events: add listener argument to listenerCount

Author: ShogunPanda

doc/api/events.md

@@ -646,16 +646,23 @@ Returns the current max listener value for the `EventEmitter` which is either
 set by [`emitter.setMaxListeners(n)`][] or defaults to
 [`events.defaultMaxListeners`][].
 
-### `emitter.listenerCount(eventName)`
+### `emitter.listenerCount(eventName, [listener])`
 
 <!-- YAML
 added: v3.2.0
+changes:
+  - version: REPLACEME
+    pr-url: REPLACEME
+    description: Added the listener argument.
 -->
 
 * `eventName` {string|symbol} The name of the event being listened for
+* `listener` {Function} The name of the listener
 * Returns: {integer}
 
 Returns the number of listeners listening to the event named `eventName`.
+If `listener` is provided, it will return `1` if the listener is found
+in the list of the listeners of the event, `0` otherwise.
 
 ### `emitter.listeners(eventName)`
 
@@ -2468,7 +2475,7 @@ to the `EventTarget`.
 [`EventTarget` error handling]: #eventtarget-error-handling
 [`Event` Web API]: https://dom.spec.whatwg.org/#event
 [`domain`]: domain.md
-[`emitter.listenerCount()`]: #emitterlistenercounteventname
+[`emitter.listenerCount()`]: #emitterlistenercounteventname-listener
 [`emitter.removeListener()`]: #emitterremovelistenereventname-listener
 [`emitter.setMaxListeners(n)`]: #emittersetmaxlistenersn
 [`event.defaultPrevented`]: #eventdefaultprevented

lib/events.js

@@ -33,6 +33,7 @@ const {
   ErrorCaptureStackTrace,
   FunctionPrototypeBind,
   FunctionPrototypeCall,
+  Number,
   NumberMAX_SAFE_INTEGER,
   ObjectDefineProperty,
   ObjectDefineProperties,
@@ -832,17 +833,32 @@ EventEmitter.prototype.listenerCount = listenerCount;
  * Returns the number of listeners listening to event name
  * specified as `type`.
  * @param {string | symbol} type
+ * @param {Function} listener
  * @returns {number}
  */
-function listenerCount(type) {
+function listenerCount(type, listener) {
   const events = this._events;
 
   if (events !== undefined) {
     const evlistener = events[type];
 
     if (typeof evlistener === 'function') {
+      if (listener) {
+        return Number(listener === evlistener);
+      }
+
       return 1;
     } else if (evlistener !== undefined) {
+      if (listener) {
+        for (let i = 0, l = evlistener.length; i < l; i++) {
+          if (evlistener[i] === listener || evlistener[i].listener === listener) {
+            return 1;
+          }
+        }
+
+        return 0;
+      }
+
       return evlistener.length;
     }
   }

test/parallel/test-events-listener-count-with-listener.js

@@ -0,0 +1,47 @@
+'use strict';
+
+const common = require('../common');
+const EventEmitter = require('events');
+const assert = require('assert');
+
+const EE = new EventEmitter();
+const handler = common.mustCall(undefined, 2);
+const anotherHandler = common.mustCall();
+
+assert.strictEqual(EE.listenerCount('event'), 0);
+assert.strictEqual(EE.listenerCount('event', handler), 0);
+assert.strictEqual(EE.listenerCount('event', anotherHandler), 0);
+
+EE.on('event', handler);
+
+assert.strictEqual(EE.listenerCount('event'), 1);
+assert.strictEqual(EE.listenerCount('event', handler), 1);
+assert.strictEqual(EE.listenerCount('event', anotherHandler), 0);
+
+EE.once('event', anotherHandler);
+
+assert.strictEqual(EE.listenerCount('event'), 2);
+assert.strictEqual(EE.listenerCount('event', handler), 1);
+assert.strictEqual(EE.listenerCount('event', anotherHandler), 1);
+
+assert.strictEqual(EE.listenerCount('another-event'), 0);
+assert.strictEqual(EE.listenerCount('another-event', handler), 0);
+assert.strictEqual(EE.listenerCount('another-event', anotherHandler), 0);
+
+EE.emit('event');
+
+assert.strictEqual(EE.listenerCount('event'), 1);
+assert.strictEqual(EE.listenerCount('event', handler), 1);
+assert.strictEqual(EE.listenerCount('event', anotherHandler), 0);
+
+EE.emit('event');
+
+assert.strictEqual(EE.listenerCount('event'), 1);
+assert.strictEqual(EE.listenerCount('event', handler), 1);
+assert.strictEqual(EE.listenerCount('event', anotherHandler), 0);
+
+EE.off('event', handler);
+
+assert.strictEqual(EE.listenerCount('event'), 0);
+assert.strictEqual(EE.listenerCount('event', handler), 0);
+assert.strictEqual(EE.listenerCount('event', anotherHandler), 0);