Observer & Singleton Patterns

What is the best way to implement unsubscribe functionality in the Observer Pattern?

class EventEmitter {
  constructor() {
    this.listeners = new Map();
  }

  on(event, callback) {
    if (!this.listeners.has(event)) {
      this.listeners.set(event, new Set());
    }
    this.listeners.get(event).add(callback);
    
    return () => {
      this.listeners.get(event)?.delete(callback);
      if (this.listeners.get(event)?.size === 0) {
        this.listeners.delete(event);
      }
    };
  }

  emit(event, data) {
    this.listeners.get(event)?.forEach(callback => callback(data));
  }
}

Remove all observers at once
Store observers in an array
Return an unsubscribe function when subscribing
This implementation demonstrates the best practice for handling unsubscribe functionality by: 1) Returning a cleanup function when subscribing, 2) Using a Set to manage unique callbacks, 3) Properly cleaning up empty event listeners, and 4) Providing a simple way to unsubscribe without maintaining external references. This approach prevents memory leaks and makes usage more intuitive.
Use a separate unsubscribe method

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