Ajax - the XMLHTTPRequest
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Ajax (Asynchronous Javascript And XML) is a
technique (or, more correctly, a combination of techniques) for submitting
server requests 'in the background' and returning information from the server to
the user without the necessity of waiting for a page load.
Ajax is actually a combination of several technologies working together to
provide this capability.
How does it work?
Instead of a
user request being made of the server via, for example, a normal HTTP POST
or GET request, such as would be made by submitting a form or clicking a
hyperlink, an Ajax script makes a request of a server by using the Javascript
XMLHTTPRequest object.
Although this object may be unfamiliar to many, in fact it behaves like a
fairly ordinary javascript object. As you may well know, when using a
javascript image object we may dynamically change the URL of the image source
without using a page refresh. XMLHTTPRequest retrieves
information from the server in a similarly invisible manner.
How is it coded?
There are a
few, relatively simple, steps to coding an Ajax application. The
description below is an attempt to describe the salient points without bogging
down the new user in too many of the technicalities.
Firstly, we need to know how to create an
XMLHTTPRequest object. The process differs slightly depending on
whether you are using Internet Explorer (5+) with ActiveX enabled, or a
standards-compliant browser such as Mozilla Firefox.
With IE, the request looks like:
http = new
ActiveXObject("Microsoft.XMLHTTP");
whereas in a
standards-compliant browser we can instantiate the object directly:
http = new
XMLHttpRequest();
NOTE ON BROWSERS:
When you're experimenting with Ajax applications, it's important to check that your programs work in all of the commonly-used browsers. It always surprises me that so many Windows users don't install other browsers - at the very least, Firefox - to try in addition to Internet explorer. It's free, has pop-up blocking, tabbed browsing, as well as privacy and security features, and a rapidly-exoanding user base. If you run Windows, you can install Firefox alongside IE and compare the results in both. Why not take a moment to get it now?
There is a more detailed discussion of the XMLHTTPRequest object here and examples of code to create the object here, which clearly demonstrates the different approaches for the different browser types.
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