With all the hubub around "ajax" these days, it's easy to understand why you'd think it's something "new" or something "Web 2.0". However, there were many sites using "ajax" years ago (Web 1.x, i guess?).

What is it?

First of all, it's not a specific "thing" - it's not a javascript library or a particular browser feature. It's a concept or a pattern. It initially stood for Asynchronous Javascript and Xml but that definition is no longer accurate. It could now be called "asynchronous code and data" - the code doesn't have to be javascript and the data doesn't have to be Xml. In fact, the most common pattern today is javascript code and data in the form of Json.

Of course, "asynchronous code and data" is really just a complicated way of saying client-server communications. In the "Web 2.0" world, the client is a "page" in the browser typically built on javascript and the Browser DOM. The server can be anything built on any platform.

In order to reduce the burden on clent-side developers, there are several javascript libraries available. These libraries are essentially a layer of abstraction over the browsers' implementations of the XmlHttpRequest object. They also typically provide a layer of abstraction over the browsers' DOM differences, but this isn't strictly related to "ajax".

Javascript Ajax Libraries


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