Jest mocking default exports - require vs import

Question

I have seen questions referring to the mocking of default exports with jest around here, but I don't think this has already been asked:

When mocking the default export of a dependency of a module that is being tested, the tests suite fails to run if the module imports the dependency with the ES6 import statement, stating TypeError: (0 , _dependency.default) is not a function It succeeds, however, if the module uses a require().default call instead.

In my understanding, import module from location directly translates to const module = require(location).default, so I am very confused why this is happening. I'd rather keep my code style consistent and not use the require call in the original module.

Is there a way to do it?

Test file with mock:

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

jest.mock('./dependency', () => {
   return {
      default: jest.fn()
   };
});

// This is what I would eventually like to call
it('calls the mocked function', () => {
   expect(dependency).toHaveBeenCalled();
});

Dependency.js

export default () => console.log('do something');

module.js (not working)

import dependency from './dependency.js';
dependency();

module.js (working)

const dependency = require('./dependency.js').default;
dependency();

Answer 1

You can use either es6 import or require js to import your js files in your jest tests.

When using es6 import you should know that jest is trying to resolve all the dependencies and also calls the constructor for the class that you are importing. During this step, you cannot mock it. The dependency has to be successfully resolved, and then you can proceed with mocks.

I should also add that as can be seen here jest by default hoists any jest.mocks to the top of the file so the order in which you place your imports does not really matter.

Your problem though is different. Your mock function assumes that you have included your js file using require js.

jest.mock('./dependecy', () => {
   return {
      default: jest.fn()
   };
});

When you import a file using require js, this is the structure it has:

So assuming I have imported my class called "Test" using require js, and it has method called "doSomething" I could call it in my test by doing something like:

const test = require('../Test');
test.default.doSomething();

When importing it using es6 import, you should do it differently though. Using the same example:

import Test from '../Test';
Test.doSomething();

EDIT: If you want to use es6 import change your mock function to:

jest.mock('./dependecy', () => jest.fn());

Answer 2

Have you tried something like this? I was dealing with the default export mocking for months until I found this.

jest.mock('./dependency', () => () => jest.fn());

The idea behind this line is that you are exporting a module that is a function. So you need to let Jest knows that it has to mock all your ./dependency file as a function, that returns a jest.fn()