Isolating with spies - Walkthrough

Steps

• 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).

can set sail

First of all, we want to stop using Port and instead use a stub we'll create a stub object that has the same interface as Port, ensuring the methods on our object are spies so we can listen in on them in our test specs. Inside Ship.test.js - in the beforeEach - we want to change the following:

beforeEach(() => {
  dover = new Port('Dover');
  calais = new Port('Calais');
  itinerary = new Itinerary([dover, calais]);
  ship = new Ship(itinerary);
});

to:

let dover;
let calais;
 
beforeEach(() => {
  dover = {
    addShip: jest.fn(),
    removeShip: jest.fn(),
    name: 'Dover',
    ships: []
  };
 
  calais = {
    addShip: jest.fn(),
    removeShip: jest.fn(),
    name: 'Calais',
    ships: []
  };
 
  itinerary = new Itinerary([dover, calais]);
  ship = new Ship(itinerary);
});

Then we can remove our assertion that port.ships no longer contains our ship. We have to do so, as the reasoning for even having this test was that we knew if port.ships did contain the ship, that our setSail method was making the call to port.removeShip. Now we're isolating our test and stubbing Port, we can do this more efficiently with a spy.

Instead of asserting on port.ships, we'll assert that our Ship instance's setSail method calls the removeShip method on our Port-like stub. Because we've tested removeShip separately inside Port.test.js, we can have complete confidence that despite test isolation, our application will have full test coverage.

This can be demonstrated can set sail test. Change your test code from:

it('can set sail', () => {
  ship.setSail();
 
  expect(ship.currentPort).toBeFalsy();
  expect(dover.ships).not.toContain(ship);
});

To:

it('can set sail', () => {
  ship.setSail();
 
  expect(ship.currentPort).toBeFalsy();
  expect(dover.removeShip).toHaveBeenCalledWith(ship);
});

gets added to port on instantiation

Our Ship constructor calls a port's addShip method in its constructor, so our Port-like stub's addShip method gets called in the beforeEach callback prior to each test running. Therefore, we just have to assert in this test that addShip was called.

Change:

it('gets added to port on instantiation', () => {
  expect(dover.ships).toContain(ship);
});

to:

it('gets added to port on instantiation', () => {
  expect(dover.addShip).toHaveBeenCalledWith(ship);
});

can dock at a different port

This one's on you! Continue with the calais stub of Port.

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