javascript - How can I mock an ES6 module import using Jest?

Question

I want to test that one of my ES6 modules calls another ES6 module in a particular way. With Jasmine this is super easy --

The application code:

// myModule.js
import dependency from './dependency';

export default (x) => {
  dependency.doSomething(x * 2);
}

And the test code:

//myModule-test.js
import myModule from '../myModule';
import dependency from '../dependency';

describe('myModule', () => {
  it('calls the dependency with double the input', () => {
    spyOn(dependency, 'doSomething');

    myModule(2);

    expect(dependency.doSomething).toHaveBeenCalledWith(4);
  });
});

What's the equivalent with Jest? I feel like this is such a simple thing to want to do, but I've been tearing my hair out trying to figure it out.

The closest I've come is by replacing the imports with requires, and moving them inside the tests/functions. Neither of which are things I want to do.

// myModule.js
export default (x) => {
  const dependency = require('./dependency'); // Yuck
  dependency.doSomething(x * 2);
}

//myModule-test.js
describe('myModule', () => {
  it('calls the dependency with double the input', () => {
    jest.mock('../dependency');

    myModule(2);

    const dependency = require('../dependency'); // Also yuck
    expect(dependency.doSomething).toBeCalledWith(4);
  });
});

For bonus points, I'd love to make the whole thing work when the function inside dependency.js is a default export. However, I know that spying on default exports doesn't work in Jasmine (or at least I could never get it to work), so I'm not holding out hope that it's possible in Jest either.

Answer 1

Edit: Several years have passed and this isn't really the right way to do this any more (and probably never was, my bad).

Mutating an imported module is nasty and can lead to side effects like tests that pass or fail depending on execution order.

I'm leaving this answer in its original form for historical purposes, but you should really use jest.spyOn or jest.mock. Refer to the jest docs or the other answers on this page for details.

Original answer follows:

---

I've been able to solve this by using a hack involving import *. It even works for both named and default exports!

For a named export:

// dependency.js
export const doSomething = (y) => console.log(y)

// myModule.js
import { doSomething } from './dependency';

export default (x) => {
  doSomething(x * 2);
}

// myModule-test.js
import myModule from '../myModule';
import * as dependency from '../dependency';

describe('myModule', () => {
  it('calls the dependency with double the input', () => {
    dependency.doSomething = jest.fn(); // Mutate the named export

    myModule(2);

    expect(dependency.doSomething).toBeCalledWith(4);
  });
});

Or for a default export:

// dependency.js
export default (y) => console.log(y)

// myModule.js
import dependency from './dependency'; // Note lack of curlies

export default (x) => {
  dependency(x * 2);
}

// myModule-test.js
import myModule from '../myModule';
import * as dependency from '../dependency';

describe('myModule', () => {
  it('calls the dependency with double the input', () => {
    dependency.default = jest.fn(); // Mutate the default export

    myModule(2);

    expect(dependency.default).toBeCalledWith(4); // Assert against the default
  });
});