Mocking and testing require.resolve calls in jest

Question

I want to mock a node's built-in function require.resolve in jest test suite. Here a working example of a problem on repl.it and the code itself:

Test subject (as an example):

const requirer = () => {
  try {
    return require.resolve('./add')
  } catch (error) {
    console.error('failed to find a module')
  }
}

module.exports = requirer

Test suite:

const requirer = require('./requirer')

describe('attempt to mock require.resolve', () => {
   it('does not work', () => {
     require.resolve = jest.fn(arg => `./${arg}`)
     console.log(
       'is require.resolve mocked',
       jest.isMockFunction(require.resolve)) // will say true

     requirer()

     expect(require.resolve).toHaveBeenCalledTimes(1)
     expect(require.resolve).toBeCalledWith('')
  })
})

Inside of a test suite declaration everything is OK (see an output of a jest.isMockFunction(require.resolve)) and mock works. But for a test subject require.resolve remains with original functionality.

Due to this issue, this isn't pure unit test.

If I, for example, mock process.exit everything works as expected.

Answer 1

So, not a perfect solution, but it is simpler to inject (DI) require.resolve into

const requirer = (resolver) => {
  try {
    return resolver('./add')
  } catch (error) {
    console.error('failed to find a module')
  }
}

module.exports = requirer

now in a test suite passing down mocked version of a require.resolve works as expected

const requirer = require('./requirer')

describe('attempt to mock require.resolve', () => {
   it('works', () => {
     require.resolve = jest.fn(arg => `./${arg}`)
     console.log(
       'is require.resolve mocked',
       jest.isMockFunction(require.resolve)) // will say true

     requirer(require.resolve)

     expect(require.resolve).toHaveBeenCalledTimes(1)
     // expect(require.resolve).toBeCalledWith('')
  })
})