unit testing - How to mock imported named function in Jest when module is unmocked

Question

I have the following module I'm trying to test in Jest:

// myModule.js

export function otherFn() {
  console.log('do something');
}

export function testFn() {
  otherFn();

  // do other things
}

As shown above, it exports some named functions and importantly testFn uses otherFn.

In Jest when I'm writing my unit test for testFn, I want to mock the otherFn function because I don't want errors in otherFn to affect my unit test for testFn. My issue is that I'm not sure the best way to do that:

// myModule.test.js
jest.unmock('myModule');

import { testFn, otherFn } from 'myModule';

describe('test category', () => {
  it('tests something about testFn', () => {
    // I want to mock "otherFn" here but can't reassign
    // a.k.a. can't do otherFn = jest.fn()
  });
});

Any help/insight is appreciated.

Answer 1

Use jest.requireActual() inside jest.mock()

jest.requireActual(moduleName)

Returns the actual module instead of a mock, bypassing all checks on whether the module should receive a mock implementation or not.

Example

I prefer this concise usage where you require and spread within the returned object:

// myModule.test.js

import { otherFn } from './myModule.js'

jest.mock('./myModule.js', () => ({
  ...(jest.requireActual('./myModule.js')),
  otherFn: jest.fn()
}))

describe('test category', () => {
  it('tests something about otherFn', () => {
    otherFn.mockReturnValue('foo')
    expect(otherFn()).toBe('foo')
  })
})

This method is also referenced in Jest's Manual Mocks documentation (near the end of Examples):

To ensure that a manual mock and its real implementation stay in sync, it might be useful to require the real module using jest.requireActual(moduleName) in your manual mock and amending it with mock functions before exporting it.