Tag: js

Jest’s support for mocks with CommonJS modules works great, they’re easy to use and it works great, as per the docs.

With ES Modules on the other hand, the docs are vague, and after spending several hours trying to get them to work with code I needed to test and not having any success, I spent a few more hours getting it to work with even the simplest example code.

This current issue describes multiple approaches which others have had varying success with, but there’s one tip mentioned that I can confirm is key to getting the mocks to work with ES modules and this is currently not mentioned in the docs (at least when I last looked) – you must await the import for the module you are mocking. This needs to be combined with the following:

1. jest.unstable_mockModule() need to be called at the top level, and must be before the import for the module you are mocking
2. You must await the import

To explain the last point, you must use this:

const { example2WithModule } = await import(‘./example.js’);

and not this:

import { example2WithModule } from ‘./example.js’;

Here’s an example of my working usage of jest.unstable_mockModule():

jest.unstable_mockModule('./example-module.js', () => ({
    exampleFunctionFromModule: jest.fn(
        () => {
            return 'mocked return!';
        }
    )
}));