Issue 7202 - Add hasListeners method to EventEmitter

test/filterNotifier.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 32 matching lines @@
   filterNotifier.on("foo", listener);
 }
 
 function removeListener(listener)
 {
   filterNotifier.off("foo", listener);
 }
 
 function compareListeners(test, testDescription, list)
 {
+  test.equal(filterNotifier.hasListeners(), list.length > 0, testDescription);
+  test.equal(filterNotifier.hasListeners("foo"), list.length > 0,
+             testDescription);
+
+  test.equal(filterNotifier.hasListeners("bar"), false, testDescription);
+
   let result1 = triggeredListeners = [];
   filterNotifier.emit("foo", {bar: true});
 
   let result2 = triggeredListeners = [];
   for (let observer of list)
     observer({bar: true});
 
   test.deepEqual(result1, result2, testDescription);
 }
 
@@ skipping 45 matching lines @@
   };
   let listener2 = listeners[1];
   addListener(listener1);
   addListener(listener2);
 
   compareListeners(test, "Initial call", [listener1, listener2]);
   compareListeners(test, "Subsequent calls", [listener2]);
 
   test.done();
 };