35. Pub-Sub Broker

class Broker {
  subs = {};
  subscribe(topic, cb) {
    if (!this.subs[topic]) this.subs[topic] = [];
    this.subs[topic].push(cb);
  }
  publish(topic, msg) {
    (this.subs[topic] || []).forEach(cb => cb(msg));
  }
}