Naming Pets - Walkthrough

const pet = new Pet('Fido');
 
pet.name;

That code should ideally set the value of pet.name as 'Fido'. However - you should have got undefined instead.

This is because the pet object doesn't have a name property.

A good unit test to cover this behaviour might look like this:

describe('constructor', () => {
  ...
 
  it('sets the name property', () => {
    const pet = new Pet('Fido');
 
    expect(pet.name).toEqual('Fido');
  });
});

Running the test should repoduce the unexpected behaviour you saw in the Node REPL:

Expected value to equal:
  "Fido"

Received:
  undefined

Again, this failing test is good - it's one more feature covered, and we can now easily see why it's not working as we'd expect.

To fix the error, we need to set a property on the object being created by the Pet function:

function Pet(name) {
  this.name = name;
}

It is important to note that constructor functions are functions, and are not objects themselves.

When you create an object literal, you would assign properties to an object like so:

const pet = {
  name: 'Fido'
};

or you could also set a property on an existing object like this:

const pet = {};
 
pet.name = 'Fido';

When using a constructor function, we have to use the keyword this.

As well as being confusingly-named, this is a really tricky topic in JavaScript.

We will be going into it in more detail in due course, but for now, you need to know that when used in a constructor function, it refers to the object that will be returned by the function.

So in the case of our Pet function, when we write this.name = name, this refers to the object returned from by calling new Pet(), so this.name refers to the name property of that object.

So our code now allows us to create objects with a given name property. That's exactly what we wanted to achieve, so pat yourselves on the back, add, commit and push your code.