HTML5, JavaScript, and jQuery 24-Hour Trainer

Lesson 35

AJAX

The web was designed to operate with the HTTP protocol. As mentioned previously, the HTTP protocol is a simple request/response protocol: The browser requests a resource, and a response is received and rendered.

This model worked well for document-centric websites because a document (an HTML page) or image could be requested and the response rendered directly in the browser. This model does not work so well for dynamic web applications.

A common requirement for web applications is to request additional information after a page has loaded. For instance, a browser-based chat application will constantly check for new ­messages and display them immediately via DOM manipulation. Without this functionality, the user is required to constantly refresh the entire web page to check for new messages.

Although it is possible to automatically refresh an entire web page at periodic intervals, the refresh process will not only cause a very noticeable delay but it will interrupt whatever the user is doing. For instance, if the user was halfway through typing a message, this message would be cleared out and lost.

In order to support web applications such as a chat application, Microsoft introduced a JavaScript feature into Internet Explorer called the XMLHttpRequest object all the way back in 1999. This allowed HTTP requests to be performed using JavaScript after the web page had loaded, and the response to be processed and incorporated directly into the DOM.

XMLHttpRequest