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

Note Don’t think that because we are using form buttons here that this is a traditional style
communication. In fact, we could have used just about any object we could click by
attaching an onclick, including links () or even structural elements like or
, but we used form buttons since the user would feel they were clickable!
We‘ve omitted the server-side script that presumably handles these requests. You could write a
simple CGI that returns a 1-pixel-by-1-pixel image if you like, or you could just return any old
content you like. Since we‘re not doing anything with the result, we don‘t even care if the server
returns an error. In fact, you don‘t even have to have a setrating.cgi to handle this request.
Instead, you could just let the server return a 404 and extract the information from your logs.
These requests will result in log lines that look something like this:
www.example.com - - [19/Mar/2004:21:05:29 -0800] "GET
/setrating.cgi?productid=2158&rating=1&user=fritz HTTP/1.0" 403 305
You could easily write a script to comb your logs for these messages, parse them, and do with
the information what you will (e.g., enter it in a database).
Note In the preceding example, you don’t necessarily have to pass user information via the CGI
parameters. If you use cookies for authentication, the cookie will be sent as usual along
with the request, and the server-side script can extract the user’s identity from the cookie.
A more elegant approach would be to have your server-side program record the data and then
actually pass back a proper response code to the browser. In this case, we could return a 204
HTTP Response code indicating no content so that the browser wouldn‘t think anything was
amiss. While this would appear the cleaner way to do things, it isn‘t really necessary. However,
don‘t play fast and loose with HTTP; there are significant pitfalls to avoid.
Encoding Parameters
Some characters are illegal in URLs, and Web servers will often choke if they are included. For
example, you can‘t have carriage returns or backspaces in a URL. If the parameters you wish
to pass the server might include problematic characters, you need to encode them. Encoding
replaces problematic characters with their ASCII values as a hexadecimal escape sequence,
for example, %0D for a new line. The Web server automatically decodes URLs it receives and
makes the decoded parameters available to CGIs and the like.
To encode a string, simply call encode() on it. We can rewrite the parameter setup code from
the previous example in this way:
var params = "productid=" + encode(productid);
params += "&rating=" + encode(rating);
params += "&user="
It‘s almost always a good idea to encode your parameters, even if you don‘t think there‘s a
chance for problematic characters to sneak in.
Other Objects
There‘s no particular reason to use Images for this RPC other than that they‘re widely
supported and simple to script. Any (X)HTML element that has a property that can be set to a
URL will work. One common technique is to use a single hidden for
communication. Whenever you wish to send a message to the server, set the ‗s src
to the message URL. We‘ll see that s are more commonly used for two-way
communication in a later section.
Redirects