2694. Event Emitter

Implement a custom EventEmitter class to support subscribing to events and emitting them later.

📄 Problem Statement

subscribe(eventName, callback): Adds a callback to the specified event. Returns an object with an unsubscribe() method to remove the listener.
emit(eventName, args): Calls all subscribed callbacks for the event in the order they were added. Returns an array of their results.

⚠️ If no callbacks exist for the event, return an empty array. Each callback is guaranteed to be unique (no duplicates).

💡 Example


const emitter = new EventEmitter();

function onClick(arg) { return `Clicked ${arg}`; }
function onHover(arg) { return `Hovered ${arg}`; }

const sub1 = emitter.subscribe("click", onClick);
const sub2 = emitter.subscribe("click", onHover);

emitter.emit("click", [1]);
// ➞ ["Clicked 1", "Hovered 1"]

sub1.unsubscribe();
emitter.emit("click", [2]);
// ➞ ["Hovered 2"]

🧠 Explanation

We store a map of events to an array of listener objects. Each subscription is wrapped in a closure so it can be cleanly removed later using unsubscribe().

On emitting an event, we iterate through all relevant callbacks, passing in the arguments and collecting their return values.

✅ Your Solution


class EventEmitter {
  constructor() {
    this.events = {};
  }

  subscribe(eventName, callback) {
    if (!this.events[eventName]) {
      this.events[eventName] = [];
    }

    const listener = { callback };
    this.events[eventName].push(listener);

    return {
      unsubscribe: () => {
        this.events[eventName] = this.events[eventName].filter(
          l => l !== listener
        );
      }
    };
  }

  emit(eventName, args = []) {
    if (!this.events[eventName]) {
      return [];
    }

    return this.events[eventName].map(listener => listener.callback(...args));
  }
}