server.js

var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);

app.get('/', function(req, res){
  res.sendFile(__dirname + '/index.html');
});

const EVENT_LOOP_TIME = 1000;
const connectedUsers = {}; // Structure is { userId: socket }

io.on('connection', function(socket){
  console.log('a user connected');
  socket.on('disconnect', function() {
    // Clean up after user
  });
  socket.on('initiateUser', function(userId) {
    // Custom event to "set up user" (keep them in memory so we can update them on our interval)
    if (!connectedUsers[userId]) {
      connectedUsers.[userId] = socket;
    }
  });
});

setInterval(() => {
  // 1. Query active games in db that our users are a part of
  const connectedUserIds = Object.keys(connectedUsers);
  const games = db.games.find({ members: { $in: connectedUserIds } }).toArray();
  // For each game, run your logic
  games.forEach((game) => {
    // Logic here
    // Then if a user needs an update, emit an event to them on their socket
    if (needsUpdate) {
      connectedUserIds.forEach((userId) => {
        const socket = connectedUsers[userId];
        socket.emit('pleaseRePoll', game._id);
      });
    }
  });
}, EVENT_LOOP_TIME);

http.listen(3000, function(){
  console.log('listening on *:3000');
});