server.ts

import * as WebSocket from "ws";

// // create WebSocket server with given port
const port: number = Number(process.env.PORT) || 8000;
const server: WebSocket.Server = new WebSocket.Server({ port: port });

// set of connected sockets
const clientSockets: Set<WebSocket> = new Set();

// init counters (0 is number of connected clients)
const counters: number[] = [0, 0, 0, 0, 0, 0, 0, 0, 0];

server.on("connection", (socket) => {
  clientSockets.add(socket); // delete client socket to set of connected sockets
  counters[0]++; // increase number of clients

  // broadcast counters to newly connected client
  broadcastCounters();

  // receive counter increment from connected client
  socket.on("message", (message) => {
    const counterIndex: number = parseInt(<string>message);
    counters[counterIndex]++;
    broadcastCounters();
  });

  // client connection closing
  socket.on("close", () => {
    clientSockets.delete(socket); // delete client socket from set of connected sockets
    counters[0]--; // decrease number of clients

    // update client counters
    broadcastCounters();
  });

  // broadcast all counters to all connected clients
  function broadcastCounters(): void {
    for (let socket of clientSockets) {
      socket.send(JSON.stringify(counters));
    }
  }
});