[teye]

"In speaking with a few colleagues, it appears Twitter is using the 301 redirect on the short URL to provide the mouseover with the full URL."

Is this just a non-technical person attempting to inject a little detail into the article? My first thought was, "How would I do this in JS?" and it took me a second to decipher that sentence.

[jaxn]

I think so.

I am not HTTP expert, but my understanding is that a 301 is a RESPONSE not a REQUEST.

What is most certainly happening is that Twitter is making a HEAD request to URLs to see if a redirect is happening. The http redirect is a RESPONSE of type 301 (permanent) or 302 (temporary).

And yes, I suspect this is happening somewhere that can be cached. It would be easy to tell by setting up a 301 redirect, posting it to twitter, and watching the server logs.

[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.