"instanceof" Operator - Determining Object Type

This section provides a quick description of the 'instanceof' operator. A tutorial example is provided on how to use the 'instanceof' operator to determine what is the type of an object, and its parent types.

So far, we have played with several types of objects, like Object, Array, Function, and Book (my own object type). And we know how to create objects with these type by using their constructor functions.

But we are given an object, how can we test its object type? JavaScript offers the "instanceof" operator to help us with this expression

   object_name instanceof constructor_name;

This "instanceof" expression returns a Boolean value indicating whether or not the specified object is an instance of the specified type (the constructor name represents the object type name).

The tutorial examples below uses the "instanceof" operator to test 3 objects used in my previous example:

<html>
<!-- Prototype_instanceof_Operator.html
   Copyright (c) 2013 by Dr. Herong Yang, herongyang.com
-->
<head>
<title>The "instanceof" Operator - Determining Object Typs</title>
</head>
<body>
<pre>
<script type="text/javascript">

// Defining object type constructor function
function Book(title, author) {
   this.title = title;
   this.author = author;
}

   // Adding properties and methods to the "prototype"
   Book.prototype.price = 9.99;
   Book.prototype.getDesc = getDescription;

// Defining a function to be used as a method
function getDescription() {
   return "\""+this.title+"\" by "+this.author;
}
   
   // Creating an object of "Book"
   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, "Object");
   showObject(Object.prototype, "Object.prototype");

function showObject(object, name) {
   document.writeln("\nShowing object \""+name+"\"");

   // Using "instanceof" to determine object type
   document.writeln("   Instance of Object: "
      +(object instanceof Object));
   document.writeln("   Instance of Function: "
      +(object instanceof Function));
   document.writeln("   Instance of Book: "
      +(object instanceof Book));
}
</script>
</pre>
</body>
</html>

Here is the output of this tutorial example:

Showing object "myBook"
   Instance of Object: true
   Instance of Function: false
   Instance of Book: true

Showing object "Book"
   Instance of Object: true
   Instance of Function: true
   Instance of Book: false

Showing object "Book.prototype"
   Instance of Object: true
   Instance of Function: false
   Instance of Book: false

Showing object "Object"
   Instance of Object: true
   Instance of Function: true
   Instance of Book: false

Showing object "Object.prototype"
   Instance of Object: false
   Instance of Function: false
   Instance of Book: false

Several interesting notes on the output

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

 Prototype-Dased Object-Oriented Programming Style

 Prototype-Based Programming Features in JavaScript

 Defining Object Constructor Functions

 Adding Properties and Methods to Objects

 "for ... in" and "for each ... in" Statements

 "prototype" Property of the Constructor Function Object

"instanceof" Operator - Determining Object Type

 "typeof" Operator and Data Types

 Inheritance of Properties and Methods through the Prototype Object Chain

 '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