Q14: Implement an Async Publish-Subscribe Bus

Create an event bus that supports asynchronous subscriber callbacks, returning a promise from publish() that resolves once all subscriber callbacks settle.

Async Event Bus

class AsyncEventBus {
  constructor() {
    this.subscribers = new Map();
  }

  subscribe(event, callback) {
    if (!this.subscribers.has(event)) this.subscribers.set(event, new Set());
    this.subscribers.get(event).add(callback);
    return () => this.subscribers.get(event).delete(callback);
  }

  async publish(event, data) {
    if (!this.subscribers.has(event)) return [];
    const callbacks = Array.from(this.subscribers.get(event));
    const promises = callbacks.map(cb => Promise.resolve().then(() => cb(data)));
    return Promise.all(promises);
  }
}

💡 Publishing wraps each callback in a Promise, allowing Promise.all to await all subscriber operations concurrently.

Key Takeaways

Returns an un-subscribe teardown closure upon registration.
Uses Promise.all to execute and await subscriber handlers concurrently during publication.

Knowledge Check

What is the return result of publishing an event in the AsyncEventBus above?

Q14: Implement an Async Publish-Subscribe Bus

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