WebSocket server with topic subscriptions (ws)

                  import { WebSocketServer } from 'ws';
import type WebSocket from 'ws';

type ClientState = { topics: Set<string> };
const state = new WeakMap<WebSocket, ClientState>();

export function startWsServer(server: import('node:http').Server) {
  const wss = new WebSocketServer({ server, maxPayload: 1024 * 64 });

  wss.on('connection', (socket) => {
    state.set(socket, { topics: new Set() });

    socket.on('message', (buf) => {
      const msg = JSON.parse(buf.toString('utf8')) as { type: string; topic?: string };
      const s = state.get(socket)!;

      if (msg.type === 'subscribe' && msg.topic) s.topics.add(msg.topic);
      if (msg.type === 'unsubscribe' && msg.topic) s.topics.delete(msg.topic);
    });
  });

  return {
    publish(topic: string, data: unknown) {
      const payload = JSON.stringify({ type: 'event', topic, data });
      for (const client of wss.clients) {
        const s = state.get(client);
        if (s?.topics.has(topic) && client.readyState === client.OPEN) {
          client.send(payload);
        }
      }
    }
  };
}