Isolating with spies ;TLDR

Read ⚡️ Stubs first.

A spy or a mock is a function that records the interactions our application has with it. In Jest, a mock function becomes a spy when we use a toHaveBeenCalled or toHaveBeenCalledWith matcher.

.toHaveBeenCalled() - expect a mock function to have been called as part of the test.

.toHaveBeenCalledWith(arg1, arg2) - expect a mock function to have been called with one or more arguments as part of the test.

Let's look at an example:

function VendingMachine () {
  this.total = 0;
}
 
VendingMachine.prototype.purchaseSnack = function purchaseSnack (snack) {
  this.total += snack.price;
 
  snack.addSale()
}
 
function Snack (name, price) {
  this.name = name
  this.price = price
  this.sales = 0
}
 
Snack.prototype.addSale = function addSale () {
  this.sales += 1
}
 
test('snack can be purchased', () => {
  const vendingMachine = new VendingMachine();
  const snack = { name: 'Milkybar', price: 0.60, addSale: jest.fn() }; // stub with spy method
 
  vendingMachine.purchaseSnack(snack);
 
  expect(snack.addSale).toHaveBeenCalled();
})

In the above example, assume that our test is part of the test suite for VendingMachine. We don't want to pass a real Snack object into our VendingMachine's purchaseSnack method, as this wouldn't be an isolated test, so instead we stub snack, ensuring the interface matches that of our Snack prototype/constructor. When our stub does get passed into purchaseSnack, the addSale method gets called, which in this case is our spy. The assertion below expect(snack.addSale).toBeCalled() therefore passes, as the addSale method was called.

Challenge

We have a problem in our Ship test suite currently in that 3 tests directly test the Port object:

Ship
  can set sail
  gets added to port on instantiation
  can dock at a different port

Refactor these tests so that they incorporate spies.

To complete this challenge, you will need to:

Note many of the changes below will happen in the beforeEach function callback. You will need to use the matcher toHaveBeenCalledWith.

• In can set sail you will need to remove the assertion on port.ships and instead assert that ship.setSail calls port.removeShip (where port is a stub, and removeShip is a method on that stub).
• In gets added to port on instantiation you will need to remove the assertion on port.ships and instead assert that port.addShip has been called (again, addShip will be a spy on a port stub).
• In can dock at a different port you will need to remove the assertion on calais.ships and instead assert that calais.addShip has been called with ship (again, addShip will be a spy on a port stub).