socket.io/examples/create-react-app-example/server.js

const io = require('socket.io')({
  cors: {
    origin: ['http://localhost:3000']
  }
});

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

  socket.on('hello!', () => {
    console.log(`hello from ${socket.id}`);
  });

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

io.listen(3001);

setInterval(() => {
  io.emit('message', new Date().toISOString());
}, 1000);