[treyp]

kind of. it's not done on the front end (js) but rather using some service on the backend.

the phrasing of that is a bit weird, but they probably test until they no longer get a 301 or 302 redirect and save the result. they print this result on the front end as a title for the link and as an HTML5 data attribute "expanded-url" on the link.

[jluxenberg]

I looked at doing this using Javascript in a browser extension a while ago, and it's actually not possible to do for a couple of reasons:

  1) XHR cross domain policy prohibits requests to domains other than the domain the page loaded from
  2) the XHR GET method always follows a 301 redirect and returns the content at the URL.  The full URL is not made available.

[RyanMcGreal]

You'd have to write something on your server that does this, and expose it to your client JS via a local URL.