Document: referrer property

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 href of the parent's Window.location in same-origin requests. In cross-origin requests, it's the origin of the parent's Window.location by default. For more information, see the Referrer-Policy: strict-origin-when-cross-origin documentation.