if(isset()) equivalent for jQuery

if(isset()) equivalent for jQuery

Hi guys, I'm very new to jQuery but i'd like to be able to code in jQuery too. I am trying to wrap a PHP class output in jQuery , so far I've managed to integration part however there's another problem when it comes to testing the code, the .click(function()) trigger doesnt seem to fit the bill properly. I have to device another way to get jQuery to listen to the PHP output and display it ( kinda like isset) in PHP. anyone has got some idea on achieving that ?