Chapter 18. Using Ajax

The term Ajax was first coined in 2005. It stands for Asynchronous JavaScript and XML, which, in simple terms, means using a set of methods built into JavaScript to transfer data between the browser and a server in the background. An excellent example of this technology is Google Maps (see Figure 18-1), in which new sections of a map are downloaded from the server when needed, without requiring a page refresh.

Using Ajax not only substantially reduces the amount of data that must be sent back and forth, it also makes web pages seamlessly dynamic—allowing them to behave more like self-contained applications. The results are a much improved user interface and better responsiveness.

What Is Ajax?

The beginnings of Ajax as used today started with the release of Internet Explorer 5 in 1999, which introduced a new ActiveX object, XMLHttpRequest. ActiveX is Microsoft’s technology for signing plug-ins that install additional software to your computer. Other browser developers later followed suit, but rather than using ActiveX, they all implemented the feature as a native part of the JavaScript interpreter.

However, even before then, an early form of Ajax had already surfaced that used hidden frames on a page that interacted with the server in the background. Chat rooms were early adopters of this technology, using it to poll for and display new message posts without requiring page reloads.

So let’s see how to implement Ajax using JavaScript.

Figure 18-1. Google Maps is ...

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