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Ajax, shorthand for Asynchronous JavaScript and XML, 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) and CSS, for marking up and styling information.
The DOM accessed with a client-side scripting language, especially ECMAScript implementations such as 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 sometimes used as the format for transferring data between the server and client, although any format will work, including preformatted HTML, plain text, JSON and even EBML. These files may be created dynamically by some form of server-side scripting.
Like DHTML, LAMP and SPA, Ajax is not a technology in itself, but a term that refers to the use of a group of technologies together.

By generating the HTML locally within the browser, and only bringing down JavaScript calls and the actual data, Ajax web pages can appear to load quickly since the payload coming down is much smaller in size. An example of this technique is a large result set where multiple pages of data exist. With Ajax, the HTML of the page, e.g., a table control and related TD and TR tags can be produced locally in the browser and not brought down with the first page of data. If the user clicks other pages, only the data is brought down, and populated into the HTML generated in the browser.

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 need 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.

While the Ajax platform is more restricted than the Java platform, current Ajax applications effectively fill part of the niche first served by Java applets: extending the browser with lightweight mini-applications.
Web applications that utilise Ajax may break the expected behavior of the browser's back button. The difference between returning to a previous state of the current, dynamically modified page versus going back to a previous static page might be a subtle one; but users generally expect that clicking the back button in web applications will move their browser to the last page it loaded, and in Ajax applications this might not be the case.

Developers have implemented various solutions to this problem. These solutions can involve 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. The World Wide Web Consortium (W3C) did not include an iframe element in its XHTML 1.1 Recommendation; the Consortium recommends the object element instead.

Another 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 '#' [7] [8]) 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. It is, however, not a complete solution.

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 [9], 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. Additionally, when an entire page is rendered there is a brief moment of re-adjustment for the eye when the 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.

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.

Using Ajax technologies in web applications provides many challenges for developers interested in adhering to WAI accessibility guidelines. In addition there are numerous development groups working on USA government projects which require strict adherence to Section 508 Compliance standards. Failure to comply with these standards can often lead to cancellation of contracts or lawsuits intended to ensure compliance.

Because of this, 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 ideally 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.

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