Adding Local Methods at the Object Level

JavaScript Tutorials - Herong's Tutorial Examples

∟Inheritance of Properties and Methods through the Prototype Object Chain

∟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:

Add the local method in the constructor function, so all objects created by this constructor will have this local method.
Add the local method to one object only, so other objects will not have this local method.

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

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

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

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

   // 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 = "herongyang.com";
   
   // 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";
   else 
      inheritance = "Inherited";
   document.writeln("   "+property+": "+inheritance+": "
      +object[property]);
}
</script>
</pre>
</body>
</html>

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 " + this.author;
}

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