import thumbWar from '../thumb-war'
test('returns winner', () => {const winner = thumbWar('Ken Wheeler', 'Kent C. Dodds')expect(['Ken Wheeler', 'Kent C. Dodds'].includes(winner)).toBe(true)
})

第1步

Mocking最简单的形式是一种称作猴子补丁（Monkey-patching）的形式。下面给出一个例子：

译注：猴子补丁是指在本地修改引入的代码，但是只能对当前运行的实例有影响。

看上面的代码，你可以注意到以下几点：1、我们必须采用import * as的形式引入utils，以便于接下来可以操作这个对象（后面会谈到，这种形式有啥坏处）。2、我们需要先把要mock的函数原始值保存起来，然后在测试后恢复原来的值，这样其他用到utils的测试才能不受这个测试用例的影响。

上面的所有操作都是为了我们能够mock getWinner函数，而实际上的mock操作只有一行代码：

import thumbWar from '../thumb-war'
import * as utils from '../utils'
test('returns winner', () => {const originalGetWinner = utils.getWinner// eslint-disable-next-line import/namespaceutils.getWinner = (...args) => {utils.getWinner.mock.calls.push(args)return args[1]}utils.getWinner.mock = {calls: []}const winner = thumbWar('Ken Wheeler', 'Kent C. Dodds')expect(winner).toBe('Kent C. Dodds')expect(utils.getWinner.mock.calls).toHaveLength(2)utils.getWinner.mock.calls.forEach(args => {expect(args).toEqual(['Ken Wheeler', 'Kent C. Dodds'])})// eslint-disable-next-line import/namespaceutils.getWinner = originalGetWinner
})

上面的代码我们加入了一个 mock 对象，用以保存被mock函数在被调用时产生的一些元数据。有了它，我们可以给出下面两个断言：

expect(utils.getWinner.mock.calls).toHaveLength(2)
utils.getWinner.mock.calls.forEach(args => {expect(args).toEqual(['Ken Wheeler', 'Kent C. Dodds'])
})

import thumbWar from '../thumb-war'
import * as utils from '../utils'
test('returns winner', () => {const originalGetWinner = utils.getWinner// eslint-disable-next-line import/namespaceutils.getWinner = jest.fn((p1, p2) => p2)const winner = thumbWar('Ken Wheeler', 'Kent C. Dodds')expect(winner).toBe('Kent C. Dodds')expect(utils.getWinner).toHaveBeenCalledTimes(2)utils.getWinner.mock.calls.forEach(args => {expect(args).toEqual(['Ken Wheeler', 'Kent C. Dodds'])})// eslint-disable-next-line import/namespaceutils.getWinner = originalGetWinner
})

import thumbWar from '../thumb-war'
import * as utils from '../utils'
test('returns winner', () => {jest.spyOn(utils, 'getWinner')utils.getWinner.mockImplementation((p1, p2) => p2)const winner = thumbWar('Ken Wheeler', 'Kent C. Dodds')expect(winner).toBe('Kent C. Dodds')utils.getWinner.mockRestore()
})

import thumbWar from '../thumb-war'
import * as utilsMock from '../utils'
jest.mock('../utils', () => {return {getWinner: jest.fn((p1, p2) => p2),}
})
test('returns winner', () => {const winner = thumbWar('Ken Wheeler', 'Kent C. Dodds')expect(winner).toBe('Kent C. Dodds')expect(utilsMock.getWinner).toHaveBeenCalledTimes(2)utilsMock.getWinner.mock.calls.forEach(args => {expect(args).toEqual(['Ken Wheeler', 'Kent C. Dodds'])})
})

other/whats-a-mock/
├── __mocks__
│   └── utils.js
├── __tests__/
├── thumb-war.js
└── utils.js

// __mocks__/utils.js
export const getWinner = jest.fn((p1, p2) => p2)

这样我们的测试可以写成：

// __tests__/thumb-war.js
import thumbWar from '../thumb-war'
import * as utilsMock from '../utils'
jest.mock('../utils')
test('returns winner', () => {const winner = thumbWar('Ken Wheeler', 'Kent C. Dodds')expect(winner).toBe('Kent C. Dodds')expect(utilsMock.getWinner).toHaveBeenCalledTimes(2)utilsMock.getWinner.mock.calls.forEach(args => {expect(args).toEqual(['Ken Wheeler', 'Kent C. Dodds'])})
})