How do I find the a referring sites URL in node?

In express 4.x:

req.get('Referrer')

This will also check both spellings of referrer so you do not have to do:

 req.headers.referrer || req.headers.referer

javascript http node.js express

If you mean how do you get it when running an express server, then it's done using the header method on your request:

req.headers.referer;// => "http://google.com"

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