events: add hasListener method

Author: ShogunPanda

doc/api/events.md

@@ -646,6 +646,18 @@ Returns the current max listener value for the `EventEmitter` which is either
 set by [`emitter.setMaxListeners(n)`][] or defaults to
 [`events.defaultMaxListeners`][].
 
+### `emitter.hasListener(eventName, listener)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `eventName` {string|symbol} The name of the event being checked
+* `listener` {Function} The listener to check
+
+Returns `true` if the listener is already register for the event,
+`false` otherwise.
+
 ### `emitter.listenerCount(eventName)`
 
 <!-- YAML

lib/events.js

@@ -850,6 +850,31 @@ function listenerCount(type) {
   return 0;
 }
 
+/**
+ * Returns true if the function is register as listener for the event
+ *
+ * @param {string | symbol} type
+ * @param {Function} listener
+ * @returns {boolean}
+ */
+EventEmitter.prototype.hasListener = function hasListener(type, listener) {
+  const list = this._events?.[type];
+
+  if (list === undefined) {
+    return false;
+  } else if (list === listener || list.listener === listener) {
+    return true;
+  } else if (typeof list !== 'function') {
+    for (let i = 0, l = list.length; i < l; i++) {
+      if (list[i] === listener || list[i].listener === listener) {
+        return true;
+      }
+    }
+  }
+
+  return false;
+};
+
 /**
  * Returns an array listing the events for which
  * the emitter has registered listeners.

test/parallel/test-events-has-listener.js

@@ -0,0 +1,40 @@
+'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.hasListener('event', handler), false);
+assert.strictEqual(EE.hasListener('event', anotherHandler), false);
+
+EE.on('event', handler);
+
+assert.strictEqual(EE.hasListener('event', handler), true);
+assert.strictEqual(EE.hasListener('event', anotherHandler), false);
+
+EE.once('event', anotherHandler);
+
+assert.strictEqual(EE.hasListener('event', handler), true);
+assert.strictEqual(EE.hasListener('event', anotherHandler), true);
+
+assert.strictEqual(EE.hasListener('another-event', handler), false);
+assert.strictEqual(EE.hasListener('another-event', anotherHandler), false);
+
+EE.emit('event');
+
+assert.strictEqual(EE.hasListener('event', handler), true);
+assert.strictEqual(EE.hasListener('event', anotherHandler), false);
+
+EE.emit('event');
+
+assert.strictEqual(EE.hasListener('event', handler), true);
+assert.strictEqual(EE.hasListener('event', anotherHandler), false);
+
+EE.off('event', handler);
+
+assert.strictEqual(EE.hasListener('event', handler), false);
+assert.strictEqual(EE.hasListener('event', anotherHandler), false);