AJAX is often cited in the context
of Web 2.0 (http://en.wikipedia.org/wiki/Web_2) as being one of the technologies that allows
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for creating next-generation web sites, which provide a friendly environment that facilitates
online information sharing and collaboration.
Technically, the only ???new??? thing about AJAX is the name. The technologies required to
create AJAX web applications have been around for a while. To perform asynchronous server
calls (send server requests in the background), we use the XMLHttpRequest object of JavaScript
(together with a few other lesser-known techniques). To update the visitor??™s page in response
to the data retrieved from the server, we use the Document Object Model (DOM) eventually
together with Cascading Style Sheets (CSS) for a modern presentation layer. And to encode
the information passed back and forth from the server, XML (the format implied by the AJAX
acronym) or other data formats can be used, with a popular alternative to XML being JavaScript
Object Model (JSON). We??™ll discuss each of these shortly, but first we??™ll show a visual representation
of how an AJAX-enabled web page works. See Figure 13-1.
Figure 13-1. Asynchronous server call using AJAX
JavaScript
JavaScript is a programming language supported by all modern web browsers, which makes it
ideal for implementing various features at the client (web browser) level.
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