Adding Local Methods at the Object Level

This section provides a quick description of adding local methods at the object level through the constructor function to all objects or directly on one object.

Now we have a good picture of where properties and methods are stored and accessed through the inheritance path. We are ready to add local methods at the object level - in objects themselves, not their prototype object. This can be done in two ways:

Let's try to add a method called "getDesc" in the "Book()" constructor function as shown in this tutorial example:

<!-- Inheritance_Add_Local_Method.html
   Copyright (c) 2013 by Dr. Herong Yang,
<title>Adding a Local Method through the Constructor</title>
<script type="text/javascript">

// Setting up my constructor, prototype and object
function Book(title, author) {
   this.title = title; = author;
   this.getDesc = getDescription;

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

   // Replacing the default prototype object
   myPrototype = new Object();
   Book.prototype = myPrototype;
   Book.prototype.price = 9.99;
   Book.prototype.constructor = Book;
   // Modifying the base prototype object
   Object.prototype.copyright = "";
   // Creating an object from my constructor
   var myBook = new Book("JavaScript Tutorials", "Herong Yang");

   // Where is this method located
   document.writeln("\nChecking Object.prototype...");
   dumpProperty(Object.prototype, "getDesc");
   document.writeln("\nChecking Book.prototype...");
   dumpProperty(Book.prototype, "getDesc");
   document.writeln("\nChecking myBook...");
   dumpProperty(myBook, "getDesc");

   // Running this method
   document.writeln("\nRunning myBook.getDesc()...");
   document.writeln("   "+myBook.getDesc());

function dumpProperty(object, property) {
   var inheritance; 
   if (object.hasOwnProperty(property)) 
      inheritance = "Local";
      inheritance = "Inherited";
   document.writeln("   "+property+": "+inheritance+": "

The output of this tutorial example confirms that "getDesc" is a local method at the object level:

Checking Object.prototype...
   getDesc: Inherited: undefined

Checking Book.prototype...
   getDesc: Inherited: undefined

Checking myBook...
   getDesc: Local: function getDescription() {
    return "\"" + this.title + "\" by " +;

Running myBook.getDesc()...
   "JavaScript Tutorials" by Herong Yang

See the next section on how to add a method at the prototype level.

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

Inheritance of Properties and Methods through the Prototype Object Chain

 Inheriting from Constructor's Prototype Object

 Changing the Constructor's Prototype Object

 Inheriting Properties from Two Level Prototype Objects

 Built-in Properties and Methods in "Object.prototype"

 Using "hasOwnProperty()" Method to Test Inherited Properties

 Setting the "constructor" Property in the "prototype" Object

Adding Local Methods at the Object Level

 Adding Inherited Methods at the Prototype Level

 Building Multiple Levels of Prototype Objects

 Prototype Object Chain Summary

 '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