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Summary • 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-sidescripting language, especially ECMAScriptimplementations 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, although any format will work, including preformatted HTML, plain text, JSON and even EBML. • 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.
History • Although the term 'Ajax' was coined in 2005, most histories of the technologies that enable Ajax start a decade earlier with Microsoft's initiatives in developing Remote Scripting. However techniques for the asynchronous loading of content on an existing web page without requiring a full reload date back as far as the IFRAME element type (introduced in Internet Explorer 3 in 1996) and the LAYER element type (introduced in Netscape 4 in 1997, abandoned during early development of Mozilla). Both element types had a src attribute that could take any external URL, and by loading a page containing javascript that manipulated the parent page, Ajax-like effects could be attained. • Microsoft's Remote Scripting (or MSRS, introduced in 1998) acted as a more elegant replacement for these techniques, with data being pulled in by a Java applet which the client side could communicate with using JavaScript. This technique worked on both Internet Explorer version 4 and Netscape Navigator version 4 onwards. Microsoft took first advantage of these techniques in Outlook Web Access supplied with the Microsoft Exchange Server 2000 release. • The web development community, first collaborating via the microsoft.public.scripting.remote newsgroup and later through blog aggregation, subsequently developed a range of techniques for remote scripting in order to enable consistent results across different browsers. Early examples include JSRS library from 2000, the introduction of the Image/Cookie technique in 2000, and the JavaScript on Demand technique in 2002. In 2002, a user-community modification to Microsoft Remote Scripting was made to replace the Java applet with XMLHttpRequest. • Remote Scripting Frameworks such as ARSCIF surfaced in 2003 not long before Microsoft introduced Callbacks in ASP.NET. • Since XMLHttpRequest is now implemented across the majority of browsers in use, alternative techniques are used infrequently. However, they are still used where wide compatibility, small implementation, or cross-site access are required. One alternative, the SVGT protocol, employs a persistent connection for continuous exchange between browser and service.
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. Google Maps, for example, performs searches in an invisible IFRAME and then pulls results back into an element on the visible web page; 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 URLfragment identifier (the portion of a URL after the '#' [1][2]) 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 [3]. 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 [4], smart preloading of data [5], and proper handling of the XMLHttpRequest object [6] users might experience delay in the interface of the web application, something which users might not expect or understand [7]. 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. • In general the potential impact of latency has not been "solved" by any of the open source Ajax toolkits and frameworks available today, such as the effect of latency variance over time [8].
Cons (cont.) • 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. • Name issues • There have been some critics of the term Ajax, claiming that Adaptive Path (the consulting firm that coined the term [9]) or other proponents are using it as a marketing vehicle for previously-used techniques [10][11][12][13]. • Accessibility • Using Ajax technologies in web applications provides many challenges for developers interested in adhering to WAIaccessibility 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 support Ajax • (Note that this is a general list, and support of Ajax applications will depend on the features the browser supports.) • MicrosoftInternet Explorer version 5.0 and above, and browsers based on it (Mac OS versions not supported) • Gecko-based browsers like Mozilla, Mozilla Firefox, SeaMonkey, Camino, Flock, Epiphany, Galeon and Netscape version 7.1 and above • Browsers implementing the KHTMLAPI version 3.2 and above, including Konqueror version 3.2 and above, AppleSafari version 1.2 and above, and Nokia's Web Browser for S60 3rd edition and above • Opera browsers version 8.0 and above, including Opera Mobile Browser version 8.0 and above
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)
