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AJAX = Asynchronous JavaScript and XML.

AJAX is not a new programming language, but a new way to use existing standards.

AJAX is the art of exchanging data with a server, and updating parts of a web page – without reloading the whole page.

Introduction

AJAX is about updating parts of a web page, without reloading the whole page.
What You Should Already Know

Before you continue you should have a basic understanding of the following:

HTML / XHTML
CSS
JavaScript / DOM

If you want to study these subjects first, find the tutorials on our Home page.
What is AJAX?

AJAX = Asynchronous JavaScript and XML.

AJAX is a technique for creating fast and dynamic web pages.

AJAX allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.

Classic web pages, (which do not use AJAX) must reload the entire page if the content should change.

Examples of applications using AJAX: Google Maps, Gmail, Youtube, and Facebook tabs.

AJAX is Based on Internet Standards

AJAX is based on internet standards, and uses a combination of:

XMLHttpRequest object (to exchange data asynchronously with a server)
JavaScript/DOM (to display/interact with the information)
CSS (to style the data)
XML (often used as the format for transferring data)

Note AJAX applications are browser- and platform-independent!

Google Suggest

AJAX was made popular in 2005 by Google, with Google Suggest.

Google Suggest is using AJAX to create a very dynamic web interface: When you start typing in Google’s search box, a JavaScript sends the letters off to a server and the server returns a list of suggestions.
Start Using AJAX Today

AJAX is based on existing standards. These standards have been used by developers for several years. Read our next chapters to see how it works!

The keystone of AJAX is the XMLHttpRequest object.
The XMLHttpRequest Object

All modern browsers support the XMLHttpRequest object (IE5 and IE6 use an ActiveXObject).

The XMLHttpRequest object is used to exchange data with a server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.
Create an XMLHttpRequest Object

All modern browsers (IE7+, Firefox, Chrome, Safari, and Opera) have a built-in XMLHttpRequest object.

Syntax for creating an XMLHttpRequest object:
variable=new XMLHttpRequest();

Old versions of Internet Explorer (IE5 and IE6) uses an ActiveX Object:
variable=new ActiveXObject(„Microsoft.XMLHTTP“);

To handle all modern browsers, including IE5 and IE6, check if the browser supports the XMLHttpRequest object. If it does, create an XMLHttpRequest object, if not, create an ActiveXObject:

The XMLHttpRequest object is used to exchange data with a server.
Send a Request To a Server

To send a request to a server, we use the open() and send() methods of the XMLHttpRequest object:
xmlhttp.open(„GET“,“ajax_info.txt“,true);
xmlhttp.send();

Method Description
open(method,url,async) Specifies the type of request, the URL, and if the request should be handled asynchronously or not.

method: the type of request: GET or POST
url: the location of the file on the server
async: true (asynchronous) or false (synchronous)
send(string) Sends the request off to the server.

string: Only used for POST requests

GET or POST?

GET is simpler and faster than POST, and can be used in most cases.

However, always use POST requests when:

A cached file is not an option (update a file or database on the server)
Sending a large amount of data to the server (POST has no size limitations)
Sending user input (which can contain unknown characters), POST is more robust and secure than GET

setRequestHeader(header,value) Adds HTTP headers to the request.

header: specifies the header name
value: specifies the header value