Observer Pattern Implementation

export class EventEmitter {
  #listeners = new Map();

  on(event, listener) {
    if (!this.#listeners.has(event)) {
      this.#listeners.set(event, new Set());
    }
    this.#listeners.get(event).add(listener);
    return () => this.off(event, listener); // returns unsubscribe fn
  }

  once(event, listener) {
    const wrapper = (...args) => {
      listener(...args);
      this.off(event, wrapper);
    };
    return this.on(event, wrapper);
  }

  off(event, listener) {
    this.#listeners.get(event)?.delete(listener);
    return this;
  }

  emit(event, ...args) {
    this.#listeners.get(event)?.forEach(l => l(...args));
    return this;
  }

  clear(event) {
    if (event) this.#listeners.delete(event);
    else this.#listeners.clear();
    return this;
  }
}