Issue 7202 - Add hasListeners method to EventEmitter

lib/events.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 35 matching lines @@
    * Removes a listener for the specified event name.
    *
    * @param {string}   name
    * @param {function} listener
    */
   off(name, listener)
   {
     let listeners = this._listeners.get(name);
     if (listeners)
     {
-      let idx = listeners.indexOf(listener);
-      if (idx != -1)
-        listeners.splice(idx, 1);
+      if (listeners.length > 1)
+      {
+        let idx = listeners.indexOf(listener);
+        if (idx != -1)
+          listeners.splice(idx, 1);
+      }
+      else if (listeners[0] === listener)
+      {
+        // We must use strict equality above for compatibility with
+        // Array.prototype.indexOf
+        this._listeners.delete(name);
+      }
     }
   }
 
   /**
    * Adds a one time listener and returns a promise that
    * is resolved the next time the specified event is emitted.
    *
    * @param {string} name
    * @returns {Promise}
    */
@@ skipping 31 matching lines @@
    */
   emit(name, ...args)
   {
     let listeners = this.listeners(name);
     for (let listener of listeners)
       listener(...args);
   }
 }
 
 exports.EventEmitter = EventEmitter;