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

There are many things to have to consider when you want to mock ES module.

1. You have to pass --experimental-vm-modules parameter to Jest
2. Use jest.unstable_mockModule instead of jest.mock function
3. You have to use await import instead of import from
4. Mock the module before you import it

I spent a lot of time fixing all of the issues I had around mocking ESM modules in both Javascript and Typescript. That's why I decided to create a repository to show a complete full-working example. You can have a look if you want: https://github.com/habibseifzadeh/jest-esm-mock-examples

Answer 2

None of the answers here seemed to work for me (the original function was always being imported rather than the mock), and it seems that ESM support in Jest is still work in progress.

After discovering this comment, I found out that jest.mock() does not actually work with regular imports, because the imports are always run before the mock (this is now also officially documented). Because of this, I am importing my dependencies using await import(). This even works with a top-level await, so I just have to adapt my imports:

import { describe, expect, it, jest } from '@jest/globals';

jest.unstable_mockModule('../dependency', () => ({
  doSomething: jest.fn()
}));

const myModule = await import('../myModule');
const dependency = await import('../dependency');

describe('myModule', async () => {
  it('calls the dependency with double the input', () => {
    myModule(2);
    expect(dependency.doSomething).toBeCalledWith(4);
  });
});

Answer 3

I tried all the solutions and none worked or were showing lots of TS errors.

This is how I solved it:

format.ts file:

import camelcaseKeys from 'camelcase-keys'
import parse from 'xml-parser'

class Format {
  parseXml (xml: string) {
    return camelcaseKeys(parse(xml), {
      deep: true,
    })
  }
}

const format = new Format()
export { format }

format.test.ts file:

import format from './format'
import camelcaseKeys from 'camelcase-keys'
import parse from 'xml-parser'

jest.mock('xml-parser', () => jest.fn().mockReturnValue('parsed'))
jest.mock('camelcase-keys', () => jest.fn().mockReturnValue('camel cased'))

describe('parseXml', () => {
  test('functions called', () => {
    const result = format.parseXml('XML')

    expect(parse).toHaveBeenCalledWith('XML')
    expect(camelcaseKeys).toHaveBeenCalledWith('parsed', { deep: true })
    expect(result).toBe('camel cased')
  })
})

Answer 4

I made some modifications on @cam-jackson original answer and side effects has gone. I used lodash library to deep clone the object under test and then made any modification I want on that object. But be ware that cloning heavy objects can have negative impact on test performance and test speed.

objectUndertest.js

const objectUnderTest = {};
export default objectUnderTest;

objectUnderTest.myFunctionUnterTest = () => {
  return "this is original function";
};

objectUndertest.test.js

import _ from "lodash";
import objectUndertest from "./objectUndertest.js";

describe("objectUndertest", () => {
  let mockObject = objectUndertest;

  beforeEach(() => {
    mockObject = _.cloneDeep(objectUndertest);
  });

  test("test function", () => {
    mockObject.myFunctionUnterTest = () => {
      return "this is mocked function.";
    };

    expect(mockObject.myFunctionUnterTest()).toBe("this is mocked function.");
  });
});

Answer 5

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
  });
});

Answer 6

Fast forwarding to 2020, I found this blog post to be the solution: Jest mock default and named export

Using only ES6 module syntax:

// esModule.js
export default 'defaultExport';
export const namedExport = () => {};

// esModule.test.js
jest.mock('./esModule', () => ({
  __esModule: true, // this property makes it work
  default: 'mockedDefaultExport',
  namedExport: jest.fn(),
}));

import defaultExport, { namedExport } from './esModule';
defaultExport; // 'mockedDefaultExport'
namedExport; // mock function

Also one thing you need to know (which took me a while to figure out) is that you can't call jest.mock() inside the test; you must call it at the top level of the module. However, you can call mockImplementation() inside individual tests if you want to set up different mocks for different tests.

Answer 7

The question is already answered, but you can resolve it like this:

File dependency.js

const doSomething = (x) => x
export default doSomething;

File myModule.js

import doSomething from "./dependency";

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

File myModule.spec.js

jest.mock('../dependency');
import doSomething from "../dependency";
import myModule from "../myModule";

describe('myModule', () => {
  it('calls the dependency with double the input', () => {
    doSomething.mockImplementation((x) => x * 10)

    myModule(2);

    expect(doSomething).toHaveBeenCalledWith(4);
    console.log(myModule(2)) // 40
  });
});

Answer 8

I solved this another way. Let's say you have your dependency.js

export const myFunction = () => { }

I create a depdency.mock.js file besides it with the following content:

export const mockFunction = jest.fn();

jest.mock('dependency.js', () => ({ myFunction: mockFunction }));

And in the test, before I import the file that has the dependency, I use:

import { mockFunction } from 'dependency.mock'
import functionThatCallsDep from './tested-code'

it('my test', () => {
    mockFunction.returnValue(false);

    functionThatCallsDep();

    expect(mockFunction).toHaveBeenCalled();

})

Answer 9

To mock an ES6 dependency module default export using Jest:

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

jest.mock('../dependency');

// If necessary, you can place a mock implementation like this:
dependency.mockImplementation(() => 42);

describe('myModule', () => {
  it('calls the dependency once with double the input', () => {
    myModule(2);

    expect(dependency).toHaveBeenCalledTimes(1);
    expect(dependency).toHaveBeenCalledWith(4);
  });
});

The other options didn't work for my case.

Answer 10

Adding more to Andreas' answer. I had the same problem with ES6 code, but I did not want to mutate the imports. That looked hacky. So I did this:

import myModule from '../myModule';
import dependency from '../dependency';
jest.mock('../dependency');

describe('myModule', () => {
  it('calls the dependency with double the input', () => {
    myModule(2);
  });
});

And added file dependency.js in the " __ mocks __" folder parallel to file dependency.js. This worked for me. Also, this gave me the option to return suitable data from the mock implementation. Make sure you give the correct path to the module you want to mock.

Answer 11

You have to mock the module and set the spy by yourself:

import myModule from '../myModule';
import dependency from '../dependency';
jest.mock('../dependency', () => ({
  doSomething: jest.fn()
}))

describe('myModule', () => {
  it('calls the dependency with double the input', () => {
    myModule(2);
    expect(dependency.doSomething).toBeCalledWith(4);
  });
});