JavaScript Tutorials - Herong's Tutorial Examples
Dr. Herong Yang, Version 2.10

Using "hasOwnProperty()" Method to Test Inherited Properties

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This section provides a quick description of the built-in default method called 'hasOwnProperty()'. A tutorial example is provided to use 'hasOwnProperty()' method to test which property is local to this object and which is inherited.

From the previous section, we learned that "Object.prototype" has a method called "hasOwnProperty()", which will be inherited by all objects in JavaScript.

From any object, you can use the "hasOwnProperty(prop)" method to test whether the specified property is local to this object.

With this nice tool, I want to revisit my "Book" object example to check different properties at different inheritance level with this tutorial example: 

<html>
<!-- Inheritance_hasOwnProperty_Method.html
   Copyright (c) 2008 by Dr. Herong Yang, http://www.herongyang.com/
-->
<head>
<title>Using "hasOwnProperty" Method</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;
}
   Book.prototype.price = 9.99;
   Object.prototype.copyright = "herongyang.com";
   var myBook = new Book("JavaScript Tutorials", "Herong Yang");

   // Dumping built-in properties at the base prototype level
   document.writeln("\nObject.prototype's built-in properties:");
   dumpProperty(Object.prototype, "constructor");
   dumpProperty(Object.prototype, "hasOwnProperty");
   dumpProperty(Object.prototype, "isPrototypeOf");
   dumpProperty(Object.prototype, "toString");
   dumpProperty(Object.prototype, "valueOf");

   // Dumping built-in properties at the my prototype level
   document.writeln("\nBook.prototype's built-in properties:");
   dumpProperty(Book.prototype, "constructor");
   dumpProperty(Book.prototype, "hasOwnProperty");
   dumpProperty(Book.prototype, "isPrototypeOf");
   dumpProperty(Book.prototype, "toString");
   dumpProperty(Book.prototype, "valueOf");

   // Dumping built-in properties at the object level
   document.writeln("\nmyBook's built-in properties:");
   dumpProperty(myBook, "constructor");
   dumpProperty(myBook, "hasOwnProperty");
   dumpProperty(myBook, "isPrototypeOf");
   dumpProperty(myBook, "toString");
   dumpProperty(myBook, "valueOf");

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>

Here is the output this tutorial example: 

Object.prototype's built-in properties:
   constructor: Local: function Object() {
    [native code]
}
   hasOwnProperty: Local: function hasOwnProperty() {
    [native code]
}
   isPrototypeOf: Local: function isPrototypeOf() {
    [native code]
}
   toString: Local: function toString() {
    [native code]
}
   valueOf: Local: function valueOf() {
    [native code]
}

Book.prototype's built-in properties:
   constructor: Local: function Book(title, author) {
    this.title = title;
    this.author = author;
}
   hasOwnProperty: Inherited: function hasOwnProperty() {
    [native code]
}
   ...

myBook's built-in properties:
   constructor: Inherited: function Book(title, author) {
    this.title = title;
    this.author = author;
}
   ...

Notice that:

  • Built-in default properties and methods are indeed inherited by "Book.prototype" and "myBook".
  • But there is one exception. "Book.prototype" has a local copy of "constructor" which points to "Book()". This tells us that JavaScript created a local property "constructor" in "Book.prototype", instead of inheriting it from "Object.prototype".

Sections in This Chapter

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

Dr. Herong Yang, updated in 2008
Using "hasOwnProperty()" Method to Test Inherited Properties