Mock Es6 classes using Jest

Update:

All this info and more has now been added to the Jest docs in a new guide, "ES6 Class Mocks."

Full disclosure: I wrote it. :-)

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The key to mocking ES6 classes is knowing that an ES6 class is a function. Therefore, the mock must also be a function.

1. Call jest.mock('./mocked-class.js');, and also import './mocked-class.js'.
2. For any class methods you want to track calls to, create a variable that points to a mock function, like this: const mockedMethod = jest.fn();. Use those in the next step.
3. Call MockedClass.mockImplementation(). Pass in an arrow function that returns an object containing any mocked methods, each set to its own mock function (created in step 2).
4. The same thing can be done using manual mocks (__mocks__ folder) to mock ES6 classes. In this case, the exported mock is created by calling jest.fn().mockImplementation(), with the same argument described in (3) above. This creates a mock function. In this case, you'll also need to export any mocked methods you want to spy on.
5. The same thing can be done by calling jest.mock('mocked-class.js', factoryFunction), where factoryFunction is again the same argument passed in 3 and 4 above.

An example is worth a thousand words, so here's the code. Also, there's a repo demonstrating all of this, here: https://github.com/jonathan-stone/jest-es6-classes-demo/tree/mocks-working

First, for your code

if you were to add the following setup code, your tests should pass:

const connectMock = jest.fn(); // Lets you check if `connect()` was called, if you want

Socket.mockImplementation(() => {
    return {
      connect: connectMock
    };
  });

(Note, in your code: Socket.mock.calls[0][1] should be [0][0], and [0][2] should be [0][1]. )

Next, a contrived example

with some explanation inline.

mocked-class.js. Note, this code is never called during the test.

export default class MockedClass {
  constructor() {
    console.log('Constructed');
  }

  mockedMethod() {
    console.log('Called mockedMethod');
  }
}

mocked-class-consumer.js. This class creates an object using the mocked class. We want it to create a mocked version instead of the real thing.

import MockedClass from './mocked-class';

export default class MockedClassConsumer {
  constructor() {
    this.mockedClassInstance = new MockedClass('yo');
    this.mockedClassInstance.mockedMethod('bro');
  }
}

mocked-class-consumer.test.js - the test:

import MockedClassConsumer from './mocked-class-consumer';
import MockedClass from './mocked-class';

jest.mock('./mocked-class'); // Mocks the function that creates the class; replaces it with a function that returns undefined.

// console.log(MockedClass()); // logs 'undefined'

let mockedClassConsumer;
const mockedMethodImpl = jest.fn();

beforeAll(() => {
  MockedClass.mockImplementation(() => {
    // Replace the class-creation method with this mock version.
    return {
      mockedMethod: mockedMethodImpl // Populate the method with a reference to a mock created with jest.fn().
    };
  });
});

beforeEach(() => {
  MockedClass.mockClear();
  mockedMethodImpl.mockClear();
});

it('The MockedClassConsumer instance can be created', () => {
  const mockedClassConsumer = new MockedClassConsumer();
  // console.log(MockedClass()); // logs a jest-created object with a mockedMethod: property, because the mockImplementation has been set now.
  expect(mockedClassConsumer).toBeTruthy();
});

it('We can check if the consumer called the class constructor', () => {
  expect(MockedClass).not.toHaveBeenCalled(); // Ensure our mockClear() is clearing out previous calls to the constructor
  const mockedClassConsumer = new MockedClassConsumer();
  expect(MockedClass).toHaveBeenCalled(); // Constructor has been called
  expect(MockedClass.mock.calls[0][0]).toEqual('yo'); // ... with the string 'yo'
});

it('We can check if the consumer called a method on the class instance', () => {
  const mockedClassConsumer = new MockedClassConsumer();
  expect(mockedMethodImpl).toHaveBeenCalledWith('bro'); 
// Checking for method call using the stored reference to the mock function
// It would be nice if there were a way to do this directly from MockedClass.mock
});