Ajax stands for Asynchronous JavaScript And XML. Although strictly speaking Ajax is not itself a technology, it mixes well-known programming techniques in an uncommon way to enable web developers to build Internet applications with much more appealing user interfaces than those to which we have become accustomed.

When using popular desktop applications, we expect the results of our work to be made available immediately, without fuss, and without our having to wait for the whole screen to be redrawn by the program. While using a spreadsheet such as Excel, for instance, we expect the changes we make in one cell to propagate immediately through the neighboring cells while we continue to type, scroll the page, or use the mouse.

Unfortunately, this sort of interaction has seldom been available to users of webbased applications. Much more common is the experience of entering data into form fields, clicking on a button or a hyperlink and then sitting back while the page slowly reloads to exhibit the results of the request. In addition, we often find that the majority of the reloaded page consists of elements that are identical to those of the previous page and that have therefore been reloaded unnecessarily; background images, logos, and menus are frequent offenders.

Ajax promises us a solution to this problem. By working as an extra layer between the user’s browser and the web server, Ajax handles server communications in the background, submitting server requests and processing the returned data. The results may then be integrated seamlessly into the page being viewed, without that page needing to be refreshed or a new one being loaded.

In Ajax applications, such server requests are not necessarily synchronized with user actions such as clicking on buttons or links. A well-written Ajax application may already have asked of the server, and received, the data required by the user—perhaps before the user even knew she wanted it. This is the meaning of the asynchronous part of the Ajax acronym.

The parts of an Ajax application that happen “under the hood” of the user’s browser, such as sending server queries and dealing with the returned data, are written in JavaScript, and XML is an increasingly popular means of coding and transferring formatted
information used by Ajax to efficiently transfer data between server and client.

We’ll look at all these techniques, and how they can be made to work together, as we work through the chapters.