How does AJAX work?

Question

What is the essence of AJAX? For example, I want to have a link on my page such that when a user clicks this link, some information is sent to my server without the reloading of the current page. Is that AJAX?

I was able to get this behavior by using isoframes. In more details I put a link (let's say a small image) in a small isoframe. When the user clicks this link the browser reloads only the page in the isoframe.

However, I think it is not an elegant way to reach the goal. I think I have to use AJAX. How does it work? Can a usage of XHTML solve the considered problem in an elegant way? Or do I need to use JavaScripts?

I do not need much. I just want to have a small link which (after clicking) sent some information to server. Let say I have a "star-image" near a message. If a user click the star (he/she likes the message) star changes the color and my server update database (to remember that user likes the message).

Answer 1

If you are totally new to AJAX (which stands for Asynchronous Javascript And XML), the AJAX entry on wikipedia is a good starting point:

Like DHTML and LAMP, AJAX is not a technology in itself, but a group of technologies. AJAX uses a combination of:

• HTML and CSS for marking up and styling information.
• The DOM accessed with JavaScript to dynamically display and interact with the information presented.
• A method for exchanging data asynchronously between browser and server, thereby avoiding page reloads. The XMLHttpRequest (XHR) object is usually used, but sometimes an IFrame object or a dynamically added tag is used instead.
• A format for the data sent to the browser. Common formats include XML, pre-formatted HTML, plain text, and JavaScript Object Notation (JSON). This data could be created dynamically by some form of server-side scripting.

As you can see, from a pure technological point of view, there is nothing really new here. Most of AJAX parts were already there in 1994 (1999 for the XMLHttpRequest object). The real novelty was to use these parts together as Google did with GMail (2004) and Google Maps (2005). Actually, both sites contributed heavily to the promotion of AJAX.

A picture being worth a thousand words, below a diagram that illustrates the communication between the client and the remote server, as well as the differences between the classic and the AJAX-powered applications:

For the orange part, you can do everything by hand (with the XMLHttpRequest object) or you can use famous JavaScript libraries like jQuery, Prototype, YUI, etc to "AJAXify" the client-side of your application. Such libraries aim to hide the complexity of JavaScript development (e.g. the cross-browser compatibility), but might be overkill for a simple feature.

On the server-side, some frameworks can help too (e.g. DWR or RAJAX if you are using Java), but all you need to do is basically to expose a service that returns only the required informations to partially update the page (initially as XML/XHTML - the X in AJAX - but JSON is often preferred nowadays).

Answer 2

The essence of AJAX is this:

Your pages can browse the web and update their own content while the user is doing other things.

That is, your javascript can send asynchronous GET and POST requests (usually via an XMLHttpRequest object) then use the results of those requests to modify its page (via Document Object Model manipulation).

Answer 3

AJAX typically involves sending HTTP requests from client to server and processing the server's response, without reloading the entire page. (Asynchronously).

Javascript generally does the submission and receives the data response from the server (traditionally XML, often other less verbose formats like JSON)

The Javascript then may update the page DOM dynamically to update the user's view.

Thus 'Asychronous Javascript And XML'.

There are other options to update the user's view without reloading the page, things like Flash and Applets, but these don't sound like good solutions for your case. Sounds like Javascript is the way to go. There's loads of good library support around, like jQuery as is used on this site, so you don't need to actually write much Javascript yourself.

Answer 4

Ajax is more than reload just a part of the page. Ajax stands for Asynchronous Javascript And Xml.

The only part of Ajax that you need is the XMLHttpRequest object from javascript. You have to use it to load and reload small part of your html as a div or any other tags.

Read that example and you will be pro sooner as you think!

<html>
<body>

<script type="text/javascript">
function ajaxFunction()
{
var xmlhttp;
if (window.XMLHttpRequest)
  {
  // code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else if (window.ActiveXObject)
  {
  // code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
else
  {
  alert("Your browser does not support XMLHTTP!");
  }
xmlhttp.onreadystatechange=function()
{
if(xmlhttp.readyState==4)
  {
  document.myForm.time.value=xmlhttp.responseText;
  }
}
xmlhttp.open("GET","time.asp",true);
xmlhttp.send(null);
}
</script>

<form name="myForm">
Name: <input type="text" name="username" onkeyup="ajaxFunction();" />
Time: <input type="text" name="time" />
</form>

</body>
</html>