Babel rewire and expect.js testing

module-testing.js

// my-module.js

function privateFunc () {
  console.log("Some convoluted logic.")

  return 1 + 2
}

export default function () {
  console.log("Hey! I'm the public interface for this module")
  
  const importantNumber = privateFunc()

  return importantNumber
}



// my-module-test.js

import expect from 'expect'
import myModule from './my-module'

// using a spy from the expect library
const privateFuncSpy = expect.createSpy().andReturn(10)
// privateFuncSpy is a function that can track how many times it was called and with what arguments

myModule.__Rewire__('privateFunc', privateFuncSpy)
// ^ using rewire along with a spy

describe('myModule', () => {
  beforeEach(() => {
    // It's a good idea to reset spys before each test
    privateFuncSpy.reset()
  })

  it('calls privateFunc', () => {
    expect(privateFuncSpy).toHaveBeenCalled()
    // Yay!!!
  })

  it('returns whatever privateFunc returns', () => {
    // I made privateFuncSpy return 10 so I can now expect that
    expect(myModule()).toEqual(10)
  })
})