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 JS file include that is used for tracking purposes..however the referrer to that JS file request is mytestsite.com...is there no way for the server handling the JS request to know that it originated from a google search?
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>
document.referrer - it's the bestA 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.
document.referrer. My point was that the server cannot know the previous site's referrer when the JS is requested (only when the HTML is requested), which is what the question was asking. The only difference in the solution is that the other answer is passing the referrer as a query string instead of as a request header. I've updated my answer to clarify the intent and solution, but generally I think people are misreading the question.
referer is among the forbidden header names which "cannot be modified programmatically".