The Document.referrer property returns the URI of the page that linked to this page.

The value is an empty string if the user navigated to the page directly (not through a link, but, for example, by using a bookmark). Because this property returns only a string, it doesn't give you document object model (DOM) access to the referring page.

Inside an <iframe>, the Document.referrer will initially be set to the same value as the href of the parent window's Window.location.