To create web applications in AJAX you have to become familiar with the JavaScript object called the XMLHttpRequest.

The XMLHttpRequest object is the main key to AJAX. It has been available since Internet Explorer 5.5 was released in July 2000, but not fully discovered before people started to talk about AJAX and Web 2.0 in 2005.

Below is given some of the methods and properties you have to become familiar with.

Different browsers use different methods to create XMLHttpRequest object.

Internet Explorer use an ActiveXObject.

Other browsers uses a built in JavaScript object which is called as XMLHttpRequest.

Here is the simplest code you can use to overcome this problem:

Above example is explained below:

First create a variable XMLHttp to use your XMLHttpRequest object.Always set the value to null.

Then test if the object window.XMLHttpRequest is available only. This object is available in newer versions of browser like Firefox, Mozilla, and Opera.

If it is available, use it to create a new object:

XMLHttp=new XMLHttpRequest().

If it's not available,  test if an object window.ActiveXObject is available. This object is available in Internet Explorer version 5.5 and later on also.

If it's available, use it to create a new object:

XMLHttp=new ActiveXObject().

Some programmer will prefer to use the newest and fastest version of the XMLHttpRequest object.

The example below try to load Microsoft's the latest version "Msxml2.XMLHTTP", available in Internet Explorer 6, before it falls back to "Microsoft.XMLHTTP", available in Internet Explorer 5.5 and later.

Above example is explained below:

First create a variable XMLHttp to use your XMLHttpRequest object. Always set the value to null.

Then try to create the object the Microsoft , available in Internet Explorer 6 and later:

XMLHttp=new ActiveXObject("Msxml2.XMLHTTP")

If this catches an error, try the older (Internet Explorer 5.5) :

XMLHttp=new ActiveXObject("Microsoft.XMLHTTP")

If  XMLHttp still has a null value, try to create an object the "standard" way:

XMLHttp=new XMLHttpRequest()

The open() method.

The open() method sets up a request to the web server.

The send() method.

The send() method sends a request to the web server.

The abort() method.

The abort() method aborts the current web server request

The readyState property defines the current state of the XMLHttpRequest object.

The code runs a query against an XML file and returns the result as HTML format.

readyState=0This stae is used, after you have created the XMLHttpRequest object, but before you have called the open() method.

readyState=1 This state is used, after you have called the open() method, but before you have called send().

readyState=2This state is used, after you have called send().

readyState=3 This state is used,after the browser has established a communication with the server, but before the server has completed the response.

readyState=4 This state is used,after the request has been completed, and the response data have been completely received from the server.

Note: Different browsers treat as ready state differently. Don't expect all browsers to report all states eqally. Some will not report 1 and 0.

For Your AJAX web applications you will actually only be interested state 4. That is when the request is completed and it is safe use the received data