I seem to have a problem with my javascript code. What I want to do is make the last line of code alert out the value of a html checkbox name which has been dynamically made in javascript. It is trying to look for prechecksub in the html but I want to the value of prechecksub and insert it into the alert if this makes sense. Probably not

Code:
function catboxclick(cat)
{
	alert("catboxclick called");
	
		var prechecksub = "checksub" + cat;
		alert(prechecksub);
	alert(document.catcheckboxes.checkcatgroup.length);
	alert(document.catcheckboxes.checksub0.length);
	
	alert(document.catcheckboxes.prechecksub.length); // this doesnt work, there is no such html element as prechecksub, it is made in JS.
}