JavaScript Tutorials - Herong's Tutorial Examples
Dr. Herong Yang, Version 2.21

Inheriting from Constructor's Prototype Object

This section provides a quick description of how object inheriting properties and methods from its constructor's prototype object. A tutorial example and diagram is provided to show relations of an object, its constructor, and the constructor's prototype object.

From previous chapters in this book, we learned that:

  • A constructor is a function. But it is also an object.
  • Each constructor has built-in default property called "prototype".
  • The constructor's "prototype" is an object.
  • Properties and methods can be added to the constructor's prototype object.
  • Each object will inherit properties and methods from its constructor's prototype object.

To demonstrate those rules, I wrote the following tutorial example to allow "myBook" to inherit the "price" property from the constructor's prototype object:

<html>
<!-- Inheritance_from_Prototype.html
   Copyright (c) 2013 by Dr. Herong Yang, herongyang.com
-->
<head>
<title>Inheritance from Prototype</title>
</head>
<body>
<pre>
<script type="text/javascript">

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

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

   // 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");

function showObject(object, name) {
   document.writeln("\n\""+name+"\" properties:");
   for (var item in object) {
      document.writeln("   "+item+": "+object[item]);
   }
}
</script>
</pre>
</body>
</html>

The output of this tutorial example confirms what we learned earlier:

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

"Book" properties:
   prototype: [object Object]

"Book.prototype" properties:
   price: 9.99

The output can also be represented with this diagram:
Inheritance from Prototype

Table of Contents

 About This JavaScript Tutorial Example 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

 References

 PDF Printing Version

Dr. Herong Yang, updated in 2013
Inheriting from Constructor's Prototype Object