Adding Inherited Methods at the Prototype Level

JavaScript Tutorials - Herong's Tutorial Examples

∟Inheritance of Properties and Methods through the Prototype Object Chain

∟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.

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

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

// Defining a function to be used as an inherited method
function getString() {
   return this.author+"/"+this.title+"/"+this.price;
}

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

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.author + "/" + this.title + "/" + this.price;
}

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