javascript to wait for answer howto...

I would like my Javascript to wait for a server answer and not to poll
every second or so.
Right now I use "window.setInterval" function and it consumes a lot of
trafic for no good reason.

I would like the server to return a value only if its different than
the one sent before otherwise my script should go to sleep and wait
for an answer.

I know it is possible. Yahoo Finance is a good example, look at the
status bar and it says waiting for http://blah...

can someone point me in the right direction please ?

thanks in advance