（多次测试应该得到相同的结果）
使用描述性快照名称
推荐将最新的snapshot更新到版本管理工具中

Async Mock实例

// user.js
import request from './request';

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

// 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));
    });
  });
}

// __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.',
            }),
    );
  });
}

// __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'));
});

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

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');
});

Timer Mock实例

使用jest.useFakeTimers()mock setTimeout, setInterval, clearTimeout, clearInterval.

runAllTimers

// timerGame.js
'use strict';

function timerGame(callback) {
  console.log('Ready....go!');
  setTimeout(() => {
    console.log('Times up -- stop!');
    callback && callback();
  }, 1000);
}

module.exports = timerGame;

// __tests__/timerGame-test.js
'use strict';

jest.useFakeTimers();

// 验证callback被调用前等待了1s
test('waits 1 second before ending the game', () => {
  const timerGame = require('../timerGame');
  timerGame();

  expect(setTimeout).toHaveBeenCalledTimes(1);
  expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 1000);
});

// 验证callback在1s后被调用
test('calls the callback after 1 second', () => {
  const timerGame = require('../timerGame');
  const callback = jest.fn();

  timerGame(callback);

  // At this point in time, the callback should not have been called yet
  expect(callback).not.toBeCalled();

  // Fast-forward until all timers have been executed
  jest.runAllTimers();

  // Now our callback should have been called!
  expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

runOnlyPendingTimers 处理timer嵌套的问题

// infiniteTimerGame.js
'use strict';

function infiniteTimerGame(callback) {
  console.log('Ready....go!');

  setTimeout(() => {
    console.log('Times up! 10 seconds before the next game starts...');
    callback && callback();

    // Schedule the next game in 10 seconds
    setTimeout(() => {
      infiniteTimerGame(callback);
    }, 10000);
  }, 1000);
}

module.exports = infiniteTimerGame;

// __tests__/infiniteTimerGame-test.js
'use strict';

jest.useFakeTimers();

describe('infiniteTimerGame', () => {
  test('schedules a 10-second timer after 1 second', () => {
    const infiniteTimerGame = require('../infiniteTimerGame');
    const callback = jest.fn();

    infiniteTimerGame(callback);

    // At this point in time, there should have been a single call to
    // setTimeout to schedule the end of the game in 1 second.
    expect(setTimeout).toHaveBeenCalledTimes(1);
    expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 1000);

    // Fast forward and exhaust only currently pending timers
    // (but not any new timers that get created during that process)
    jest.runOnlyPendingTimers();

    // At this point, our 1-second timer should have fired it's callback
    expect(callback).toBeCalled();

    // And it should have created a new timer to start the game over in
    // 10 seconds
    expect(setTimeout).toHaveBeenCalledTimes(2);
    expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 10000);
  });
});

advanceTimersByTime(msToRun) 设置向前运行多少毫秒

it('calls the callback after 1 second via advanceTimersByTime', () => {
  const timerGame = require('../timerGame');
  const callback = jest.fn();

  timerGame(callback);

  // At this point in time, the callback should not have been called yet
  expect(callback).not.toBeCalled();

  // Fast-forward until all timers have been executed
  jest.advanceTimersByTime(1000);

  // Now our callback should have been called!
  expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

naseeihity / LearnToLearn

前端测试工具Jest——使用 #50