What is AJAX ?

AJAX (Asynchronous JavaScript and XML), is a group of interrelated web development techniques used to create interactive web applications or rich Internet applications. With AJAX, your JavaScript can communicate directly with the server, using the JavaScript XMLHttpRequest object. With this object, your JavaScript can trade data with a web server, without reloading the page.

The use of Ajax has led to an increase in interactive animation on web pages. AJAX uses asynchronous data transfer (HTTP requests) between the browser and the web server, allowing web pages to request small bits of information from the server instead of whole pages. Despite the name, the use of JavaScript and XML is not actually required, nor do the requests need to be asynchronous.

Prior Technologies To Learn:

* XHTML and CSS for presentation
* XML and XSLT for the interchange, manipulation and display of data
* Document Object Model for dynamic display & interaction with data
* XMLHttpRequest object for asynchronous communication
* JavaScript to bring these technologies together

Since then, however, there have been a number of developments in the technologies used in an Ajax application, and the definition of the term Ajax. In particular, it has been noted that:

* Apart from JavaScript, other languages such as VBScript and EGL Programming Language are also capable of the required functionality.

* XML is not required for data interchange and therefore XSLT is not required for the manipulation of data. JavaScript Object Notation (JSON) is often used as an alternative format for data interchange, although other formats such as preformatted HTML or plain text can also be used.

Advantages

* In many cases, related pages on a website consist of much content that is common between them. Using traditional methods, that content would have to be reloaded on every request. However, using Ajax, a web application can request only the content that needs to be updated, thus drastically reducing bandwidth usage and load time.

* The use of asynchronous requests allows the client's Web browser UI to be more interactive and to respond quickly to inputs, and sections of pages can also be reloaded individually.

* The use of Ajax can reduce connections to the server, since scripts and style sheets only have to be requested once.

Disadvantages

* Pages dynamically created using successive Ajax requests do not automatically register themselves with the browser's history engine, so clicking the browser's "back" button may not return the user to an earlier state of the Ajax-enabled page, but may instead return them to the last full page visited before it.

* Dynamic web page updates also make it difficult for a user to bookmark a particular state of the application.

* Because most web crawlers do not execute JavaScript code, web applications should provide an alternative means of accessing the content that would normally be retrieved with Ajax, to allow search engines to index it.

* Any user whose browser does not support Ajax or JavaScript, or simply has JavaScript disabled, will not be able to use its functionality. Similarly, devices such as mobile phones, PDAs, and screen readers may not have support for JavaScript or the XMLHttpRequest object.