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What is AJAX?

What is AJAX?

One of the most recent buzz words going around the Internet is "AJAX". What is AJAX? Is it a new and extremely powerful programming language? Is it an all-purpose cleaner? In plain-simple English, what exactly is AJAX? Well it's neither a new programming Language nor an all-purpose cleaner, it's mainly a new way of using the programming languages we already know.

In the early days of the Web, the only way to make a webpage interactive was for the user to enter data into the webpage and submit it back to the server. The server would respond by creating a new webpage, which it would send back to the user, the user's browser would load the new webpage.

DHTML (Dynamic HTML), introduced in 1997, made it possible to create interactivity on the client machine. Data and Java Script programming is sent in or along with the webpage. Java Script is used to detect events on the webpage, such as user clicks, and in response, use the XML data to modify HTML elements on the webpage. The user could interact with the webpage without having to submit it back to the server, and without having to load a new webpage. This gave the user quicker response and saved a lot of Internet bandwidth and server processing time.

The problem with DHTML is that you can send only a limited amount of data along with the initial webpage. For example, you can't send an entire database with millions of records. That would take too long. So if the scope of the user's interactivity went outside of the data sent with the initial webpage, you still needed to make a request to the server and load the new webpage returned by the server.

AJAX (Asynchronous Java Script and XML) uses Java Script in a webpage to submit requests to the server, receive XML data from the server, and modify HTML elements on the webpage, without having to load a new webpage. This greatly expands the functionality of the users webpage without the overhead of loading a new webpage after each request to the server.

AJAX functionality became available in 1999 when Microsoft implemented the XMLHttpRequest ActiveX Object in Internet Explorer 5. The Mozilla project followed by implementing the XMLHttpRequest functionality natively in the Mozilla 1.0 browser.

Creating an instance of the XMLHttpRequest object for Internet Explorer:

var req = new ActiveXObject("Microsoft.XMLHTTP");

Creating an instance of the XMLHttpRequest object for Mozilla:

var req = new XMLHttpRequest();

Creating an XMLHttpRequest object requires a branch structure (not shown) to acomodate for browser differences in the way the objects are created.

To make a request, tell the XMLHttpRequest object the kind of request, the url you want to request, the function to be called when the request is made, and what, (if any) information you want sent along in the body of the request.

req.open("GET", "test.txt", true);

req.onreadystatechange = function() 
{
  if (req.readyState == 4) 
  {
    alert(req.responseText)
  }
}
req.send(null);

This code checks the readyState property, and when its value is 4, indicating the load is complete, it displays a message to the user.

So, in plain-simple English, AJAX allows a webpage to submit data to the server and retrieve new XML data or load new Java Script functionality, without loading a new webpage. It's a way to use languages we already know, to provide functionality similar to a Web service, without the steep SOAP, WSDL, UDDI, alphabet soup learning curve.

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• What is AJAX?
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