Inheriting Properties from Two Level Prototype Objects

This section provides a quick description of how constructor's prototype objects form a two-level inheritance. A tutorial example is provided to show object property inheritance from first and send level prototype objects.

Now we know how an object inherits properties from its constructor's prototype object.

Since the constructor's prototype object can also inherit properties from its own constructor's prototype object, an object can inherit properties from its own prototype object and the prototype's prototype object.

To show you this two-level inheritance feature, I wrote this tutorial example below to allow "myBook" inherit properties from the "price" property from the first level prototype and the "copyright" property from the second level prototype:

<!-- Inheritance_from_Second_Level_Prototype.html
   Copyright (c) 2013 by Dr. Herong Yang,
<title>Inheriting Properties from the Second-Level Prototype Object
<script type="text/javascript">

// Define the constructor: "Book"
function Book(title, author) {
   this.title = title; = author;

   // Adding a property to the "prototype"
   Book.prototype.price = 9.99;

   // Adding a property to the prototype's "prototype"
   // Book.prototype's constructor is Object
   Object.prototype.copyright = "";

   // Creating an object: "myBook"
   var myBook = new Book("JavaScript Tutorials", "Herong Yang");

   // Showing the object, constructor, and prototype
   showObject(myBook, "myBook");
   showObject(Book, "Book");
   showObject(Book.prototype, "Book.prototype");
   showObject(Object.prototype, "Object.prototype");

function showObject(object, name) {
   document.writeln("\n\""+name+"\" properties:");
   for (var item in object) {
      document.writeln("   "+item+": "+object[item]);

The output of this tutorial example confirms that JavaScript allows objects to inherit properties from its parent prototype objects of multiple levels:

"myBook" properties:
   title: JavaScript Tutorials
   author: Herong Yang
   price: 9.99

"Book" properties:
   prototype: [object Object]

"Book.prototype" properties:
   price: 9.99

"Object.prototype" properties:

The diagram below shows you how properties from prototype objects of two levels are inherited by the object.
Two-Level Prototype Inheritance

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