2694-event-emitter.ts

// Title: Event Emitter
// Description:
//     Design an EventEmitter class. This interface is similar (but with some differences) to the one found in Node.js or the Event Target interface of the DOM.
//     The EventEmitter should allow for subscribing to events and emitting them.
//     Your EventEmitter class should have the following two methods:
//         subscribe -
//             This method takes in two arguments: the name of an event as a string and a callback function.
//             This callback function will later be called when the event is emitted.
//             An event should be able to have multiple listeners for the same event.
//             When emitting an event with multiple callbacks, each should be called in the order in which they were subscribed. An array of results should be returned.
//             You can assume no callbacks passed to subscribe are referentially identical.
//             The subscribe method should also return an object with an unsubscribe method that enables the user to unsubscribe.
//             When it is called, the callback should be removed from the list of subscriptions and undefined should be returned.
//         emit -
//             This method takes in two arguments: the name of an event as a string and an optional array of arguments that will be passed to the callback(s).
//             If there are no callbacks subscribed to the given event, return an empty array.
//             Otherwise, return an array of the results of all callback calls in the order they were subscribed.
// Link: https://leetcode.com/problems/event-emitter/

type Callback = (...args: any[]) => any;
type Subscription = {
  unsubscribe: () => void
};

class EventEmitter {
  readonly subscriberBuckets: Map<string, Set<Callback>> = new Map();

  subscribe(eventName: string, callback: Callback): Subscription {
    if (!this.subscriberBuckets.has(eventName)) this.subscriberBuckets.set(eventName, new Set());

    this.subscriberBuckets.get(eventName).add(callback);

    return {
      unsubscribe: () => {
        this.subscriberBuckets.get(eventName).delete(callback);
      }
    };
  }

  emit(eventName: string, args: any[] = []): any {
    /*
      The iteration over a Set visits elements in insertion order.
      See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set#iterating_sets
    */
    return Array.from(this.subscriberBuckets.get(eventName) ?? [])
      .map((callback) => callback(...args));
  }
}

/**
 * const emitter = new EventEmitter();
 *
 * // Subscribe to the onClick event with onClickCallback
 * function onClickCallback() { return 99 }
 * const sub = emitter.subscribe('onClick', onClickCallback);
 *
 * emitter.emit('onClick'); // [99]
 * sub.unsubscribe(); // undefined
 * emitter.emit('onClick'); // []
 */