Newbie question: JS to capture URL respondent came from?

I'm using a single anonymous survey link and asking people from around the web to take my survey. I'd like to capture which domain people are coming from. Because of the number of sites involved, using a query string at the end of the survey link is not an option. I'm hoping to use JavaScript to capture it but don't know:

• how to write the code


• where to insert the code