Isolating with stubs - Walkthrough

Steps

• Look for any places in the test suite where objects are passed into methods where it would be straightforward to stub those objects.
• Replace them with stubs.
• Run your tests again to ensure they still pass (might be better to run tests after every change).

Identifying areas to stub

We want to stub out any remaining objects that aren't related to a test suite. The easiest way to find this is to go through a test suite such as Ship.test.js and look for any reference to other constructors such as Port and Itinerary that don't cover the object under test (which of course is Ship).

Looking through the test suites, the only file I can see is Ship.test.js, where Itinerary has been used in the following places:

Ship
  with a port and itinerary
    beforeEach
  can dock at a different port

Replace with stubs

We'll tackle the first area together, which is the beforeEach:

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);
});

Here you want to stub out the Itinerary instance:

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

Remember, our stub has to match the interface of the object we replace, and any properties we instantiate on our original object in order for the test to function have to be duplicated on the stub. Itinerary normally has a ports property, which is set to an array of ports we pass in on instantiation. Therefore, in our stub we add a ports property, and set it to a value of an array containing our Port instance.

Now ride solo and tackle can dock at a different port, stubbing out Itinerary.

Run your tests again to ensure they still pass (might be better to run tests after every change).

(They should).

Add, commit with a meaningful message, and push to GitHub.