External javascript variable problem

Question

QUESTION: Why would an external javascript variable be undefined if I set it equal to all the links in a page?
BACKGROUND: I have a simple image swap function that works when the script is all in the html. But when I tried taking the script to an external javascript file, when I gather all the anchors on the page. The variable that is supposed to refer to the anchors is considered undefined in the middle of the script.
EXAMPLE: I have place an example online. You may check it out. Please.
Thanks!
Message was edited by: UteFanJason

Answer 1

So I checked out that link, and some information on scope. I got the function to work. Check it out.
I did some more reading, and it turns out that the onload event is when the entire DOM has finished loading. (That is according to a book put out a couple of years ago so it could be different now).
This is what I changed the the code to:
function prepare_thumbs() {
    var main_img = document.getElementById('main_img');
    links = document.getElementsByTagName('a');
    for (x=0; x<links.length; x++) {
        links[x].onclick = function() {
            main_img.setAttribute('src', this.getAttribute('id'));
            return false;
window.onload = prepare_thumbs;
Again, thanks for the help!