Bummer! This is just a preview. You need to be signed in with a Basic account to view the entire video.

Preview

Start a free Basic trial
to watch this video

Sign up for Treehouse

Explanation: Building Constructor Methods

2:45 with Ashley Boucher

Here an explanation for how we'll build the constructor methods for the three classes used in our library application.

Instructions for Workspace

Your instructions are as follows:

1) In the Library.js file, add a constructor method to the Library class. In the constructor method, initialize the following properties: books, patrons.

2) In the Book.js file, add a constructor method to the Book class. In the constructor method, initialize the following properties: title, author, isbn.

3) In the Patron.js file, add a constructor method to the Patron class. In the constructor method, initialize the following properties: name, email, currentBook.

Don't forget to add parameters for any values that need to be passed in to each of the constructor methods.

  • 0:00

    Welcome back, Ashley here.

  • 0:01

    Let's move forward with our library app.

  • 0:03

    So far we've declared three classes, library, book and

  • 0:06

    patron, each one in their own js file.

  • 0:09

    When I'm building out an object oriented app, I like to follow my rough plan and

  • 0:13

    build out my constructor methods for each class first.

  • 0:16

    As a quick recap a constructor method is where we tell the class, the properties.

  • 0:20

    And the property's initial values that any new object

  • 0:23

    of that class type should have available to them.

  • 0:27

    Building the constructor methods first let's me instantiate a few test objects

  • 0:30

    right off the bat.

  • 0:31

    That I can plug some data into and play with as I go.

  • 0:34

    This gives me a way to make sure that my code is working properly along the way.

  • 0:38

    So that's what we'll be doing next.

  • 0:40

    Building out the constructor methods for the library, book and patron classes.

  • 0:44

    Let's start with the library class.

  • 0:46

    To recap, according to our rough plan, the library class has two properties,

  • 0:50

    books and patrons.

  • 0:51

    These are going to be initialized to empty arrays in our constructor method.

  • 0:55

    That's because the library doesn't have any books or

  • 0:57

    patrons yet, it's a clean slate.

  • 0:59

    A user of our library system can add books and

  • 1:02

    patrons to these arrays, through the methods relayed out in our rough.

  • 1:06

    AddBook and addPatron.

  • 1:08

    This means we don't have to pass anything into our constructor method for

  • 1:11

    the library class.

  • 1:13

    As for our book class, we're going to be initializing a few properties

  • 1:16

    in the constructor method as well.

  • 1:18

    Just so you know,

  • 1:19

    properties don't have to be initialized inside a constructor method.

  • 1:23

    They can be initialized, like any variable, in a method on a class as well.

  • 1:27

    If that's the case, that means an object of that class type

  • 1:30

    will only have those properties if that method is called on.

  • 1:35

    Our rough plan says that the book class will have the properties for

  • 1:37

    title, author and isbn.

  • 1:40

    All of these values will need to be passed in.

  • 1:43

    Finally, let's talk about the patron class.

  • 1:45

    The properties initialized in the constructor method are name,

  • 1:48

    email and currentBook.

  • 1:50

    The values for the name and

  • 1:51

    email properties will be passed in when a patron object is created.

  • 1:55

    In the real world, the patron will be providing that information to the library

  • 1:59

    employee who keys it into the system.

  • 2:01

    The currentBook property, however, is a little different.

  • 2:04

    A patron won't have checked out a book when they are first entered into

  • 2:07

    the system.

  • 2:08

    So when the new patron object is created,

  • 2:10

    the value of the currentBook property should probably just be null.

  • 2:14

    We can set that directly inside the constructor method

  • 2:16

    without passing in a value.

  • 2:18

    Later on, when a patron does borrow a book, the checkOut method can update

  • 2:22

    the current book property so it holds the book that the patron is borrowing.

  • 2:26

    Let's add these constructor methods to our class files, then afterwards we can create

  • 2:30

    some test objects and see what they look like and how they work so far.

  • 2:34

    Head to the attached work space to see the updated read me with new instructions.

  • 2:38

    Or see the teachers notes attached to this video.

  • 2:41

    Once you're finished or

  • 2:41

    if you're having trouble, check out the solution following this video.

You need to sign up for Treehouse in order to download course files.

Sign up