presence.js

const Redis = require('ioredis');
const express = require('express');
const app = express();
const http = require('http');
const server = http.createServer(app);
const io = require('socket.io')(server);

const redis = new Redis();

const redisAdapter = require('socket.io-redis');
io.adapter(redisAdapter({ host: 'localhost', port: 6379 }));

const key = 'sia:viewers';
// clear key on restart
redis.del(key);

const notify = async () => {
  const viewers = await redis.scard(key);
  io.emit('update', { viewers });
}

io.on('connection', async (socket) => {
  console.log('socket connected..');
  if (!await redis.sismember(key, socket.id)) {
    await redis.sadd(key, socket.id);
  }

  // notify others
  notify();

  socket.on('disconnect', async () => {
    await redis.srem(key, socket.id);
    notify();
  })
});

server.listen(3000, () => {
  console.log('server started');
});