Observer & Singleton Patterns

How should the Observer Pattern be implemented to handle different types of events?

class EventSubject {
  #handlers = new Map();

  on(eventType, handler) {
    if (!this.#handlers.has(eventType)) {
      this.#handlers.set(eventType, new Set());
    }
    this.#handlers.get(eventType).add(handler);
    return () => this.off(eventType, handler);
  }

  off(eventType, handler) {
    const handlers = this.#handlers.get(eventType);
    if (handlers) {
      handlers.delete(handler);
      if (handlers.size === 0) {
        this.#handlers.delete(eventType);
      }
    }
  }

  emit(eventType, data) {
    this.#handlers.get(eventType)?.forEach(handler => handler(data));
  }
}

Use separate observer classes for each event type
Create a new subject for each event type
Use a single handler for all events
Use a Map to store different event type handlers
This implementation effectively handles different event types by: 1) Using a Map to store handlers for different event types, 2) Organizing handlers by event type in Sets for uniqueness, 3) Providing type-specific subscription and emission, and 4) Properly cleaning up empty handler sets. This approach maintains good separation of concerns while keeping the implementation flexible.

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