This looks like an implementation of STUN - there are no miracles, if two machines are behind NATs someone need to broker the connection.

Hmm thanks, so that says:

> To broker connections, PeerJS connects to a PeerServer. Note that no peer-to-peer data goes through the server; The server acts only as a connection broker.

> If you don't want to run your own PeerServer, we offer a free cloud-hosted version of PeerServer.

So I suppose there's still a server, but it's shared and run by two folks based on donations: https://peerjs.com/peerserver

Without data going through the server it should be pretty cheap to host, right?

That is why there are public STUN servers available by google that any one can use: stun.l.google.com:19302 stun1.l.google.com:19302 stun2.l.google.com:19302 stun3.l.google.com:19302 stun4.l.google.com:19302

I used peerjs years ago (2013~). It was a whole lot of pain back then... I assume things have gotten better, but damn was it painful! I ended up forking and fixing/maintaining my own fork because it was too slow to get things fixed in master.