Memory Management & Garbage Collection

What critical method is missing from this EventEmitter to prevent memory leaks?
class EventEmitter {
  constructor() {
    this.events = {};
  }
  
  on(event, listener) {
    if (!this.events[event]) {
      this.events[event] = [];
    }
    this.events[event].push(listener);
    return this;
  }
  
  emit(event, ...args) {
    if (this.events[event]) {
      this.events[event].forEach(listener => listener(...args));
    }
    return this;
  }
}
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