If you've started testing your JavaScript code, you will have come across the need to test whether a particular function is being called, or perhaps you have needed to assert what happens when a particular function returns a particular value or throws an error.

In your tests, given you have a function you want to test that takes another function as an argument, you may want to check whether the function you passed in is being called when you call the parent function. So for example, given:

function parent(bool, func) {
  if (bool) {
    func()
  }
}

you would test whether the passed in func was called using a Spy. In our test suite at work, we use the Mocha, Chai and Sinon libraries so this would look something like this for us:

it('should call the given function once if bool is true', () => {
  // 1. set up the spy
  const spy = sinon.spy()

  // 2. call the function being tested passing in the spy
  parent(true, spy)

  // 3. test our expectation
  expect(spy).to.have.been.calledOnce
})

it('should NOT call the given function if bool is false', () => {
  const spy = sinon.spy()
  parent(false, spy)
  expect(spy).not.to.have.been.called
})

Alternatively it may be the case that the output of the function being passed in may be important to the function being tested. For example, given:

function parent(func) {
  return func() ? 'So true' : 'Really false'
}

you would test this function by passing in a Stub as the func argument and controlling what the stub returns when it is called; something like this:

it('should return "So true" when the given function returns true', () => {
  // 1. set up the stub
  const stub = sinon.stub()

  // 2. define what the stub returns when called
  stub.returns(true)

  // 3. call the function being tested passing in the stub
  const output = parent(stub)

  // 4. test our expectation
  expect(output).to.equal('So true')
})

it('should return "Really false" when the given function returns false', () => {
  const stub = sinon.stub()
  stub.returns(false)
  const output = parent(stub)
  expect(output).to.equal('Really false')
})

These are the basics of how you would use spies and stubs in your tests. We have only scratched the surface of what libraries like Sinon give you, but I believe this is a good starting point to be able to understand the concept.