AJAX allows asynchronous communication between the browser and server without reloading the page. It uses the XMLHttpRequest object to make HTTP requests and retrieve data from the server in the background. This allows parts of the web page to be updated independently without interfering with the display and behavior of the existing page.

1. AJAX
AJAX stands for Asynchronous JavaScript And XML.
Any server side technology that supports JavaScript also
supports AJAX.
AJAX is a browser technology, and is therefore independent of
web server platforms.
AJAX is not a programming language, so you don’t have to
learn any new technology. AJAX can be implemented by using
existing standards (JavaScript and XML) in a different way.
AJAX uses HTTP requests for this. With AJAX, JavaScript
communicates directly with the server, through the JavaScript
XMLHttpRequest object (XML over HTTP)

2. AJAX to request a data from the server we need
• 1. Create an XMLHttpRequest object.
• 2. Then using this object, request data from the server.
• 3. JavaScript will then monitor for the changing of state of the
request.
• 4. If the response is successful, then the content from the
data store requested will be returned as response (response
can be in the form of a String or XML).
• 5. Use the response in your web page.

3. 1.Create an XMLHttpRequest object
• JavaScript has a built-in XMLHttpRequest object
• For Internet Explorer use the ActiveXObject

4. var req;
XMLHttpRequest for all browsers :
if(window.XMLHttpRequest)
{
//For Firefox, Safari, Opera
req = new XMLHttpRequest();
}
else if(window.ActiveXObject)
{
//For IE 5
req = new ActiveXObject(“Microsoft.XMLHTTP”);
}
else if(window.ActiveXObject)
{
//For IE 6+
req = new ActiveXObject(“Msxml2.XMLHTTP”);
}
Else
{
//Error for an old browser
alert(‘Your browser is not IE 5 or higher, or Firefox or Safari or Opera’);
}

5. 2. Request for a web page
• After creating the XMLHttpRequest we now need to send the web request
using the open method. We also need to specify the HttpRequest method,
GET or POST. Use the following code to send the request.
• Use the following code to send the request.
• req.open(“GET”,”somedata.php”);
• req.send(null);
• Here, req is the XMLHttpRequest object. It will request to the server for
somedata.php using GET method. The open function also has a third
parameter, an optional boolean parameter. You should set that to true :
• req.open(“GET”,”somedata.php”,true);
• req.send(null);
• Both of the above is correct.

6. 3. Monitor for the response of the request
• For doing this you can assign a function to
req.onreadystatechange (Here, req is the
XMLHttpRequest object), like below.
• req.onreadystatechange=function()
{
if(req.readyState==4 && req.status == 200)
{
var resp = req.responseText;
}
}

7. like this
• req.onreadystatechange = handleResponse;
• function handleResponse()
{
if(req.readyState == 4 && req.status == 200)
{
//returned text from the PHP script
var response = req.responseText;
}
}

8. • The readyState property holds the status of the server’s response.
Each time the readyState changes, the onreadystatechange
function will be executed. Here are the possible values for the
readyState property:
• State Description
• 0 The request is not initialized
• 1 The request has been set up
• 2 The request has been sent
• 3 The request is in process
• 4 The request is complete
• And status is the status of the HTTP Request, like 500 Internal
Server Error, 400 Bad Request, 401 Unauthorized, 403
Forbidden, 404 Not Found etc. 200 means no error.

9. 4. Get the response
• The response will be as string or as XML. The data sent back
from the server can be retrieved with the responseText
property as string. Use responseXML for getting the response
as XML.

10. 5. Use the response on your web page
• You can use the response you got from the XMLHttpRequest
in your web page/application. You can either set a value of a
text field or use the returned HTML from the web request as
innerHTML for a <div></div> tag or <span></span> tag (See
below for the implementation of this)