Adding Inherited Methods at the Prototype Level

This section provides a quick description of adding inherited methods at the constructor's prototype object to allow all objects created by the constructor to share.

In the previous section, we learned how to add a local method to all objects through the constructor function.

However if a method needs to be used by all objects, it's better to add it as an inherited method in the prototype object. This saves the extra storage required in each object to store a local method.

Let's try to add a method called "toString" in the "Book.prototype" object. This "toString method will be inherited by all objects created by "Book()". By the way, "Book.prototype.toString" will override "Object.prototype.toString" in the inheritance tree.

<!-- Inheritance_Add_Inherited_Method.html
   Copyright (c) 2013 by Dr. Herong Yang,
<title>Adding a Inherited Method in the Prototype</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 local 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;

   // Adding an inherited method in the prototype
   Book.prototype.toString = getString;

// Defining a function to be used as an inherited method
function getString() {

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

   // Where are these methods located
   document.writeln("\nChecking Object.prototype...");
   dumpProperty(Object.prototype, "getDesc");
   dumpProperty(Object.prototype, "toString");
   document.writeln("\nChecking Book.prototype...");
   dumpProperty(Book.prototype, "getDesc");
   dumpProperty(Book.prototype, "toString");
   document.writeln("\nChecking myBook...");
   dumpProperty(myBook, "getDesc");
   dumpProperty(myBook, "toString");

   // Running these methods
   document.writeln("\nRunning myBook.getDesc()...");
   document.writeln("   "+myBook.getDesc());
   document.writeln("\nRunning myBook.toString()...");
   document.writeln("   "+myBook.toString());

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 "toString" is an inherited method at the prototype level:

Checking Object.prototype...
   getDesc: Inherited: undefined
   toString: Local: function toString() {
    [native code]

Checking Book.prototype...
   getDesc: Inherited: undefined
   toString: Local: function getString() {
    return + "/" + this.title + "/" + this.price;

Checking myBook...
   getDesc: Local: function getDescription() {
    return "\"" + this.title + "\" by " +;
   toString: Inherited: function getString() {
    return + "/" + this.title + "/" + this.price;

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

Running myBook.toString()...
   Herong Yang/JavaScript Tutorials/9.99

Now we know where local methods and inherited methods are located. Here is my updated diagram of my object inheritance tree structure:
Adding Local and Inherited Properties and Methods

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