If I search for something on google and click on a result (mytestsite.com), the referer to that site will be URL of the google search.
Now on that site, there is a J
I'm a little unclear on what you are trying to do, but you can grab the referrer with JavaScript using:
document.referrer
...and pass it along to the server in your request for the JS file. Several ways to do this...here's one:
<script>
var e = document.createElement("script");
e.src = 'someJSfile.js?referrer='+document.referrer;
e.type="text/javascript";
document.getElementsByTagName("head")[0].appendChild(e);
</script>
A script tag will always refer to the document that is sourcing it. If you're doing something special on the server you might want to consider using a session or cookies.