User:FatalError/Ajax (programming)

"AJAX" redirects here. For other uses, see 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. JavaScript is the scripting language in which Ajax function calls are usually made. Data is retrieved using the XMLHttpRequest object, or through the use of Remote Scripting in browsers that do not support it. In any case, it is not required that the content be formatted in XML or retrieved asynchronously.

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 as an ActiveX control in Internet Explorer 5, and developers of Mozilla and Safari followed soon after with native versions of the object. On April 5, 2006, the XMLHttpRequest object became an official web standard.

Advantages

• In 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. A user of a web application that uses Ajax will get the feeling that changes are happening instantaneously.

• The used 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 triggering the "back" function of the user's browser might not bring the desired result; rather than returning the user to an earlier state of the Ajax-enabled page, the back button would return the user to the last page visited before the Ajax-enabled page. 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.

Response-time

Network latency, the interval between user request and server response, must be considered carefully during Ajax development. Without clear feedback to the user, preloading of data and proper handling of the XMLHttpRequest object, users might experience delays in the interface of the web application, something which they might not expect or understand. Additionally, when an entire page is rendered there is a brief moment of re-adjustment for the eye as content changes. The lack of this re-adjustment with smaller portions of the screen changing makes the latency more apparent. The use of visual feedback (such as throbbers) to alert the user of background activity and/or preloading of content and data are often-suggested solutions to these latency issues.

Search engine optimization

Websites that use Ajax to load data which should be indexed by search engines must be careful to provide equivalent data for clients without JavaScript, as search engines do not generally execute the JavaScript code required for Ajax functionality. This problem is not specific to Ajax, as the same issue occurs with sites that provide dynamic data as a full-page refresh in response to, say, a form submit. (The general problem is sometimes called the deep web.)Using mod_rewrite, this issue can be resolved.

Reliance on JavaScript and the DOM

Ajax relies on JavaScript and the browser's Document Object Model (DOM), which is often implemented differently by different browsers or versions of a particular browser. Because of this, sites that use JavaScript may need to be tested in multiple browsers to check for compatibility issues. It's common to see JavaScript code written twice—one part for Internet Explorer, another part for Mozilla compatibles—although this is less true with the release of IE7 and with the now-common use of JavaScript libraries such as Prototype or jQuery, which abstract browser-specific differences from the web developer.

The level of IDE support for JavaScript used to be poor, although it is changing with more wide-spread use of tools like Drosera, Firebug, Microsoft Script Debugger and Venkman.

Finally, if the user has switched off JavaScript support, pages designed to use Ajax may not function.

Web analytics

Many web analytics solutions expect a new page to be loaded whenever new or updated content is displayed to the user, and after each step in a process such as a check-out. Since Ajax alters this process, developers must take care to instrument a page or a portion thereof so that it can be accurately tracked. Analytics systems which can track events other than simple page views, such as clicks of buttons or links, are most able to accommodate Ajax-using sites.

Accessibility

Main article: WAI-ARIA

Non-Ajax users would ideally continue to load and manipulate the whole page as a fall back, enabling the developers to preserve the experience of users in non-Ajax environments (including all relevant accessibility concerns) while giving those with capable browsers a much more responsive experience. For this reason it is advised to first develop a full application without Ajax, and implement Ajax enhancements as an addition only (see: Progressive enhancement methodology). The same counts for JavaScript in general, which can be disabled in most browsers.

Because many people consider it unreasonable to advise against the use of JavaScript in modern web applications, the W3C Web Accessibility Initiative (WAI) is working on a specification for Accessible Rich Internet Applications (ARIA). In effect, ARIA allows web pages, or portions of web pages, to declare themselves as applications rather than as documents. This allows Ajax interactions and other types of JavaScript-enabled UI controls to interact with assistive technology, like screen readers for the vision-impaired.

One early user agent implementation is Firefox 3, which maps WAI-ARIA to the accessibility API of the underlying operating system, making it possible for screen readers to treat ARIA widgets in much the same way as the widgets found in client-side software. Another early implementation is Fire Vox, an extension that turns Firefox into a self-voicing browser. Since Fire Vox operates directly on the DOM and does not use the operating system's accessibility API, it can handle WAI-ARIA features, such as live regions, which have no mapping to traditional desktop applications.

Ajax libraries and toolkits are also adopting ARIA. Notable examples include Google Web Toolkit, Backbase and Dojo Toolkit. In addition, AxsJAX, a JavaScript library which focuses on web-application accessibility, includes several abstractions built on top of ARIA.

See also

• Ajax framework
• Progressive enhancement
• Reverse Ajax
• Rich Internet application
• XMLHttpRequest

External links

• AJAX at the Open Directory Project.
• Jesse James Garrett. “Ajax: A New Approach to Web Applications”, Adaptive Path
• Ajax Tutorial with get, post, text and XML examples.
• Attacking AJAX Applications Presentation on Ajax Security issues given at the Black Hat security conference.