Isolating with dummies ;TLDR

This is the first step of several, covering how we can isolate our unit tests so that a single test-suite only tests the object at hand (e.g. in Ship.test.js we only want to test Ship - however we are using real Port and Itinerary objects to do so). For a recap on test doubles, you can rewatch the lecture here.

A dummy is a test double that acts as a named placeholder - we pass it into a method when that method has a required parameter and only intends to pass around the passed argument (i.e. it doesn't expect to call one of its methods or access a property). A good example might be if you have a method that adds an object to an array, and then you assert on the array that the object exists. You aren't actually using the object in any way - just passing it around.

In Ports have Ships chapter we created a ship variable with the value of an empty object to use as a placeholder for Ship in Port tests. The ship is a dummy as Port does not access any of its properties or methods, it just adds it to the ships array.

In Jest, we can also use jest.fn() to create a dummy. E.g:

const basket = new Basket()
const product = jest.fn()
 
basket.addProduct(product)
 
expect(basket.products).toContain(product)

Challenge

Incorporate dummies into your test suites in places where they could be used instead of the actual objects.

To complete this challenge, you will need to:

• 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).

Recommended Reading

• TDD Terminology Simplified - there are some concepts in here we don't cover, but a useful glossary for many of the terms used around TDD.