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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.

Welcome back, Ashley here. 0:00
Let's move forward with our library app. 0:01
So far we've declared three classes, library, book and 0:03
patron, each one in their own js file. 0:06
When I'm building out an object oriented app, I like to follow my rough plan and 0:09
build out my constructor methods for each class first. 0:13
As a quick recap a constructor method is where we tell the class, the properties. 0:16
And the property's initial values that any new object 0:20
of that class type should have available to them. 0:23
Building the constructor methods first let's me instantiate a few test objects 0:27
right off the bat. 0:30
That I can plug some data into and play with as I go. 0:31
This gives me a way to make sure that my code is working properly along the way. 0:34
So that's what we'll be doing next. 0:38
Building out the constructor methods for the library, book and patron classes. 0:40
Let's start with the library class. 0:44
To recap, according to our rough plan, the library class has two properties, 0:46
books and patrons. 0:50
These are going to be initialized to empty arrays in our constructor method. 0:51
That's because the library doesn't have any books or 0:55
patrons yet, it's a clean slate. 0:57
A user of our library system can add books and 0:59
patrons to these arrays, through the methods relayed out in our rough. 1:02
AddBook and addPatron. 1:06
This means we don't have to pass anything into our constructor method for 1:08
the library class. 1:11
As for our book class, we're going to be initializing a few properties 1:13
in the constructor method as well. 1:16
Just so you know, 1:18
properties don't have to be initialized inside a constructor method. 1:19
They can be initialized, like any variable, in a method on a class as well. 1:23
If that's the case, that means an object of that class type 1:27
will only have those properties if that method is called on. 1:30
Our rough plan says that the book class will have the properties for 1:35
title, author and isbn. 1:37
All of these values will need to be passed in. 1:40
Finally, let's talk about the patron class. 1:43
The properties initialized in the constructor method are name, 1:45
email and currentBook. 1:48
The values for the name and 1:50
email properties will be passed in when a patron object is created. 1:51
In the real world, the patron will be providing that information to the library 1:55
employee who keys it into the system. 1:59
The currentBook property, however, is a little different. 2:01
A patron won't have checked out a book when they are first entered into 2:04
the system. 2:07
So when the new patron object is created, 2:08
the value of the currentBook property should probably just be null. 2:10
We can set that directly inside the constructor method 2:14
without passing in a value. 2:16
Later on, when a patron does borrow a book, the checkOut method can update 2:18
the current book property so it holds the book that the patron is borrowing. 2:22
Let's add these constructor methods to our class files, then afterwards we can create 2:26
some test objects and see what they look like and how they work so far. 2:30
Head to the attached work space to see the updated read me with new instructions. 2:34
Or see the teachers notes attached to this video. 2:38
Once you're finished or 2:41
if you're having trouble, check out the solution following this video. 2:41

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