Ajax Basics

Ajax
Ajax (asynchronous JavaScript and XML), or AJAX, is a group of interrelated web development techniques used for creating interactive web applications. With Ajax, web applications can retrieve data from the server asynchronously in the background without interfering with the display and behavior of the existing page. Data is retrieved using the XMLHttpRequest object or through the use of Remote Scripting in browsers that do not support it. Despite the name, the use of JavaScript, XML, and asynchrony is not required
History
While the term Ajax was coined in 2005,techniques for the asynchronous loading of content date back to 1996, when Internet Explorer introduced the IFrame element.Microsoft's Remote Scripting, introduced in 1998, acted as a more elegant replacement for these techniques, with data being pulled in by a Java applet with which the client side could communicate using JavaScript. In 2002, Microsoft created the XMLHttpRequest object as an ActiveX control in Internet Explorer 5, and developers of Mozilla and Safari followed soon after with native versions of the object. It did not become an official web standard until April 5, 2006, when the World Wide Web Consortium (W3C) released a specification for the object.
Technologies
The term Ajax has come to represent a broad group of web technologies that can be used to implement a web application that communicates with a server in the background, without interfering with the current state of the . In the article that coined the term Ajax, Jesse James Garrett explained that it referred specifically to several technologies:
XHTML and CSS for presentation
the Document Object Model for dynamic display of and interaction with data
XML and XSLT for the interchange and manipulation of data, respectively
the 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:
JavaScript is not the only client-side scripting language that can be used for implementing an Ajax application. Other languages such as VBScript are also capable of the required functionality.
the XMLHttpRequest object is not necessary for asynchronous communication. It has been noted that IFrames are capable of the same effect.
XML is not required for data interchange and therefore XSLT is not required for manipulation of data. Garrett proposed the use of Javascript Object Notation as an alternative format for data interchange.
Advantages
many cases, the 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.
Because only sections of pages need to be reloaded, Ajax allows for much more responsive web applications,[10] giving users the feeling that changes are happening instantaneously.
The use of Ajax can reduce connections to the server, since scripts and style sheets only have to be requested once.
Disadvantages

Dynamically created pages do not register themselves with the browser's history engine, so clicking the browser's "back" button would not return the user to an earlier state of the Ajax-enabled page, but would instead return them to the last page visited before it. Workarounds include the use of invisible IFrames to trigger changes in the browser's history.
Dynamic Web page updates also make it difficult for a user to bookmark a particular state of the application. Solutions to this problem exist, many of which use the URL fragment identifier (the portion of a URL after the '#') to keep track of, and allow users to return to, the application in a given state.
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.[12]
Any user whose browser does not support Ajax or JavaScript, or simply has JavaScript disabled, will not be able to use its functionality.[12] Similarly, devices such as mobile phones, PDAs, and screen readers may not have support for JavaScript or the XMLHttpRequest object.Also, screen readers that are able to use Ajax may not properly read the dynamically generated content.
The same origin policy prevents Ajax from being used across domains,although the W3C has a draft that would enable this functionality.