Request.referrer
  The referrer read-only property of the
  Request interface is set by the user agent to be the referrer of the
  Request. (e.g., client, no-referrer, or a URL.)
    Note: If referrer's value is no-referrer,
    it returns an empty string.
  
Value
A DOMString representing the request's referrer.
Examples
  In the following snippet, we create a new request using the
  Request() constructor (for an image file in the same directory as
  the script), then save the request referrer in a variable:
var myRequest = new Request('flowers.jpg');
var myReferrer = myRequest.referrer; // returns "about:client" by default
Specifications
| Specification | 
|---|
| Fetch Standard # ref-for-dom-request-referrer① | 
Browser compatibility
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