ES6 Socket.io server structure

server.ts

import { createServer } from 'http';
import { Server, Socket } from 'socket-io';
import express from 'express';
import * as Socket from './socket';

const app = express();
const httpServer = createServer(app);
const io = new Server(httpServer, {
  // options
});

const { joinRoom } = Socket.default(io);

io.on('connection', (socket: Socket) => {
  socket.on('JOIN_ROOM', joinRoom);
});

httpServer.listen(4000, () => {
  console.log('Listening on port 4000');
});

socket.ts

import { Server, Socket } from 'socket-io';

export default function (io: Server) {

  // roomId parameter and callback function 
  const joinRoom = function (roomId: string, cb: (res: boolean) => void) {
    const socket: Socket = this; // hence, an arrow function will not work

    socket.join(roomId);
    io.in(roomId).emit('USER_JOIN', socket.id);

    cb(true);
  };

  // other functions

  return {
    joinRoom
  };

};