events.listenerCount

History

events.listenerCount(emitterOrTarget, eventName): integer

Attributes

emitterOrTarget:EventEmitter | EventTarget

eventName:string | symbol

Returns:integer

Returns the number of registered listeners for the event named eventName.

For EventEmitters this behaves exactly the same as calling .listenerCount on the emitter.

For EventTargets this is the only way to obtain the listener count. This can be useful for debugging and diagnostic purposes.

import { EventEmitter, listenerCount } from 'node:events';

{
  const ee = new EventEmitter();
  ee.on('event', () => {});
  ee.on('event', () => {});
  console.log(listenerCount(ee, 'event')); // 2
}
{
  const et = new EventTarget();
  et.addEventListener('event', () => {});
  et.addEventListener('event', () => {});
  console.log(listenerCount(et, 'event')); // 2
}

JavaScript

const { EventEmitter, listenerCount } = require('node:events');

{
  const ee = new EventEmitter();
  ee.on('event', () => {});
  ee.on('event', () => {});
  console.log(listenerCount(ee, 'event')); // 2
}
{
  const et = new EventTarget();
  et.addEventListener('event', () => {});
  et.addEventListener('event', () => {});
  console.log(listenerCount(et, 'event')); // 2
}

JavaScript