Defining Object Constructor Functions

This section provides a quick description on defining object constructor functions. A tutorial example is provided on creating a 'Book' constructor with 3 properties and 1 method.

The first thing to learn about prototype-based programming style in JavaScript is the constructor function. When writing a constructor function, we need to remember:

Here is a tutorial example that defines a constructor function called "Book(...)", which defines 3 properties and 1 method:

<html>
<!-- Prototype_Constructor_Function.html
   Copyright (c) 2008 HerongYang.com, All Rights Reserved.
-->
<head>
<title>Defining Object Constructor Functions</title>
</head>
<body>
<pre>
<script type="text/javascript">

// Defining the constructor function for a new object type
function Book(title, author) {
   document.writeln("\nConstructing a \"Book\" object...");
   this.title = title;
   this.author = author;
   this.price = undefined;
   this.getDesc = getDescription;
}

// Defining a function to be used as a method
function getDescription() {
   return "\""+this.title+"\" by "+this.author;
}

   // Creating an object of "Book"
   var myBook = new Book("JavaScript Tutorials", "Herong Yang");
   document.writeln("\nChecking the \"Book\" object");
   document.writeln("   Type: "+(typeof myBook));
   document.writeln("   Instance of \"Book\": "
      +(myBook instanceof Book));

   // Testing properties and methods
   document.writeln("   myBook.title: "+myBook.title);
   document.writeln("   myBook.author: "+myBook.author);
   document.writeln("   myBook.price: "+myBook.price);
   document.writeln("   myBook.getDesc(): "+myBook.getDesc());

   // Trying to call a constructor function as a normal function
   Book("JavaScript Cookbook", "Yosef Cohen");
</script>
</pre>
</body>
</html>

The output of the tutorial example is printed below:

Constructing a "Book" object...

Checking the "Book" object
   Type: object
   Instance of "Book": true
   myBook.title: JavaScript Tutorials
   myBook.author: Herong Yang
   myBook.price: undefined
   myBook.getDesc(): "JavaScript Tutorials" by Herong Yang

Constructing a "Book" object...

Notice that the last test statement proves that a constructor function is just a normal function, and you can call it directly. But calling a constructor function directly will not produce any result.

As a quick summary:

Table of Contents

 About This Book

 Introduction to JavaScript

 ECMAScript Language Specification and JavaScript Dialects

 Data Types, Variables and Expressions

 Flow Control Statements

 Creating, Accessing, and Manipulating Arrays

 Defining and Calling Functions

 Web Browser Supporting JavaScript

 Server-Side and Client-Side Web Scripting

 Introduction to Objects

Defining Your Own Object Types

 Prototype-Dased Object-Oriented Programming Style

 Prototype-Based Programming Features in JavaScript

Defining Object Constructor Functions

 Adding Properties and Methods to Objects

 "for ... in" and "for each ... in" Statements

 "prototype" Property of the Constructor Function Object

 "instanceof" Operator - Determining Object Type

 "typeof" Operator and Data Types

 Inheritance of Properties and Methods through the Prototype Object Chain

 'jrunscript' - JavaScript Shell Command from JDK

 Using Functions as "Function" Objects

 Introduction to Built-in Object Types

 W3C's Document Object Model (DOM) Specifications

 References

 Full Version in PDF/EPUB