How to augment instances of a mocked constructor in Jest

Question

I'd like to augment, but not completely replace, instances of a mocked constructor in a Jest unit test.

I want to add a few values to the instance, but keep the auto-mocked goodness of Jest.

For example:

A.js

module.exports = class A {
  constructor(value) {
    this.value = value;
  }
  getValue() {
    return this.value;
  }
}

To get some auto-mock awesomeness:

jest.mock('./A');

With the automock, instances have a mocked .getValue() method, but they do not have the .value property.

A documented way of mocking constructors is:

// SomeClass.js
module.exports = class SomeClass {
  m(a, b) {}
}

// OtherModule.test.js
jest.mock('./SomeClass');  // this happens automatically with automocking
const SomeClass = require('./SomeClass')
const mMock = jest.fn()
SomeClass.mockImplementation(() => {
  return {
    m: mMock
  }
})

const some = new SomeClass()
some.m('a', 'b')
console.log('Calls to m: ', mMock.mock.calls)

Using that approach for A:

jest.mock('./A');

const A = require('./A');

A.mockImplementation((value) => {
  return { value };
});

it('does stuff', () => {
  const a = new A();
  console.log(a); // -> A { value: 'value; }
});

The nice thing about that is you can do whatever you want to the returned value, like initialize .value.

The downsides are:

You don't get any automocking for free, e.g. I'd need to add .getValue() myself to the instance
You need to have a different jest.fn() mock function for each instance created, e.g. if I create two instances of A, each instance needs its own jest.fn() mock functions for the .getValue() method
SomeClass.mock.instances is not populated with the returned value (GitHub ticket)

One thing that didn't work (I was hoping that maybe Jest did some magic):

A.mockImplementation((value) => {
  const rv = Object.create(A.prototype); // <- these are mocked methods
  rv.value = value;
  return rv;
});

Unfortunately, all instances share the same methods (as one would expect, but it was worth a shot).

My next step is to generate the mock, myself, via inspecting the prototype (I guess), but I wanted to see if there is an established approach.

Thanks in advance.

tiffon tiffon · Accepted Answer · 2019-02-24T06:23:00

Turns out this is fixed (as of jest 24.1.0) and the code in the question works, as expected.

To recap, given class A:

A.js

module.exports = class A {
  constructor(value) {
    this.value = value;
  }
  setValue(value) {
    this.value = value;
  }
}

This test will now pass:

A.test.js

jest.mock('./A');

const A = require('./A');

A.mockImplementation((value) => {
  const rv = Object.create(A.prototype); // <- these are mocked methods
  rv.value = value;
  return rv;
});

it('does stuff', () => {
  const a = new A('some-value');
  expect(A.mock.instances.length).toBe(1);
  expect(a instanceof A).toBe(true);
  expect(a).toEqual({ value: 'some-value' });
  a.setValue('another-value');
  expect(a.setValue.mock.calls.length).toBe(1);
  expect(a.setValue.mock.calls[0]).toEqual(['another-value']);
});

How to augment instances of a mocked constructor in Jest

2 Answers