First, a history lesson:

The misspelling of referrer originated in the original proposal by computer scientist Phillip Hallam-Baker to incorporate the field into the HTTP specification. The misspelling was set in stone by the time of its incorporation into the Request for Comments standards document RFC 1945; document co-author Roy Fielding has remarked that neither “referrer” nor the misspelling “referer” were recognized by the standard Unix spell checker of the period. “Referer” has since become a widely used spelling in the industry when discussing HTTP referrers; usage of the misspelling is not universal, though, as the correct spelling “referrer” is used in some web specifications such as the Document Object Model.

Source: HTTP referer - Wikipedia

…so back in the beginnings of the web, someone made a spelling mistake, and now everyone has to deal with it. (It feels like if every web server and browser agreed to support both, we could eventually kill off the misspelling. I’m sure there’s been an effort to do this, but clearly it hasn’t gotten very far.)

Anyway, if you’re a JavaScript developer, you can use document.referrer (document.referer doesn’t even exist).

If you do HTML, it’s rel="noreferrer" according to the W3C spec.

If you’re a Ruby developer, you can use both and even enforce referrer using Rubocop:

  EnforcedStyle: referrer

More info here: Rails cops: referer vs referrer - Issue #3105 - rubocop-hq/rubocop.