JavaScript has had the XMLHttpRequest object for a few years now, but
it's really only started getting wide attention recently, due to some
showoff web applications that make every developer who sees them think,
"I want my site to do that!"
A few high-profile Google applications in particular made a splash with
it: Suggest, and Maps, and Gmail. Now it has become a buzzword, and may even
be the first JavaScript object with its own fan website. Date.com doesn't count, although I did
have a scintillating chat with a lady there once about the
getTimeZoneoffset method.
So what is this fancy object that everybody wants a piece of? In brief,
it's a solution to one of the big annoyances of web interfaces.
Generally, the user inputs some data or makes a choice, and clicks a
button that sends the data to the server. The server takes a look at
the data and sends back a whole new web page, which is loaded into the
browser. Reloading a page every time you want to do something is
annoying, disjunctive and time-consuming for the user. XMLHttpRequest
(I'm going to call it XHR, which sounds nice and high tech) moves that
browser-server interaction to behind the scenes, so the user can just
keep on playing with the same page, even while elements on the page are
talking to the server! Go take a look at Google Suggest if I've lost
you — it's a nice, eloquent example of this.
JavaScript has always been able to sneakily trigger a server-side
script without anything happening in the browser by using a few classic
tricks. This one, for example: onSubmit='runascript = new
Image(); runascript.src="myscript.php?" + this.value'. That sort
of chicanery is good, maybe, for caching form data to a file on the
server, but it doesn't return any information to the JavaScript that
calls it, so its usefulness is limited. XHR, on the other hand, can get
a full parcel of data back from the script it calls. Hence the "XML"
part of the name — which really is just there for looks, kind of
like the "Java" part of Javascript, because the returned data can be
plain text or whatever you like, if XML is overkill or just not your
cup of tea.
This opens up millions of exciting possibilities. Every form
submission, every JavaScript event, and heaven knows what else, can now
interact with server-side databases and processing power. Data
retrieval, password authentication, image generation — you
name it, XHR can trigger it. AdaptivePath.com calls this sort of
asynchronized application Ajax, which is a spiffy, spiffy name.
Look for it.
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