javascript - jest.mock(): How to mock ES6 class default import using factory parameter

Question

Mocking ES6 class imports

I'd like to mock my ES6 class imports within my test files.

If the class being mocked has multiple consumers, it may make sense to move the mock into __mocks__, so that all the tests can share the mock, but until then I'd like to keep the mock in the test file.

Jest.mock()

jest.mock() can mock imported modules. When passed a single argument:

jest.mock('./my-class.js');

it uses the mock implementation found in the __mocks__ folder adjacent to the mocked file, or creates an automatic mock.

The module factory parameter

jest.mock() takes a second argument which is a module factory function. For ES6 classes exported using export default, it's not clear what this factory function should return. Is it:

1. Another function that returns an object that mimics an instance of the class?
2. An object that mimics an instance of the class?
3. An object with a property default that is a function that returns an object that mimics an instance of the class?
4. A function that returns a higher-order function that itself returns 1, 2 or 3?

The docs are quite vague:

The second argument can be used to specify an explicit module factory that is being run instead of using Jest's automocking feature:

I'm struggling to come up with a factory definition that can function as a constructor when the consumer imports the class. I keep getting TypeError: _soundPlayer2.default is not a constructor (for example).

I've tried avoiding use of arrow functions (since they can't be called with new) and having the factory return an object that has a default property (or not).

Here's an example. This is not working; all of the tests throw TypeError: _soundPlayer2.default is not a constructor.

Class being tested: sound-player-consumer.js

import SoundPlayer from './sound-player'; // Default import

export default class SoundPlayerConsumer {
  constructor() {
    this.soundPlayer = new SoundPlayer(); //TypeError: _soundPlayer2.default is not a constructor
  }

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

Class being mocked: sound-player.js

export default class SoundPlayer {
  constructor() {
    // Stub
    this.whatever = 'whatever';
  }

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

The test file: sound-player-consumer.test.js

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

// What can I pass as the second arg here that will 
// allow all of the tests below to pass?
jest.mock('./sound-player', function() { 
  return {
    default: function() {
      return {
        playSoundFile: jest.fn()
      };
    }
  };
});

it('The consumer should be able to call new() on SoundPlayer', () => {
  const soundPlayerConsumer = new SoundPlayerConsumer();
  expect(soundPlayerConsumer).toBeTruthy(); // Constructor ran with no errors
});

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

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(SoundPlayer.playSoundFile).toHaveBeenCalledWith(coolSoundFileName);
});

What can I pass as the second arg to jest.mock() that will allow all of the tests in the example pass? If the tests need to be modified that's okay - as long as they still test for the same things.

Answer 1

Updated with a solution thanks to feedback from @SimenB on GitHub.

---

Factory function must return a function

The factory function must return the mock: the object that takes the place of whatever it's mocking.

Since we're mocking an ES6 class, which is a function with some syntactic sugar, then the mock must itself be a function. Therefore the factory function passed to jest.mock() must return a function; in other words, it must be a higher-order function.

In the code above, the factory function returns an object. Since calling new on the object fails, it doesn't work.

Simple mock you can call new on:

Here's a simple version that, because it returns a function, will allow calling new:

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

Note: Arrow functions won't work

Note that our mock can't be an arrow function because we can't call new on an arrow function in Javascript; that's inherent in the language. 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.

Keeping track of usage (spying on the mock)

Not throwing errors is all well and good, but we may need to test whether our constructor was called with the correct parameters.

In order to track calls to the constructor, we can replace the function returned by the HOF with a Jest mock function. We create it with jest.fn(), and then we specify its implementation with mockImplementation().

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

This will let us inspect usage of our mocked class, using SoundPlayer.mock.calls.

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.

Because a new mock object will be created during our tests, SoundPlayer.playSoundFile.calls won't help us. To work around this, we populate playSoundFile with another mock function, and store a reference to that same mock function in our test file, so we can access it during tests.

let mockPlaySoundFile = jest.fn();
jest.mock('./sound-player', () => {
  return jest.fn().mockImplementation(() => { // Works and lets you check for constructor calls
    return { playSoundFile: mockPlaySoundFile }; // Now we can track calls to playSoundFile
  });
});

Complete example

Here's how it looks in the test file:

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

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

it('The consumer should be able to call new() on SoundPlayer', () => {
  const soundPlayerConsumer = new SoundPlayerConsumer();
  expect(soundPlayerConsumer).toBeTruthy(); // Constructor ran with no errors
});

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

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]).toEqual(coolSoundFileName);
});