Prototype: Ajax.Request w/asynchronous: false

You are correct Wes, callbacks are not called when asynchronous is set
to false. About a month or two ago I actually ran into this same
problem and went wandering around the code for Prototype. If I
remember correctly, there was some code that basically said "If
asynchronous is false, then don't bother with the callbacks".

I can't speak for the Prototype developers, but I suspect the
reasoning for doing that was because of how a synchronous call would
be used. When I used it in my own code it was basically this way:

/* Some initialization code */
var req = Ajax.Request(...) // Synchronous call
/* Do some stuff with the data returned in 'req' */

After the request finishes, you can do things like call
req.responseIsFailure() to determine if you did or did not get a 200
back from the server. Also, req.transport is the original
XMLHTTPRequest object, so req.transport.responseText will give you the
data sent back from the server.

Generally speaking, do not ever use synchronous ajax calls in
production code, as the browser will block completely.
As in, if the connection hangs for some reason, the browser is
basically dead meat and hangs (depends on the browser, though).

Best,
Thomas

This is correct, and can be pretty hard on the user... If you make an
Ajax.Request with asynchronous set to false, and it hangs people using
Firefox will not be able to properly close their browsers. Firefox
currently has a bug open for this very issue. Luckily IE ans Safari
will kill the request no matter where it is and close the browser just
fine but I wouldn't leave out all of those Firefox users...

You might want to delve into the onXXX (onFailure, onLoaded,
onComplete, onException, etc) properties of the Ajax.Request call. You
can check for all sorts of status' form the AJAX request.
http://www.sergiopereira.com/articles/prototype.js.html#Ajax.Request
(Scroll down a bit.)

You can assign an anonymous function to these properties detailing
(using JavaScript) how to handle these situations. This way you should
be able to capture enough of the issues and goals of your AJAX
request, without making it a synchronous call. For example you could
make an onComplete that looks like this:

onComplete: function( responseText )
{
    // Passes the data from recieved form the server on the first AJAX
call to a function invoking the second AJAX call.
    makeSecondAjaxCall( responseText );
}

Hope that helps.
-John