Jest: mocking externally dependencies

inject_behaviour.js

// TestSubject.js
const ExternalDependency = require('./ExternalDependency')

module.exports = class TestSubject {
  constructor () {
    this.externalDependency = new ExternalDependency()
  }

  isExternalDependencyMocked () {
    return this.externalDependency.isMocked
  }

  isSpied() {
    this.externalDependency.isSpied()
  }
}

// TestSubject.spec.js
const spy = jest.fn()
const mock = (value) => {
  return () => {
    return class ExternalDependency {
      constructor () {
        this.isMocked = value
      }
      isSpied () {
        spy()
      }
    }
  }

}
jest.resetModules()
jest.doMock('./ExternalDependency', mock('MyMock'))
const ExternalDependencyMocked = require('./ExternalDependency')
const TestSubjectMocked = require('./TestSubject')

describe ('TestSubject', () => {
  it ('tested injected', () => {
    const testSubject = new TestSubjectMocked()
    expect(testSubject.isExternalDependencyMocked()).toEqual('MyMock')
    testSubject.isSpied()
    expect(spy).toHaveBeenCalled()
  })
})

mocking.js

// __mock__/ExternalDependency.js
module.exports = class ExternalDependency {
  constructor () {
    this.isMocked = true
  }
}

// ExternalDependency.js
module.exports = class ExternalDependency {
  constructor () {
    this.isMocked = false
  }
}

// TestSubject.js
const ExternalDependency = require('./ExternalDependency')

module.exports = class TestSubject {
  constructor () {
    this.externalDependency = new ExternalDependency()
  }

  isExternalDependencyMocked () {
    return this.externalDependency.isMocked
  }
}

// TestSubject.spec.js
const TestSubject = require('./TestSubject')

const mock = () => {
  return class ExternalDependency {
    constructor () {
      this.isMocked = true
    }
  }
}

describe ('TestSubject', () => {
  it ('will require and instantiate the real ExternalDependency', () => {
    const testSubject = new TestSubject()
    expect(testSubject.isExternalDependencyMocked()).toEqual(false)
  })
  it ('or the mocked one here', () => {
    jest.resetModules()
    jest.mock('./ExternalDependency')
    // This works fine too:
    // jest.doMock('./ExternalDependency', mock) // you really need doMock here!
    const TestSubjectMocked = require('./TestSubject')
    const testSubject = new TestSubjectMocked()
    expect(testSubject.isExternalDependencyMocked()).toEqual(true)
  })
  it ('and back to the real', () => {
    const testSubject = new TestSubject()
    expect(testSubject.isExternalDependencyMocked()).toEqual(false)
  })
})