Jest mock is undefined when inspecting it

Question

I have a module that uses a second module. They're both custom modules, not NPM packages.

I want to make sure that moduleUnderTest calls a specific method from foo.

So I'm using jest.mock and I pass it a function that has the same signature as foo but contains jest spy functions instead of the actual implementation.

I was under the impression that using jest.mock with the mockFoo object would inject the mocked foo instead of the actual module when moduleUnderTest requires foo.

If I inspect what foo is from within moduleUnderTest while the test is running, I can see that it is indeed my mocked foo.

But when I get to the expect, mockedFoo.met1 becomes undefined.

Why is that?

// ../foo/foo.js
const foo = arg => {
  console.log(arg)
  return {
    met1: () => {},
    met2: () => {},
  }
}

module.exports = foo

// ..foo/index.js
// I am doing it this way so I can put the actual implementation in 
// several different files, but can require the whole module with 
// require('../foo') rather than require('../foo/foo')
module.exports = require('./foo')

// ./moduleUnderTest.js
const foo = require('../foo')('hi')

require('util').inspect(foo) // it seems that foo is indeed the mocked
  // version here[0]

const moduleUnderTest = () => {
  foo.met1()
}

module.exports = moduleUnderTest

// ./moduleUnderTest.test.js
const moduleUnderTest = require('./moduleUnderTest')
const mockFoo = () => ({
  met1: jest.fn(),
  met2: jest.fn(),
})
jest.mock('../foo')

test('foo.met1 is called', () => {
  moduleUnderTest()

  expect(mockFoo.met1).toHaveBeenCalledTimes(1) // NOPE![1] 
})

// [0]
//   { met1: 
//     { [Function: mockConstructor]
//       _isMockFunction: true,
//       getMockImplementation: [Function],
//       mock: [Getter/Setter],
//       mockClear: [Function],
//       mockReset: [Function],
//       mockReturnValueOnce: [Function],
//       mockResolvedValueOnce: [Function],
//       mockRejectedValueOnce: [Function],
//       mockReturnValue: [Function],
//       mockResolvedValue: [Function],
//       mockRejectedValue: [Function],
//       mockImplementationOnce: [Function],
//       mockImplementation: [Function],
//       mockReturnThis: [Function],
//       mockName: [Function],
//       getMockName: [Function],
//       mockRestore: [Function] },
//    met2: 
//     { [Function: mockConstructor]
//       _isMockFunction: true,
//       getMockImplementation: [Function],
//       mock: [Getter/Setter],
//       mockClear: [Function],
//       mockReset: [Function],
//       mockReturnValueOnce: [Function],
//       mockResolvedValueOnce: [Function],
//       mockRejectedValueOnce: [Function],
//       mockReturnValue: [Function],
//       mockResolvedValue: [Function],
//       mockRejectedValue: [Function],
//       mockImplementationOnce: [Function],
//       mockImplementation: [Function],
//       mockReturnThis: [Function],
//       mockName: [Function],
//       getMockName: [Function],
//       mockRestore: [Function] } }

// [1]
//  expect(jest.fn())[.not].toHaveBeenCalledTimes()
//  
//  jest.fn() value must be a mock function or spy.
//  Received: undefined

Solution

I found what the issue is.

Because foo is instantiated by executing the function, calling mockFoo.met1 returns undefined since mockFoo is a function. For mock1 to be defined, I'd have to call mockFoo().met1. But at that point I'm getting a new instance of mockFoo which is different from the one loaded and called by moduleUnderTest.

To fix the problem, I have to declare and set the mock this way:

const mockMet1 = jest.fn()
const mockMet2 = jest.fn()
const mockFoo = () => ({
  met1: mockMet1,
  met2: mockMet2,
})
const moduleUnderTest = require('./moduleUnderTest')
jest.mock('../foo', mockFoo)

// ...snip...
afterEach(() => { // resets the called count etc
  mockMet1.resetMock()
  mockMet2.resetMock()
})
// ...snip...
expect(mockMet1).toHaveBeenCalledTimes(1)
// ...snip...

Comments

[Vivek Iyer]

This leads to an error - The second argument of 'jest.mock' must be an inline function. How do we resolve this ?