Port Object - Walkthrough

Steps

• Discuss with your classmates how the domain model might be added to in this scenario.
• Create a new test file, which should describe a new Port object.
• Create a new test spec to check the new Port object can be instantiated.
• Write the code that makes this test pass.
• Create a new test spec to check the new Port object has a name property.
• Write the code that makes this test pass.
• Refactor the Ship test suite so a Ship takes a Port object instead of a string and check tests still pass.
• Create a new test spec inside Ship.test.js to test a dock method.
• Write the code that makes this test pass.
• Add, commit with a meaningful message, and push to GitHub.

Domain Model

As a cruise ship captain,
So I can get passengers to a new destination,
I want a ship to be able to dock at a different port.

The domain model might now look like this:

:exclamation: Note that startingPort has disappeared from Ship and currentPort has been introduced instead. It makes more sense - now a ship can be at different ports - that this property be renamed. Make sure you rename this property in your Ship constructor and tests, and run your tests after to ensure everything still passes.

Create a new test file Port.test.js, which should describe Port

:exclamation: You should be able to do this without the walkthrough. Once you've passed the test, then proceed.

Create a new test spec to check the new object can be instantiated.

:exclamation: You should be able to do this without the walkthrough. Once you've passed the test, then proceed.

Write the code to make the test pass

Create a new test spec to check the new Port object has a name property.

:exclamation: You should be able to do this without the walkthrough. Once you've passed the test, then proceed.

Write the code to make the test pass

Refactor the Ship test suite so a Ship takes a Port object instead of a string and check tests still pass.

Now we have a Port object, it makes no sense to pass a string of 'Dover' to each Ship we instantiate in our tests. Instead, we can instantiate a new Port passing in 'Dover' as its name, and we can pass the new instance into Ship.

First, we require in Port into our Ship.test.js file:

Then we'll look for places where we've instantiated a Ship and we'll pass in a Port instance instead of a string. We also have to change any expectations that expect 'Dover':

This is called dependency inversion - the premise that an object (in this case Ship) can depend on other objects, but it shouldn't know what they are. All Ship knows is that it's receiving an argument passed into it's constructor, and its assigning it as a property. It doesn't know it's a Port object. This is good as it means a Port is substitutable and as such our code is more modular.

We should - after a cheeky Port :wine_glass: refactor - still be green on our tests:

Create a new test spec inside Ship.test.js to test a dock method.

When we call the dock method on a Ship, we want to change the state on our ship a.k.a the ship's currentPort property needs to be updated accordingly. Lets see how the test might look:

Here we create a Port: dover, and pass it to a new Ship as part of the setup process.

Then - as part of the exercise process - we create a new Port: calais, and we call ship.dock passing in calais.

Finally - as party of the verify process - we expect the Ship's currentPort to be calais.

Run your tests. They should fail with :red_circle: TypeError: ship.dock is not a function.

Write the code that makes this test pass.

In our Ship code, we make no further assumptions about what currentPort is. Because we've followed the dependency inversion principle, we can just assign to properties whatever gets passed into our constructor and methods as arguments. Therefore, dock really is as simple as re-assigning to the currentPort property:

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