Isolating with dummies - Walkthrough

Steps

Look for any places in the test suite where objects are passed into methods where those methods don't use the object.
Replace them with dummies.
Run your tests again to ensure they still pass (might be better to run tests after every change).

There are three tests where we could utilise dummies:

Port
  can add a ship
  can remove a ship

Itinerary
  can have ports

Have a look at these tests and see if you can see why and where test dummies could be incorporated.

We'll do one together - can add a ship:

Change:

const ship = {};

To:

const ship = jest.fn();

You should be able to add dummies to the other two tests without the walkthrough.

(They should).