What the heck is AJAX?

Asynchronous JavaScript And XML, or its acronym, Ajax (Pronounced A-jacks), is a Web development technique for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes, so that the entire Web page does not have to be reloaded each time the user makes a change. This is meant to increase the Web page's interactivity, speed, and usability.

The Ajax technique uses a combination of:

• XHTML (or HTML), CSS, for marking up and styling information.
• The DOM accessed with a client-side scripting language, especially ECMAScript implementations like JavaScript and JScript, to dynamically display and interact with the information presented.
• The XMLHttpRequest object to exchange data asynchronously with the web server. In some Ajax frameworks and in certain situations, an IFrame object is used instead of the XMLHttpRequest object to exchange data with the web server.
• XML is commonly used as the format for transferring data back from the server.

Ajax is not a technology in itself, but a term that refers to the use of a group of technologies together.

Pros, cons and criticism

Pros

Interactivity

Ajax applications are mainly executed on the user's machine, by manipulating the current page within their browser using document object model methods. Ajax can be used for a multitude of tasks such as updating or deleting records; expanding web forms; returning simple search queries; or editing category trees -- all without the requirement to fetch a full page of HTML each time a change is made. Generally only small requests are required to be sent to the server, and relatively short responses are sent back. This permits the development of more interactive applications featuring more responsive user interfaces due to the use of DHTML techniques.

Portability

Ajax applications use well-documented features present in all major browsers on most existing platforms. Though this situation could feasibly change in the future, at the moment, Ajax applications are effectively cross-platform.

While the Ajax platform is more restricted than the Java platform, current Ajax applications effectively fill part of the one-time niche of Java applets: extending the browser with lightweight mini-applications.

Cons and criticism

Usability criticisms

One major complaint voiced against the use of Ajax in web applications is that it might easily break the expected behavior of the browser's back button. The different expectations between returning to a page which has been modified dynamically versus the return to a previous static page might be a subtle one. Users generally expect that clicking the back button in web applications will move to their last page loaded, and in Ajax applications this might not be the case.

Developers have implemented various solutions to this problem, most of which revolve around creating or using invisible IFRAMEs to invoke changes that populate the history used by a browser's back button. It is possible to track user behaviour via callbacks which are called whenever the back button is pressed, restoring the application state that existed at the time.

A related issue is that dynamic web page updates 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. This is possible because many browsers allow JavaScript to update the fragment identifier of the URL dynamically, so that Ajax applications can maintain it as the user changes the application's state. This solution also improves back-button support.

Response-time concerns

Network latency — or the interval between user request and server response — needs to be considered carefully during Ajax development. Without clear feedback to the user, smart preloading of data, and proper handling of the XMLHttpRequest object users might experience delay in the interface of the web application, something which users might not expect or understand. The use of visual feedback to alert the user of background activity and/or preloading of content and data are often suggested solutions to these latency issues.

JavaScript

While no browser plug-in is required for Ajax, it requires users to have JavaScript enabled in their browsers. This applies to all browsers that support Ajax except for Microsoft Internet Explorer 6 and below which additionally require ActiveX to be enabled, as the XMLHttpRequest object is implemented with ActiveX in this browser. Internet Explorer 7, however, will implement this interface as a native JavaScript object and hence does not need ActiveX to be enabled for Ajax to work.

As with DHTML applications, Ajax applications must be tested rigorously to deal with the quirks of different browsers and platforms. A number of programming libraries have become available as Ajax has matured that can help ease this task. Likewise, techniques have been developed to assist in designing applications which degrade gracefully and offer alternative functionality for users without JavaScript enabled.

Accessibility

Using Ajax technologies in web applications provides many challenges for developers interested in adhering to WAI accessibility guidelines. Developers need to provide fallback options for users on other platforms or browsers, as most methods of Ajax implementation rely on features only present in desktop graphical browsers.

Web developers use Ajax in some instances to provide content only to specific portions of a web page, allowing data manipulation without incurring the cost of re-rendering the entire page in the web browser. Non-Ajax users would optimally continue to load and manipulate the whole page as a fallback, allowing 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.

Browsers that do not support Ajax

This is a list of browsers that definitely do not support Ajax

• Opera 7 and below
• Microsoft Internet Explorer 4.0 and below
• Text-based browsers like Lynx and Links
• Browsers for the visually impaired (speech-synthesizing, braille)