Tài liệu JavaScript & DHTML Cookbook

It may be difficult to imagine that a technology born as recently as 1995 would have
had enough of a life cycle to experience a rise and fall in popularity, followed now by
an amazing renaissance. Client-side scripting, begun initially with JavaScript embedded
in Netscape Navigator 2, has experienced such a roller coaster ride. A number of
early incompatibilities among major browsers caused many a content author’s head
to ache. But we learned to live with it, as a long period of stability in one platform—
Internet Explorer 6, in particular—meant that we could use our well-worn compatibility
workarounds without cause for concern. Another stabilizing factor was the
W3C DOM Level 2 specification, which remained a major target for browser makers
not following Microsoft’s proprietary ways. Mozilla, Safari, and Opera used the
W3C DOM as the model to implement, even if Microsoft didn’t seem to be in a
hurry to follow suit in all cases.
Two factors have contributed to the rebirth of interest in JavaScript and Dynamic
HTML. The first is the wide proliferation of broadband connections. Implementing
large client-side applications in JavaScript can take a bunch of code, all of which
must be downloaded to the browser. At dial-up speeds, piling a 50–75 kilobyte script
onto a page could seriously degrade perceived performance; at broadband speeds,
nobody notices the difference.
But without a doubt, the major attraction these days is the now widespread availability
in all mainstream browsers of a technology first implemented by Microsoft: the
XMLHttpRequest object. It’s a mouthful (leading some to refer to it as, simply, XHR),
but it allows background communication between the browser and server so that a
script can request incremental data from the server and update only a portion of a
page. It is far more efficient than downloading a bunch of data with the page and less
visually disruptive than the old submit-and-wait-for-a-new-page process. To help put
a label on the type of applications one can build with this technology, the term Asynchronous
JavaScript and XML (Ajax) was coined. In truth, Ajax is simply a catchy
handle for an existing technology.