Unless they use a proxy every internet interaction contains the originators ip address. It's how you are able to respond to it.
Assuming you are developing node.js on a server.
That's more of a function of your web server's access logs (or your hosted service's analytics, if in the cloud) than Node.js itself.
req.ip, but also look at the x-forwarded-for header if you are using a proxy server. This header can have multiple ip's when you have say nginx terminate ssl and use cloudflare.
are the users using http/https to access the app?
Would it be possible to use fetch to call out to the whatismyip.com API? Or scrape it?
https://www.whatismyip.com/api/