simulate position:fixed with bottom (jquery)

var genuineBottom = parseInt($("#alertwindow-'.$view->getParam("uniqid").'").css("bottom").slice(0,-2),10);
					if (genuineBottom == undefined || genuineBottom == NaN) 
					{
						genuineBottom = 0;
					}
							
					$(window).scroll(function() {
						offset = $($("#alertwindow-'.$view->getParam("uniqid").'")).offset();	
						var newTop = $(window).height() - $("#alertwindow-'.$view->getParam("uniqid").'").height() + $(window).scrollTop() - genuineBottom;		
						$("#alertwindow-'.$view->getParam("uniqid").'").css("top", newTop + "px");
					});