Live User Count with NodeJS and Socket.io

client.html

<script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/2.3.0/socket.io.js"></script>
<script>
  var socket = io("http://localhost:3000", {
    // setting transports to websocket, and not polling
    transports: ["websocket"],
  });

  socket.on("reconnect_attempt", () => {
    // setting transports to websocket, and not polling
    socket.io.opts.transports = ["websocket"];
  });

  socket.on("userCount", console.log);
  socket.on("error", console.error);
</script>

LiveUserCount.js

const WHITELISTED_ORIGINS = ["https://adeka.lk"];
const PORT = process.env.PORT || 3000;

const io = require("socket.io")(PORT);

let userCount = 0;

// Checks if the socket connection is from a whitelisted origin
const isNotWhitelisted = (origin) => {
  return WHITELISTED_ORIGINS.indexOf(origin) === -1;
};

// io.emit can be used for broadcasting to all sockets
const emitUserCount = () => {
  io.emit("userCount", { userCount });
};

io.on("connection", (socket) => {
  if (isNotWhitelisted(socket.handshake.headers.origin)) {
    console.log("Origin not white listed:", socket.handshake.headers.origin);
    return;
  }

  // increment user count when on socket connections
  userCount++;
  emitUserCount();

  // decrement user count when socket disconnects
  socket.on("disconnect", () => {
    userCount--;
    emitUserCount();
  });
});

console.log("Live User Count server started on %d", PORT);