JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

The HTTP response code 204 ―No Content,‖ which we alluded to earlier, is a very useful, but
not well-known feature of the HTTP protocol. When a server returns a 204 (as opposed to, say,
a 200 ―OK‖ or 404 ―File not found‖), it is a signal to the browser that for some reason the server
doesn‘t have any data to send in response, so the browser should take no action. Even though
the browser makes the request to the server, pointing your browser at a URL that returns a 204
has no visual effect: the browser appears to do nothing.
We can use HTTP 204 response codes to our advantage by making an RPC call via a
JavaScript redirect to a URL that returns a 204. To set this up, configure your Web server (or a
cgi script) to return a 204 for a particular URL, for example, setrating.cgi. Then write your
JavaScript to redirect the browser to this URL with the parameters of the message you wish to
send.
The only part of the previous example that needs to be changed to use this technique is the
sendURL() function, which will now redirect instead of loading a fake image:
function sendURL(url)
{
}
window.location = url;
To send an RPC, the JavaScript constructs the URL and then redirects the browser to it. When
the server receives the request, it returns a 204, which causes the browser to cancel navigation
since there‘s no page to navigate to. The user is none the wiser, though some browsers may
show a slight indication of browser activity hinting at the behind-the-scenes transmission.
When many developers first encounter this technique, they‘re often quite skeptical that it could
actually work in a wide array of browsers. The authors have verified that it works in Internet
Explorer 3 and later, Netscape 4 and later, and Mozilla-based browsers, and we wouldn‘t be
surprised to hear if it was nearly universally supported.
One advantage this technique has over image-based techniques is that it is often perceived by
users as less ―scary.‖ Many users rightly worry about ―Web bugs,‖ invisible images placed on
pages that are fetched from third-party servers in order to track browsing habits. Some users
examining the page are probably more likely to feel comfortable with a redirect than an
undercover image fetch.
A disadvantage of this technique is that if the user‘s browser doesn‘t properly implement HTTP,
it could actually navigate to a blank page as a result of the redirect. Though this is extremely
uncommon, some developers may feel it is a compelling reason to use images instead.
Two-Way Communication
Communicating something from the browser to the server without a round trip is a useful
technique, but much more can be accomplished if JavaScript can also receive a return value. In
this section, we cover a number of ways to implement two-way communication (true RPC), from
the primitive to the mature. The techniques we discuss are not, however, the only mechanism
by which to do RPC. You could use JavaScript to pull in other kinds of dynamic content, to
control a Java applet that talks to the server, to drive an ActiveX control that handles
networking, or to leverage proprietary browser enhancements such as IE‘s data source
features.
Images
Since the height and width properties of an Image are automatically filled in by the browser
once an image has been downloaded, your server can return images of varying dimensions to