Ajax (programming)

Asynchronous JavaScript and XML or Ajax

is a web development technique for creating interactive web applications using a combination of:

XHTML (or HTML) and CSS for marking up and styling information

The Document Object Model manipulated through JavaScript to dynamically display and interact with the information presented

The XMLHttpRequest object to exchange data asynchronously with the web server. (XML is commonly used, although any format will work, including preformatted HTML, plain text, JSON and even EBML). In some Ajax frameworks and in some situations, an IFrame object is used instead of the XMLHttpRequest object to exchange data with the web server.

Like DHTML, LAMP, or SPA, Ajax is not a technology in itself, but a term that refers to the use of a group of technologies together. In fact, derivative/composite technologies based substantially upon Ajax, such as AFLAX, are already appearing.

For a number of tasks, only small amounts of data must be transfered between the client and the server, allowing a number of Ajax applications to perform almost as well as (interpreted) applications executed completely on the user's machine. This has the effect that pages need only be updated in the user's browser, rather than having to be entirely refreshed. “Every user's action that normally would generate an HTTP request takes the form of a JavaScript call to the Ajax engine instead”, wrote Jesse James Garrett, in the essay that first defined the term. “Any response to a user action that doesn’t require a trip back to the server — such as simple data validation, editing data in memory, and even some navigation — the engine handles on its own. If the engine needs something from the server in order to respond — if it’s submitting data for processing, loading additional interface code, or retrieving new data — the engine makes those requests asynchronously, usually using XML, without stalling a user’s interaction with the application.”

Traditional web applications essentially submit forms, completed by a user, to a web server. The web server does some processing, and responds by sending a new web page back. Because the server must send a whole new page each time, applications run more slowly and awkwardly than their native counterparts.

Ajax applications, on the other hand, can send requests to the web server to retrieve only the data that is needed, and may use SOAP or some other XML-based web services dialect. On the client, JavaScript processes the web server response, and may then modify the document's content through the DOM to show the user that an action has been completed. The result is a more responsive interface, since the amount of data interchanged between the web browser and web server is vastly reduced. Web server processing time is also saved, since much of it is done on the client.

The earliest form of asynchronous remote scripting, Microsoft's Remote Scripting, was developed before XMLHttpRequest existed, and made use of a dedicated Java applet. Thereafter, remote scripting was extended by Netscape DevEdge at around 2001/2002 by use of an IFRAME instead of a Java applet.