A ".js" file is a text file including ONLY JavaScript commands. There is no need to type in:
"<!--Begin", "//End-->", or even the script tags in that document. All you need to do is place this on your page to activate:
   <SCRIPT LANGUAGE=JAVASCRIPT SRC=fileLocation.js><SCRIPT>
Best of all, that one page of script is useable on multiple pages, and multiple times per page. Here are some examples: