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:

<!-- Prototype_Constructor_Function.html
   Copyright (c) 2013 by Dr. Herong Yang,
<title>Defining Object Constructor Functions</title>
<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; = author;
   this.price = undefined;
   this.getDesc = getDescription;

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

   // 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(" ";
   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");

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 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:

  • Constructor equals to function. A constructor is a function. And a function is a constructor.
  • A constructor can be used to create a new object with the "new" operator.
  • Any constructor can be called to execute as a function.
  • Any function can be used to create objects as a constructor.
  • 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


     PDF Printing Version