JavaScript Tutorials - Herong's Tutorial Examples - 2.33, by Herong Yang
"typeof" Operator and Data Types
This section provides a quick description of the 'typeof' operator and data types supported in JavaScript. A tutorial example is provided to list all data types number, string, boolean, object, function, and undefined.
In the previous section, we learned how to use the "instanceof" operator. But JavaScript offers another operator called "typeof", which works like "instanceof". Here is a comparison of them:
1. "object_name instanceof constructor_name" returns true if the specified object is an instance of the specified type.
2. "typeof expression" returns the data type name of the specified expression. Remember that JavaScript supports several data types: "number", "string", "boolean", "object", "function", and "undefined".
To show you those data types and how to use "typeof", I wrote this tutorial example:
<html> <!-- Prototype_typeof_Operator.html Copyright (c) 2008 HerongYang.com. All Rights Reserved. --> <head> <title>"typeof" Operator</title> </head> <body> <pre> <script type="text/javascript"> // Creating an object of "Book" var myBook = new Book("JavaScript Tutorials", "Herong Yang"); // Show different data types showDatatype(9, "9"); showDatatype(3.14, "3.14"); showDatatype("Hello", "\"Hello\""); showDatatype(true, "true"); showDatatype(undefined, "undefined"); showDatatype(null, "null"); showDatatype(myBook, "myBook"); showDatatype(Book, "Book"); function showDatatype(x, name) { document.writeln(name+": "+(typeof x)); } function Book(title, author) { this.title = title; this.author = author; } </script> </pre> </body> </html>
Here is the output of this tutorial example. Note that the data type of "null" is "object".
9: number 3.14: number "Hello": string true: boolean undefined: undefined null: object myBook: object Book: function
Table of Contents
ECMAScript Language Specification and JavaScript Dialects
Data Types, Variables and Expressions
Creating, Accessing, and Manipulating Arrays
Defining and Calling Functions
Web Browser Supporting JavaScript
Server-Side and Client-Side Web Scripting
►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