Is there a way to verify if a page exists in Javascript so that you can load another document if the page is not available?

If the page is on the same domain as the script you want to test with, then you can use the XMLHTTP object (the object used in the 'AJAX' features) to check the response code.

If it is not on the same server, you'll run into problems with browser security levels so it's not worth the hassle.