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 79 matching lines @@
   }
 
   /**
    * Returns a copy of the array of listeners for the specified event.
    *
    * @param {string} name
    * @returns {Array.<function>}
    */
   listeners(name)
   {
-    let listeners = this._listeners.get(name);
+    let listeners = this._listeners.size > 0 ? this._listeners.get(name) : null;
     return listeners ? listeners.slice() : [];
+  }
+
+  /**
+   * Checks whether there are any listeners for the specified event.
+   *
+   * @param {string} [name] The name of the event. If omitted, checks whether
+   *   there are any listeners for any event.
+   * @returns {boolean}
+   */
+  hasListeners(name)
+  {
+    return this._listeners.size > 0 &&
+           (typeof name == "undefined" || this._listeners.has(name));
   }
 
   /**
    * Calls all previously added listeners for the given event name.
    *
    * @param {string} name
    * @param {...*}   [args]
    */
   emit(name, ...args)
   {
-    let listeners = this.listeners(name);
-    for (let listener of listeners)
-      listener(...args);
+    let listeners = this._listeners.size > 0 ? this._listeners.get(name) : null;
+    if (listeners)
+    {
+      for (let listener of listeners.slice())
+        listener(...args);
+    }
   }
 }
 
 exports.EventEmitter = EventEmitter;