System Design Quiz Practice

class PubSub {
  constructor() { this.subscribers = new Map(); }
  subscribe(topic, cb) {
    if (!this.subscribers.has(topic)) this.subscribers.set(topic, new Set());
    this.subscribers.get(topic).add(cb);
  }
  publish(topic, msg) {
    const subs = this.subscribers.get(topic);
    if (!subs) return;
    for (const cb of subs) cb(msg);
  }
}
const bus = new PubSub();
bus.subscribe("x", (m) => console.log("A:", m));
bus.subscribe("x", (m) => console.log("B:", m));
bus.publish("x", 42);