ES6 类模拟

Jest 可用于模拟你从文件导入的想要测试的ES6类。

ES6类是带有一些语法糖的构造函数。 因此，ES6类的任何 模拟都必须是一个函数或者一个真实的 ES6类(又是另一个函数)。 So you can mock them using mock functions.

一个ES6类的例子

We'll use a contrived example of a class that plays sound files, SoundPlayer, and a consumer class which uses that class, SoundPlayerConsumer. We'll mock SoundPlayer in our tests for SoundPlayerConsumer.

sound-player.js

export default class SoundPlayer {
  constructor() {
    this.foo = 'bar';
  }

  playSoundFile(fileName) {
    console.log('Playing sound file ' + fileName);
  }
}

sound-player-consumer.js

import SoundPlayer from './sound-player';

export default class SoundPlayerConsumer {
  constructor() {
    this.soundPlayer = new SoundPlayer();
  }

  playSomethingCool() {
    const coolSoundFileName = 'song.mp3';
    this.soundPlayer.playSoundFile(coolSoundFileName);
  }
}

4 种方式去模拟 ES6 类

自动模拟

Calling jest.mock('./sound-player') returns a useful "automatic mock" you can use to spy on calls to the class constructor and all of its methods. It replaces the ES6 class with a mock constructor, and replaces all of its methods with mock functions that always return undefined. Method calls are saved in theAutomaticMock.mock.instances[index].methodName.mock.calls.

备注

If you use arrow functions in your classes, they will not be part of the mock. 这样做的原因是箭头函数不能代表一个对象的原型，它们仅仅是是一个函数的引用。

如果你不需要替换class的实现，这是最简单的设置方式。 例如：

import SoundPlayer from './sound-player';
import SoundPlayerConsumer from './sound-player-consumer';
jest.mock('./sound-player'); // SoundPlayer is now a mock constructor

beforeEach(() => {
  // Clear all instances and calls to constructor and all methods:
  SoundPlayer.mockClear();
});

it('We can check if the consumer called the class constructor', () => {
  const soundPlayerConsumer = new SoundPlayerConsumer();
  expect(SoundPlayer).toHaveBeenCalledTimes(1);
});

it('We can check if the consumer called a method on the class instance', () => {
  // Show that mockClear() is working:
  expect(SoundPlayer).not.toHaveBeenCalled();

  const soundPlayerConsumer = new SoundPlayerConsumer();
  // Constructor should have been called again:
  expect(SoundPlayer).toHaveBeenCalledTimes(1);

  const coolSoundFileName = 'song.mp3';
  soundPlayerConsumer.playSomethingCool();

  // mock.instances is available with automatic mocks:
  const mockSoundPlayerInstance = SoundPlayer.mock.instances[0];
  const mockPlaySoundFile = mockSoundPlayerInstance.playSoundFile;
  expect(mockPlaySoundFile.mock.calls[0][0]).toBe(coolSoundFileName);
  // Equivalent to above check:
  expect(mockPlaySoundFile).toHaveBeenCalledWith(coolSoundFileName);
  expect(mockPlaySoundFile).toHaveBeenCalledTimes(1);
});

手动模拟

Create a manual mock by saving a mock implementation in the __mocks__ folder. 这使得你可以指定实现，这个实现将被test文件使用。

__mocks__/sound-player.js

// Import this named export into your test file:
export const mockPlaySoundFile = jest.fn();
const mock = jest.fn().mockImplementation(() => {
  return {playSoundFile: mockPlaySoundFile};
});

export default mock;

Import the mock and the mock method shared by all instances:

sound-player-consumer.test.js

import SoundPlayer, {mockPlaySoundFile} from './sound-player';
import SoundPlayerConsumer from './sound-player-consumer';
jest.mock('./sound-player'); // SoundPlayer is now a mock constructor

beforeEach(() => {
  // Clear all instances and calls to constructor and all methods:
  SoundPlayer.mockClear();
  mockPlaySoundFile.mockClear();
});

it('We can check if the consumer called the class constructor', () => {
  const soundPlayerConsumer = new SoundPlayerConsumer();
  expect(SoundPlayer).toHaveBeenCalledTimes(1);
});

it('We can check if the consumer called a method on the class instance', () => {
  const soundPlayerConsumer = new SoundPlayerConsumer();
  const coolSoundFileName = 'song.mp3';
  soundPlayerConsumer.playSomethingCool();
  expect(mockPlaySoundFile).toHaveBeenCalledWith(coolSoundFileName);
});

Calling jest.mock() with the module factory parameter

jest.mock(path, moduleFactory) takes a module factory argument. 模块工厂是一个函数，这个函数会返回 mock。

为了模拟 constructor 构造函数，模块工厂必须返回一个构造函数。 也就是说，模块工厂必须返回一个函数，而这个函数会返回另外一个函数 - 高阶函数(HOF)。

import SoundPlayer from './sound-player';
const mockPlaySoundFile = jest.fn();
jest.mock('./sound-player', () => {
  return jest.fn().mockImplementation(() => {
    return {playSoundFile: mockPlaySoundFile};
  });
});

注意

Since calls to jest.mock() are hoisted to the top of the file, Jest prevents access to out-of-scope variables. By default, you cannot first define a variable and then use it in the factory. Jest will disable this check for variables that start with the word mock. However, it is still up to you to guarantee that they will be initialized on time. Be aware of Temporal Dead Zone.

For example, the following will throw an out-of-scope error due to the use of fake instead of mock in the variable declaration.

// Note: this will fail
import SoundPlayer from './sound-player';
const fakePlaySoundFile = jest.fn();
jest.mock('./sound-player', () => {
  return jest.fn().mockImplementation(() => {
    return {playSoundFile: fakePlaySoundFile};
  });
});

The following will throw a ReferenceError despite using mock in the variable declaration, as the mockSoundPlayer is not wrapped in an arrow function and thus accessed before initialization after hoisting.

import SoundPlayer from './sound-player';
const mockSoundPlayer = jest.fn().mockImplementation(() => {
  return {playSoundFile: mockPlaySoundFile};
});
// results in a ReferenceError
jest.mock('./sound-player', () => {
  return mockSoundPlayer;
});

Replacing the mock using mockImplementation() or mockImplementationOnce()

You can replace all of the above mocks in order to change the implementation, for a single test or all tests, by calling mockImplementation() on the existing mock.

Jest.mock 的调用会被提升到文件顶部。 You can specify a mock later, e.g. in beforeAll(), by calling mockImplementation() (or mockImplementationOnce()) on the existing mock instead of using the factory parameter. 这能让你在你需要时，可以在不同 test 之间改变mock的实现。

import SoundPlayer from './sound-player';
import SoundPlayerConsumer from './sound-player-consumer';

jest.mock('./sound-player');

describe('When SoundPlayer throws an error', () => {
  beforeAll(() => {
    SoundPlayer.mockImplementation(() => {
      return {
        playSoundFile: () => {
          throw new Error('Test error');
        },
      };
    });
  });

  it('Should throw an error when calling playSomethingCool', () => {
    const soundPlayerConsumer = new SoundPlayerConsumer();
    expect(() => soundPlayerConsumer.playSomethingCool()).toThrow();
  });
});

深入：理解模拟的构造函数

Building your constructor function mock using jest.fn().mockImplementation() makes mocks appear more complicated than they really are. 这一章节会给你展示如何创建你自己的模拟，这会阐述模拟是如何工作的。

手工模拟是另外一个ES6类

If you define an ES6 class using the same filename as the mocked class in the __mocks__ folder, it will serve as the mock. 这个类将会被取代真实的类。 这将使得你可以为这个类注入一个 test 实现，但是这样做不能让你监测它的调用。

对于我们的例子来讲，这个 mock 也许会像这有：

__mocks__/sound-player.js

export default class SoundPlayer {
  constructor() {
    console.log('Mock SoundPlayer: constructor was called');
  }

  playSoundFile() {
    console.log('Mock SoundPlayer: playSoundFile was called');
  }
}

使用模块工厂函数进行模拟

The module factory function passed to jest.mock(path, moduleFactory) can be a HOF that returns a function*. This will allow calling new on the mock. 同样的，这种方式让你为测试注入不同的实现，但是不能让你去监测调用。

* 模块工厂函数必须返回一个函数

为了模拟 constructor 构造函数，模块工厂必须返回一个构造函数。 也就是说，模块工厂必须返回一个函数，而这个函数会返回另外一个函数 - 高阶函数(HOF)。

jest.mock('./sound-player', () => {
  return function () {
    return {playSoundFile: () => {}};
  };
});

备注

The mock can't be an arrow function because calling new on an arrow function is not allowed in JavaScript. So this won't work:

jest.mock('./sound-player', () => {
  return () => {
    // Does not work; arrow functions can't be called with new
    return {playSoundFile: () => {}};
  };
});

This will throw TypeError: _soundPlayer2.default is not a constructor, unless the code is transpiled to ES5, e.g. by @babel/preset-env. (ES5 没有箭头函数或者类，所以他们都会被编译成简单的函数)

Mocking a specific method of a class

Lets say that you want to mock or spy on the method playSoundFile within the class SoundPlayer. A simple example:

// your jest test file below
import SoundPlayer from './sound-player';
import SoundPlayerConsumer from './sound-player-consumer';

const playSoundFileMock = jest
  .spyOn(SoundPlayer.prototype, 'playSoundFile')
  .mockImplementation(() => {
    console.log('mocked function');
  }); // comment this line if just want to "spy"

it('player consumer plays music', () => {
  const player = new SoundPlayerConsumer();
  player.playSomethingCool();
  expect(playSoundFileMock).toHaveBeenCalled();
});

Static, getter and setter methods

Lets imagine our class SoundPlayer has a getter method foo and a static method brand

export default class SoundPlayer {
  constructor() {
    this.foo = 'bar';
  }

  playSoundFile(fileName) {
    console.log('Playing sound file ' + fileName);
  }

  get foo() {
    return 'bar';
  }
  static brand() {
    return 'player-brand';
  }
}

You can mock/spy on them easily, here is an example:

// your jest test file below
import SoundPlayer from './sound-player';

const staticMethodMock = jest
  .spyOn(SoundPlayer, 'brand')
  .mockImplementation(() => 'some-mocked-brand');

const getterMethodMock = jest
  .spyOn(SoundPlayer.prototype, 'foo', 'get')
  .mockImplementation(() => 'some-mocked-result');

it('custom methods are called', () => {
  const player = new SoundPlayer();
  const foo = player.foo;
  const brand = SoundPlayer.brand();

  expect(staticMethodMock).toHaveBeenCalled();
  expect(getterMethodMock).toHaveBeenCalled();
});

Keeping track of usage (spying on the mock)

注入一个测试的实现是非常有帮助的，但是你可能也需要去测试这个类的构造函数以及上面的方法被调用是是否传入了正确的参数。

监测构造函数

为了监测构造函数的调用，我们需要把高阶函数HOF替换成Jest 的模拟函数。 Create it with jest.fn(), and then specify its implementation with mockImplementation().

import SoundPlayer from './sound-player';
jest.mock('./sound-player', () => {
  // Works and lets you check for constructor calls:
  return jest.fn().mockImplementation(() => {
    return {playSoundFile: () => {}};
  });
});

This will let us inspect usage of our mocked class, using SoundPlayer.mock.calls: expect(SoundPlayer).toHaveBeenCalled(); or near-equivalent: expect(SoundPlayer.mock.calls.length).toBeGreaterThan(0);

模拟非default export 出的类

If the class is not the default export from the module then you need to return an object with the key that is the same as the class export name.

import {SoundPlayer} from './sound-player';
jest.mock('./sound-player', () => {
  // Works and lets you check for constructor calls:
  return {
    SoundPlayer: jest.fn().mockImplementation(() => {
      return {playSoundFile: () => {}};
    }),
  };
});

监测我们类上的方法

Our mocked class will need to provide any member functions (playSoundFile in the example) that will be called during our tests, or else we'll get an error for calling a function that doesn't exist. 但是我们也许同时需要监测这些方法的调用，去确保他们调用的时候传入的参数是我们期望的。

在测试文件里每次 mock 的构造函数被调用会返回一个新的对象。 To spy on method calls in all of these objects, we populate playSoundFile with another mock function, and store a reference to that same mock function in our test file, so it's available during tests.

import SoundPlayer from './sound-player';
const mockPlaySoundFile = jest.fn();
jest.mock('./sound-player', () => {
  return jest.fn().mockImplementation(() => {
    return {playSoundFile: mockPlaySoundFile};
    // Now we can track calls to playSoundFile
  });
});

如果使用手动模拟，代码如下：

__mocks__/sound-player.js

// Import this named export into your test file
export const mockPlaySoundFile = jest.fn();
const mock = jest.fn().mockImplementation(() => {
  return {playSoundFile: mockPlaySoundFile};
});

export default mock;

Usage is similar to the module factory function, except that you can omit the second argument from jest.mock(), and you must import the mocked method into your test file, since it is no longer defined there. Use the original module path for this; don't include __mocks__.

在每个 test 之间清除 mock

To clear the record of calls to the mock constructor function and its methods, we call mockClear() in the beforeEach() function:

beforeEach(() => {
  SoundPlayer.mockClear();
  mockPlaySoundFile.mockClear();
});

完成示例

Here's a complete test file which uses the module factory parameter to jest.mock:

sound-player-consumer.test.js

import SoundPlayer from './sound-player';
import SoundPlayerConsumer from './sound-player-consumer';

const mockPlaySoundFile = jest.fn();
jest.mock('./sound-player', () => {
  return jest.fn().mockImplementation(() => {
    return {playSoundFile: mockPlaySoundFile};
  });
});

beforeEach(() => {
  SoundPlayer.mockClear();
  mockPlaySoundFile.mockClear();
});

it('The consumer should be able to call new() on SoundPlayer', () => {
  const soundPlayerConsumer = new SoundPlayerConsumer();
  // Ensure constructor created the object:
  expect(soundPlayerConsumer).toBeTruthy();
});

it('We can check if the consumer called the class constructor', () => {
  const soundPlayerConsumer = new SoundPlayerConsumer();
  expect(SoundPlayer).toHaveBeenCalledTimes(1);
});

it('We can check if the consumer called a method on the class instance', () => {
  const soundPlayerConsumer = new SoundPlayerConsumer();
  const coolSoundFileName = 'song.mp3';
  soundPlayerConsumer.playSomethingCool();
  expect(mockPlaySoundFile.mock.calls[0][0]).toBe(coolSoundFileName);
});