Yep I actually cannot control the status code at all. Part of what made this a fun and hacky project was the fact that I was actually redirecting to the actual URLs by catching the 404s. So, the only way i could think of catching the infinite loop was to supply the domain. That being said, I do appreciate the feedback and it's always great to learn more!
I think you could catch the infinitive loop client-side as well by comparing what the URL to redirect to is, and comparing it with the current URL. If they are the same, prevent the redirect. Although you would still be able to create loops by using two issues instead. Maybe just comparing the full hostname? If it's nlsn.cf, don't redirect.
Doesn't work; eg, make http://someothershorten.er/foo -> http://nlsn.cf/1234 -> http://someothershorten.er/foo.
By the way, your feedback is good for author improves. Thank you.
You might want to detect infinite loops as well :) https://nlsn.cf/6
Last note, the status code seems to be incorrectly set to 404 instead of any of the redirection status code. If you're hosting it on GitHub pages, I'm not sure you can actually control it. If the status code was correctly set + the Location header, I think all browsers would be able to detect the infinite loop automatically and prevent it.