How to mock an exported const in jest

Question

I have a file that relies on an exported const variable. This variable is set to true but if ever needed can be set to false manually

Answer 1

Thanks to @Luke I was able to expand on his answer for my needs. I had the requirements of:

• Only mocking certain values in the file - not all
• Running the mock only inside a single test.

Turns out that doMock() is like mock() but doesn't get hoisted. In addition requireActual() can be used to grab original data.

My config.js file - I need to mock only part of it

export const SOMETHING = 'blah'
export const OTHER = 'meh'

My test file

// import { someFunc } from  'some/file' // This won't work with doMock - see below
describe('My test', () => {

  test('someFunc() does stuff', async () => {

    // Here I mock the config file which gets imported somewhere deep in my code
    jest.doMock('config.js', () => {

      // Grab original
      const originalModule = jest.requireActual('config')

      // Return original but override some values
      return {
        __esModule: true, // Depends on your setup
        ...originalModule,
        SOMETHING: 'boom!'
      }
    })

    // Because `doMock` doesn't get hoisted we need to import the function after
    const { someFunc } = await import(
      'some/file'
    )

    // Now someFunc will use the original config values but overridden with SOMETHING=boom!
    const res = await someFunc()
  })
})

Depending on other tests you may also need to use resetModules() somewhere such as beforeAll or afterAll.

Docs:

• doMock
• requireActual
• resetModules