Ajax Basic Introduction
Ajax (Asynchronous JavaScript and XML) is a method of building interactive applications for the Web that process user requests immediately. Ajax combines several programming tools including JavaScript, dynamic HTML (DHTML), Extensible Markup Language (XML), cascading style sheets (CSS), the Document Object Model (DOM), and the Microsoft object, XMLHttpRequest.
Ajax allows content on Web pages to update immediately when a user performs an action, unlike an HTTP request, during which users must wait for a whole new page to load. For example, a weather forecasting site could display local conditions on one side of the page without delay after a user types in a zip code.
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 page. In the article that coined the term Ajax, Jesse James Garrett explained that the following technologies are incorporated:
HTML (or XHTML) and CSS for presentation
The Document Object Model (DOM) for dynamic display of and interaction with data
XML for the interchange of data, and XSLT for its manipulation
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. 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,[8] although other formats such as preformatted HTML or plain text can also be used.
Asynchronous HTML and HTTP (AHAH) involves using XMLHTTPRequest to retrieve (X)HTML fragments, which are then inserted directly into the Web page.