System Design Quiz Practice

class NotificationRouter {
  constructor() { this.channels = []; this.delivered = new Set(); }
  subscribe(c) { this.channels.push(c); }
  publish(event) {
    for (const c of this.channels) {
      const key = `${c.name}:${event.id}`;
      if (this.delivered.has(key)) continue;
      this.delivered.add(key);
      c.deliver(event);
    }
  }
}
const r = new NotificationRouter();
r.subscribe({ name: "email", deliver: (e) => console.log("sent:", e.id) });
r.publish({ id: "evt1" });
r.publish({ id: "evt1" });