What is AJAX ( Asynchronous Java Script )

AJAX, web applications are fun to build. They are like the fancy sportscar of Websites. Web applications allow the designer and developer to get together and solve a problem for their customers that the customers might not have even know they had. That's how the blogging tools like MovableType and Blogger came about after all. I mean, before Blogger, did you know you needed an online tool to build your Web site blog? But most Web applications are slow and tedious. Even the fastest of them has lots of free time for your customers to go get a coffee, work on their dog training, or (worst of all) head off to a faster Web site. It's that dreaded hourglass! You click a link and the hourglass appears as the Web application consults the server and the server thinks about what it's going to send back to you.

Ajax is Here to Change That
Ajax (sometimes called Asynchronous JavaScript and XML) is a way of programming for the Web that gets rid of the hourglass. Data, content, and design are merged together into a seamless whole. When your customer clicks on something on an Ajax driven application, there is very little lag time. The page simply displays what they're asking for. If you don't believe me, try out Google Maps for a few seconds. Scroll around and watch as the map updates almost before your eyes. There is very little lag and you don't have to wait for pages to refresh or reload.
What is Ajax?

Ajax is a way of developing Web applications that combines:

* XHTML and CSS standards based presentation
* Interaction with the page through the DOM
* Data interchange with XML and XSLT
* Asynchronous data retrieval with XMLHttpRequest
* JavaScript to tie it all together

In the traditional Web application, the interaction between the customer and the server goes like this:

1. Customer accesses Web application
2. Server processes request and sends data to the browser while the customer waits
3. Customer clicks on a link or interacts with the application
4. Server processes request and sends data back to the browser while the customer waits

There is a lot of customer waiting.

Ajax Acts as an Intermediary
The Ajax engine works within the Web browser (through JavaScript and the DOM) to render the Web application and handle any requests that the customer might have of the Web server. The beauty of it is that because the Ajax engine is handling the requests, it can hold most information in the engine itself, while allowing the interaction with the application and the customer to happen asynchronously and independently of any interaction with the server.
Asynchronous

This is the key. In standard Web applications, the interaction between the customer and the server is synchronous. This means that one has to happen after the other. If a customer clicks a link, the request is sent to the server, which then sends the results back.

With Ajax, the JavaScript that is loaded when the page loads handles most of the basic tasks such as data validation and manipulation, as well as display rendering the Ajax engine handles without a trip to the server. At the same time that it is making display changes for the customer, it is sending data back and forth to the server. But the data transfer is not dependent upon actions of the customer.