Jasmine/JavaScript: Test that a new object was created

My goal it to assert that new Bassist() was called by the start() method and create a mock object, instead.

This the second test is currently failing with the error "Expected spy constructor to have been called."

class Bassist {
  constructor() {
    console.log('bassist joined')
  }
}

class Drummer {
  constructor() {
    console.log('drummer joined')
  }
}

class Band {
  constructor(name) {
    this.name = name
    this.bassist = null
    this.drummer = null
  }

  start() {
    this.bassist = new Bassist()
    this.drummer = new Drummer()
  }
}

describe('Band', () => {
  describe('start()', () => {
    it('sets the bassist property', () => {
      let band = new Band('The Foobars')
      band.start()
      expect(band.bassist).not.toBeNull()
    })

    it('creates a new bassist', () => {
      spy = spyOn(Bassist.prototype, 'constructor')
      band = new Band('The Foobars')
      band.start()
      expect(spy).toHaveBeenCalled()
      console.log('expect was called')
    })
  })
})

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Solution

After a lot of trial and error (and searching), I wasn't able to override the call to new Bassist() and replace it with a spy object. Instead, I created a separate method within the Band class to handle Bassist object creation, and then put a spy on it.

Who knows if this is good practice, but it works for me.

class Bassist {
  constructor() {
    console.log('bassist joined')
  }
}

class Band {
  constructor(name) {
    this.name = name
    this.bassist = null
  }

  start() {
    this.bassist = this.createBassist()
  }

  createBassist() {
    return new Bassist()
  }
}

describe('Band', () => {
  let band, createBassistSpy, bassistSpy

  beforeEach(() => {
    // setup spies
    bassistSpy = jasmine.createSpyObj('bassist', ['shred', 'quit', 'otherSickMethod'])
    createBassistSpy = spyOn(Band.prototype, 'createBassist')
     .and.returnValue(bassistSpy)

    // init subject being tested
    band = new Band('The Foobars')
  })

  describe('new', () => {
    it('initializes bassist to null', () => {
      expect(band.bassist).toBe(null)
    })
  })

  describe('start()', () => {
    it('calls the createBassist() method', () => {
      band.start()
      expect(createBassistSpy).toHaveBeenCalled()

      // Not a great test, but ensures we are working with the
      // bassistSpy object, and not something else.
      expect(band.bassist).toEqual(bassistSpy)
    })

    it('sets the bassist property', () => {
      band.start()
      expect(band.bassist).not.toBeNull()
    })
  })
})

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