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

pcResult = true;
If you enter a misspelled word, for example, ―absquatalate,‖ the server should return the
corrected spelling:
rpcComplete = true;
rpcResult = "absquatulate";
This example is shown in action in Figure 19-2.
Figure 19-2: Spellchecking using RPC
One aspect of the previous script is particularly noteworthy: If we want to send an RPC request
while one is already pending, we must wait for the first request to complete. Once the browser
has begun loading the first RPC‘s , we can‘t stop it, and if we start another RPC
while it‘s loading we run the risk of clobbering the return values. There are ways around this
problem. For example, you can keep arrays of timers and return values in order to manage
multiple requests concurrently. Another, more elegant approach is to use a callback.
Callbacks
A callback is a function that will be called when an RPC completes. The idea is simple: instead
of periodically checking whether an RPC has completed, have the script returned by the server
call a function instead.
Using a callback simplifies our spelling correction script:
var commandURL = "http://demos.javascriptref.com/checkspelling.php?";
// Sends the RPC given in url. The server will return JavaScript that
calls
// RPCComplete() with the result.
function sendRPC(url)
{