Ajax

Ajax (also AJAX; pronounced /ˈeɪdʒæks/; an acronym for asynchronous JavaScript and XML)^[1] is a group of interrelated web development methods used on the client-side to create interactive web applications. With Ajax, web applications can send data to, and retrieve data from, a server asynchronously (in the background) without interfering with the display and behavior of the existing page. Data are usually retrieved using the XMLHttpRequest object. Despite the name, the use of XML is not needed (JSON is often used instead), and the requests need not be asynchronous.^[2]

Like DHTML and LAMP, Ajax is not one technology, but a group of technologies. Ajax uses a combination of HTML and CSS to mark up and style information. The DOM is accessed with JavaScript to dynamically display, and to allow the user to interact with the information presented. JavaScript and the XMLHttpRequest object provide a method for exchanging data asynchronously between browser and server to avoid full page reloads.

History

In the 1990s, most web sites were based on complete HTML pages; each user action required that the page be re-loaded from the server (or a new page loaded). This process is inefficient, as reflected by the user experience: all page content disappears then reappears, etc. Each time a page is reloaded due to a partial change, all of the content must be re-sent instead of only the changed information. This can place additional load on the server and use excessive bandwidth.

Asynchronous loading of content first became practical when Java applets were introduced in the first version of the Java language in 1995. These allow compiled client-side code to load data asynchronously from the web server after a web page is loaded.^[3] In 1996, Internet Explorer introduced the iframe element to HTML, which also enabled asynchronous loading.^[4] In 1999, Microsoft utilized its iframe technology to dynamically update the news stories and stock quotes on the default page for Internet Explorer (http://home.microsoft.com) and in April 2000 filed a patent on the basic Ajax technology, which was granted in June 2006.^[5] In 1999, Microsoft created the XMLHTTP ActiveXcontrol in Internet Explorer 5, which was later adopted by Mozilla, Safari, Opera and other browsers as the XMLHttpRequest JavaScript object.^[4][6] Microsoft has adopted the native XMLHttpRequest model as of Internet Explorer 7, though the ActiveX version is still supported. The utility of background HTTP requests to the server and asynchronous web technologies remained fairly obscure until it started appearing in full scale online applications such as Outlook Web Access (2000)^[7] and Oddpost (2002), and later, Google made a wide deployment of Ajax with Gmail (2004) and Google Maps (2005).^[8]

The term Ajax was coined on February 18, 2005 by Jesse James Garrett in an article entitled "Ajax: A New Approach to Web Applications".^[1]

On April 5, 2006 the World Wide Web Consortium (W3C) released the first draft specification for the XMLHttpRequest object in an attempt to create an official web standard.^[8]

[edit]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 page. In the article that coined the term Ajax,^[1] 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. 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.^[2][9] (However, JavaScript is the most popular language for Ajax programming due to its inclusion in and compatibility with the majority of modern web browsers.) Also, 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,^[10] although other formats such as preformatted HTML or plain text can also be used.^[11]