More Javascript help, please

[cesy]

I have a link, and when you click on it, it calls edit_tagcloud(tagname);.
I then have a function defined:
function edit_tagcloud(newtag)
{
if (! newtag) return;
var cur_taglist = document.getElementById('prop_taglist').value;
if (cur_taglist.length > 0) {
cur_taglist = cur_taglist + ", " + newtag;
} else {
cur_taglist = newtag;
}
document.getElementById('prop_taglist').value = cur_taglist;
}
What I'm aiming for is that tagname is added to the list already in prop_taglist, which is the id for the tags input box. However, the code above doesn't work - nothing happens when I click on the link. Can anyone tell me what I'm doing wrong, or give me any tips on how to debug Javascript like this?