The AJAX Core Engine

AJAX is not a particular technology or product. It refers to a number of client features, and related
development techniques, that make Web applications look like desktop applications. AJAX
doesn’t require any plug-in modules either and is not browser specific. Virtually any browser
released in the past five years can serve as a great host for AJAX-based applications. AJAX development
techniques revolve around one common software element—the XMLHttpRequest object.
The availability of this object in the object model of most browsers is the key to the current
ubiquity and success of AJAX applications. In addition to XMLHttpRequest, a second factor
contributes to the wide success of AJAX—the availability of a rich document object model in
virtually any browser.

Originally introduced with Internet Explorer 5.0, the XMLHttpRequest object is an internal
object that the browser publishes to its scripting engine. In this way, the script code found in
any client page—typically, JavaScript code—can invoke the object and take advantage of its
functionality.

The XMLHttpRequest object allows script code to send HTTP requests and handle their
response. Functionally speaking, and despite the XML in the name, the XMLHttpRequest
object is nothing more than a tiny object designed to place HTTP calls via script in a
non-browser-led way. When users click the submit button of a form, or perform any
action that ends up invoking the submit method on the DOM’s form object, the browser
kicks in and takes full control of the subsequent HTTP request. From the user’s perspective,
the request is a black box whose only visible outcome is the new page being displayed. The
client script code has no control over the placement and outcome of the request.