Перейти до основного змісту
Версія: 28.x

Імітації класів ES6

Jest може застосовуватись для імітації класів ES6, які імпортуються у файли, що підлягають тестуванню.

ES6 класи - це функції-конструктори з деяким синтаксичним цукром. Тому, будь-яка імітація класу ES6 повинна бути функцією або класом ES6 (який, знову ж таки, є функцією). Ви можете імітувати їх за допомогою функцій-імітацій.

Приклад ES6 класу​

Ми використаємо спеціальний приклад класу, який відтворює звукові файли, SoundPlayer і клас SoundPlayerConsumer, який його використовує. Ми імітуватимемо SoundPlayer в наших тестах для 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 способи створення імітації класу ES6​

Автоматична імітація​

Виклик jest.mock('./sound-player') повертає корисну "автоматичну імітацію", яку можна використовувати для слідкування за викликами конструктору класів і всіма його методами. Він замінює клас ES6 на конструктор-імітацію та замінює всі його методи на функції-імітації, які завжди повертають undefined. Виклики методів зберігаються в theAutomaticMock.mock.instances[index].methodName.mock.calls.

note

Якщо використовувати стрілкові функції у ваших класах, вони не будуть частиною імітації. The reason for that is that arrow functions are not present on the object's prototype, they are merely properties holding a reference to a function.

If you don't need to replace the implementation of the class, this is the easiest option to set up. Наприклад:

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

beforeEach(() => {
  // Очистимо всі екземпляри та виклики конструктору та всіх методів:
  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', () => {
  // Покажемо, що mockClear() працює:
  expect(SoundPlayer).not.toHaveBeenCalled();

  const soundPlayerConsumer = new SoundPlayerConsumer();
  // Конструктор мав знову бути викликаний:
  expect(SoundPlayer).toHaveBeenCalledTimes(1);

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

  // mock.instances доступний з автоматичними імітаціями:
  const mockSoundPlayerInstance = SoundPlayer.mock.instances[0];
  const mockPlaySoundFile = mockSoundPlayerInstance.playSoundFile;
  expect(mockPlaySoundFile.mock.calls[0][0]).toBe(coolSoundFileName);
  // Перевірка, аналогічна вищезазначеній:
  expect(mockPlaySoundFile).toHaveBeenCalledWith(coolSoundFileName);
  expect(mockPlaySoundFile).toHaveBeenCalledTimes(1);
});

Ручні імітації​

Create a manual mock by saving a mock implementation in the __mocks__ folder. This allows you to specify the implementation, and it can be used across test files.

__mocks__/sound-player.js
// Імпортуйте цей іменований export у ваш тестовий файл:
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 тепер є конструктором-імітацією

beforeEach(() => {
  // Очистимо всі екземпляри та виклики конструктору та всіх методів:
  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);
});

Виклик jest.mock() з заводським параметром модуля​

jest.mock(path, moduleFactory) takes a module factory argument. A module factory is a function that returns the mock.

In order to mock a constructor function, the module factory must return a constructor function. In other words, the module factory must be a function that returns a function - a higher-order function (HOF).

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

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.

// Зверніть увагу: цей варіант не спрацює
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};
});
// результатом буде ReferenceError
jest.mock('./sound-player', () => {
  return mockSoundPlayer;
});

Заміна імітації з використанням mockImplementation() або 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.

Calls to jest.mock are hoisted to the top of the code. 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. This also allows you to change the mock between tests, if needed:

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();
  });
});

Імітації функцій-конструкторів у деталях​

Building your constructor function mock using jest.fn().mockImplementation() makes mocks appear more complicated than they really are. This section shows how you can create your own mocks to illustrate how mocking works.

Ручна імітація, що сама є класом ES6​

If you define an ES6 class using the same filename as the mocked class in the __mocks__ folder, it will serve as the mock. This class will be used in place of the real class. This allows you to inject a test implementation for the class, but does not provide a way to spy on calls.

For the contrived example, the mock might look like this:

__mocks__/sound-player.js
export default class SoundPlayer {
  constructor() {
    console.log('Mock SoundPlayer: constructor was called');
  }

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

Імітація за допомогою заводських параметрів модуля​

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. Again, this allows you to inject different behavior for testing, but does not provide a way to spy on calls.

* Функція заводського модуля повинна повертати функцію​

In order to mock a constructor function, the module factory must return a constructor function. In other words, the module factory must be a function that returns a function - a higher-order function (HOF).

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

Імітація не може бути стрілковою функцією, тому що в JavaScript виклик new для стрілкової функції не допускається. 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 doesn't have arrow functions nor classes, so both will be transpiled to plain functions.)

Імітація певного методу класу​

Припустимо, ви хочете імітувати або слідкувати за методом playSoundFile всередині класу SoundPlayer. A simple example:

// знизу представлений ваш тестовий jest файл
import SoundPlayer from './sound-player';
import SoundPlayerConsumer from './sound-player-consumer';

const playSoundFileMock = jest
  .spyOn(SoundPlayer.prototype, 'playSoundFile')
  .mockImplementation(() => {
    console.log('mocked function');
  }); // закоментуйте цей рядок, якщо потрібне лише слідкування

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

Статичні методи, getter та setter​

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';
  }
}

Ви можете з легкістю імітувати/слідкувати за ними, як показано в прикладі:

// 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)​

Injecting a test implementation is helpful, but you will probably also want to test whether the class constructor and methods are called with the correct parameters.

Spying on the constructor​

In order to track calls to the constructor, replace the function returned by the HOF with a Jest mock function. 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: () => {}};
  });
});

Це дозволить нам перевірити використання нашого класу-імітації, використовуючи SoundPlayer.mock.calls: expect(SoundPlayer).toHaveBeenCalled(); або майже ідентичний: expect(Sounder.mock.calls.length).toBeGreaterThan(0);

Mocking non-default class exports​

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: () => {}};
    }),
  };
});

Spying on methods of our class​

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. But we'll probably want to also spy on calls to those methods, to ensure that they were called with the expected parameters.

A new object will be created each time the mock constructor function is called during tests. 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
  });
});

The manual mock equivalent of this would be:

__mocks__/sound-player.js
// Імпортуйте цей іменований export у ваш тестовий файл:
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__.

Cleaning up between tests​

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();
});

Complete example​

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();
  // Переконуємось, що конструктор створив об'єкт:
  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);
});