Jest: Mock ES6 Module with both default and named export

Question

I have an ES6 module with both a default and a named export:

/** /src/dependency.js **/

export function utilityFunction() { return false; }

export default function mainFunction() { return 'foo'; }

Its being used by a second ES6 module:

/** /src/myModule.js **/

import mainFunction, { utilityFunction } from './dependency';

// EDIT: Fixed syntax error in code sample
// export default myModule() {
export default function myModule() {
    if (!utilityFunction()) return 2;
    return mainFunction();
}

I'm trying to write a unit test for myModule.js using Jest. But when I try to mock both the named and the default import, Jest appears to only mock the named import. It continues to use the actual implementation of the default import, and doesn't allow me to mock it, even after I call .mockImplementation(). Here's the code I'm trying to use:

/** 
  * Trying to mock both named and default import. 
  * THIS DOESN'T WORK.
  */

/** /tests/myModule.test.js **/

import mainFunction, { utilityFunction } from '../src/dependency';
import myModule from '../src/myModule';

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

mainFunction.mockImplementation(() => 1);
utilityFunction.mockImplementation(() => true);

describe('myModule', () => {
    it('should return the return value of mainFunction when the result of utilityFunction is true', () => {
        expect(myModule()).toEqual(1); // FAILS - actual result is 'foo'
    });
});

This behavior seems really strange to me, because when mocking JUST default imports or JUST named imports, this API works fine. For example, in the case where myModule.js only imports a default import, this is pretty easily done:

/**
  * Trying to mock just the default import. 
  * THIS WORKS.
  */

/** /src/myModule.js **/

import mainFunction from './dependency';

// EDIT: Fixed syntax error in code sample
// export default myModule() {
export default function myModule() {
    return mainFunction();
}


/** /tests/myModule.test.js **/
// If only mainFunction is used by myModule.js

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

jest.mock('../src/dependency');
mainFunction.mockImplementation(() => 1);

describe('myModule', () => {
    it('should return the return value of mainFunction', () => {
        expect(myModule()).toEqual(1); // Passes
    });
});

In the case where only the named 'utilityFunction' export is used, its also pretty easy to mock the import:

/**
  * Trying to mock both named and default import. 
  * THIS WORKS.
  */
/** /src/myModule.js **/

import { utililtyFunction } from './dependency';

// EDIT: Fixed syntax error in code sample
// export default myModule()
export default function myModule() {
    return utilityFunction();
}


/** /tests/myModule.test.js **/
// If only utilityFunction is used by myModule.js

import { utilityFunction } from '../src/dependency';
import myModule from '../src/myModule';

jest.mock('../src/dependency);
utilityFunction.mockImplementation(() => 'bar');

describe('myModule', () => {
    it('should return the return value of utilityFunction', () => {
        expect(myModule()).toEqual('bar'); // Passes
    });
});

Is it possible to mock both the named and default import using Jest? Is there a different syntax that I can use to achieve the result I want, where I import both named and default values from a module and am able to mock them both?

Hi, Did you tried my answer? I think that's what you're looking for. Also, If it works, could you put the best answer? — Albert Olivé

djfdev djfdev · Accepted Answer · 2018-02-14T23:53:04

You have a syntax error ... the function keyword is omitted from the default export in myModule.js. Should look like this:

import mainFunction, { utilityFunction } from './dependency';

export default function myModule() {
    if (!utilityFunction()) return 2;
    return mainFunction();
}

I'm not sure how you got the test to run otherwise, but I just tried this out locally and it passed.

Jest: Mock ES6 Module with both default and named export

2 Answers