Ajax (programming)

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Ajax (asynchronous JavaScript and XML), or AJAX, is a group of interrelated web development techniques used for creating interactive web applications or rich Internet 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.[1] Data is retrieved using the XMLHttp object or through the use of Remote Scripting in browsers that do not support it. Despite the name, the use of JavaScript, XML, or its asynchronous use is not required.
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 1999, Microsoft created the XMLHttpRequest object 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 World Wide Web Consortium (W3C) released the first draft specification for the object in an attempt to create an official web standard.

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,[3] Jesse James Garrett explained that it refers specifically to these technologies:

* XHTML and CSS for presentation
* the Document Object Model for dynamic display of and interaction with data
* XML and XSLT for the interchange and manipulation of data, respectively
* 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.
* 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, although other formats such as preformatted HTML or plain text can also be used.


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 and load time.
* The use of asynchronous requests allows the client's Web browser UI to be more interactive and to respond quickly to inputs, and sections of pages can also be reloaded individually. Users may perceive the application to be faster or more responsive, even if the application has not changed on the server side.
* The use 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 clicking the browser's "back" button would not return the user to an earlier state of the Ajax-enabled page, but would instead return them to the last page visited before it. Workarounds include the use of invisible IFrames to trigger changes in the browser's history and changing the anchor portion of the URL (following a #) when AJAX is run and monitoring it for changes.
* 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.
* Because most web crawlers do not execute JavaScript code, web applications should provide an alternative means of accessing the content that would normally be retrieved with Ajax, to allow search engines to index it.
* Any user whose browser does not support Ajax or JavaScript, or simply has JavaScript disabled, will not be able to use its functionality.[12] Similarly, devices such as mobile phones, PDAs, and screen readers may not have support for JavaScript or the XMLHttpRequest object. Also, screen readers that are able to use Ajax may still not be able to properly read the dynamically generated content.
* The same origin policy prevents Ajax from being used across domains, although the W3C has a draft that would enable this functionality.
* The lack of a standards body behind Ajax means there is no widely adopted best practice to test Ajax applications. Testing tools for Ajax often do not understand Ajax event models, data models, and protocols.
* Opens up another attack vector for hackers that web developers might not fully test for.
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