Mock an entire module with Jest in Javascript

Question

I searched for a very long time how to mock any module with jest (like rewire does). I finally manage to do it this way, and it work like a charm :

jest.mock('common/js/browser-utils', () => ({ openBrowser: jest.fn() })); const { openBrowser: openBrowserSpy } = jest.requireMock( 'common/js/browser-utils' );

But i wonder if there is another fast way to do so ? I saw the genMockFromModule method but i never makes it work (maybe it's not for that usage.)

What i want is simple : mocking a module by a jest.fn() (or any auto mechanism), then being able to access this jest.fn() in my tests (here: openBrowserSpy) to expect(assertions) on it

Brian Adams · Accepted Answer · 2019-03-16 22:28:28Z

You can just auto-mock the module using jest.mock:

jest.mock('common/js/browser-utils');

The docs could probably be improved with a better description of what "auto-mocked version" means, but what happens is that Jest keeps the API surface of the module the same while replacing the implementation with empty mock functions.

Complete example

browser-utils.js

export const openBrowser = () => { /* do something */ };

code.js

import { openBrowser } from './browser-utils'; export const func = () => { /* do stuff */ openBrowser(); /* do other stuff */ }

code.test.js

jest.mock('./browser-utils'); // create an auto-mock of the module import { openBrowser } from './browser-utils'; // openBrowser is already an empty mock function import { func } from './code'; test('func', () => { func(); expect(openBrowser).toHaveBeenCalled(); // Success! });

Bonus: Mock single function

To mock a single function you can use jest.spyOn like this:

import * as browserUtils from './browser-utils'; import { func } from './code'; test('func', () => { const spy = jest.spyOn(browserUtils, 'openBrowser'); spy.mockImplementation(); // replace implementation with empty mock function (optional) func(); expect(spy).toHaveBeenCalled(); // Success! });

Thank you very much, that was exactly what i searched for. What blows my mind was to do a jest.mock BEFORE any import.
@oopinou you're welcome! ...and actually in this case it doesn't matter if jest.mock comes first or not since babel-jest hoists calls to jest.mock to the top of the code block so any jest.mock calls always happen first.
Actually i'm not using babel but typescript, which has the same kind of behaviour i suppose
Can anyone speak to the difference between using jest.mock and jest.requireMock?

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