socket.io/examples/expo-example/server/index.js

import { Server } from 'socket.io';

const io = new Server({
  cors: {
    origin: ['http://localhost:8081']
  }
});

io.on('connection', (socket) => {
  console.log(`connect: ${socket.id}`, socket.request.headers);

  socket.on('disconnect', () => {
    console.log(`disconnect: ${socket.id}`);
  });
});

io.listen(3000);