Time for action – adding prototypes

Let's now add some methods to our class. Here we will use the prototype object to define them.

  1. Create a new script called hello-world-prototype.js and fill it with this:
    #!/usr/bin/env seed

print("Hello world")

var Book = function(isbn, title) {
   this.isbn = isbn;
   this.title = title;
}

Book.prototype = {
   printTitle: function(){
      print("Title is " + this.title);
   },

   printISBN: function() {
      print("ISBN is " + this.isbn);
   }
}

var book = new Book("1234", "A good title");
book.printTitle();
book.printISBN();
  2. Run it.
  3. See the values printed:
    Hello world
Title is A good title
ISBN is 1234

What just happened?

In a JavaScript object, prototype is a special object that holds all the properties and methods inside a class or an object. So what we do here is to fill in the prototype with our own methods.

var Book = function(isbn, title) {
   this.isbn = isbn;
   this.title = title;
}

In this code, we have our constructor like before:

Book.prototype = {

Then, we start the declaration of the prototype, ready to fill it in with our own method definitions:

   printTitle: function(){
      print("Title is " + this.title);
   },

Here, we put our first method as described with a function body:

   printISBN: function() {
      print("ISBN is " + this.isbn);
   }

Note

We use a colon instead of an equals sign to define the method, and we put a comma at the end of our method, meaning that there will come another method or member declaration after this line. Remember our experience from the previous code when defining the book object in a different way?

Then, our next method comes along. Here we end the definition without putting a comma.

var book = new Book("1234", "A good title");

After that, we declare a book variable by constructing the Book object with specified arguments.

book.printTitle();
book.printISBN();

Finally, we use our method by just calling it (note the brackets after the name of the method).

Have a go hero – adding more methods

Why don't we add more methods? Let's say we need these methods:

Tip

Don't forget about the colon and the comma!