Ever needed to check the IP address of a user in your (client-side) JS code? Me too.
I know there are existing services out there for doing this, but some of them can be blocked by privacy tools and the like (and felt like overkill sometimes). So here's a file you can add into your functions directory for projects being hosted on Netlify that will get the user's IP address for you.
Once this is in place, you should be able to make requests against /.netlify/functions/ip on your hostname to get a JSON object with the user's IP.
Depending on your needs you may want to poke around with the Access-Control-Allow-Origin header in there if you want to restrict access to a particular hostname.