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DesignPatternsintermediate

Observer — Listener count

📖 Usage Example
emitter.on(handler1);
emitter.on(handler2);
emitter.listeners.length; // 2

Track how many subscribers are registered by checking the listeners array length.

Observer Pattern — Tracking Listener Count

An observer system can track how many listeners are subscribed to each event.

Challenge: Implement an EventEmitter with a listenerCount(eventName) method that returns the number of subscribers for that event.

Example:

const emitter = new EventEmitter()
emitter.listenerCount('data') // 0
emitter.on('data', () => {})
emitter.listenerCount('data') // 1
emitter.on('data', () => {})
emitter.listenerCount('data') // 2
💡 Hints (3)
  • • Use listenerCount to return the length of the listener array
  • • Initialize empty array if event has no listeners yet
  • • Keep track of listeners per event name