events: handle inherited properties properly

As of now, the events module considers inherited properties as one of the
valid types, by default.

    > process.version
    'v3.0.0'
    > events.EventEmitter.listenerCount(new events.EventEmitter(), 'toString')
    1

This patch makes sure that the inherited properties are considered as
normal types and they will not be counted unless explicitly added.

    > process.version
    'v4.0.0-pre'
    > events.EventEmitter.listenerCount(new events.EventEmitter(), 'toString')
    0
    > const emitter = new events.EventEmitter();
    undefined
    > emitter.on('toString', function() {});
    EventEmitter {
      domain:
       Domain {
         domain: null,
         _events: { error: [Function] },
         _eventsCount: 1,
         _maxListeners: undefined,
         members: [] },
      _events: { toString: [Function] },
      _eventsCount: 1,
      _maxListeners: undefined }
    > events.EventEmitter.listenerCount(emitter, 'toString')
    1

lib/events.js

@@ -31,7 +31,7 @@ EventEmitter.init = function() {
   }
 
   if (!this._events || this._events === Object.getPrototypeOf(this)._events) {
-    this._events = {};
+    this._events = Object.create(null);
     this._eventsCount = 0;
   }
 
@@ -198,7 +198,7 @@ EventEmitter.prototype.addListener = function addListener(type, listener) {
 
   events = this._events;
   if (!events) {
-    events = this._events = {};
+    events = this._events = Object.create(null);
     this._eventsCount = 0;
   } else {
     // To avoid recursion in the case that type === "newListener"! Before
@@ -285,7 +285,7 @@ EventEmitter.prototype.removeListener =
 
       if (list === listener || (list.listener && list.listener === listener)) {
         if (--this._eventsCount === 0)
-          this._events = {};
+          this._events = Object.create(null);
         else {
           delete events[type];
           if (events.removeListener)
@@ -308,7 +308,7 @@ EventEmitter.prototype.removeListener =
         if (list.length === 1) {
           list[0] = undefined;
           if (--this._eventsCount === 0) {
-            this._events = {};
+            this._events = Object.create(null);
             return this;
           } else {
             delete events[type];
@@ -335,11 +335,11 @@ EventEmitter.prototype.removeAllListeners =
       // not listening for removeListener, no need to emit
       if (!events.removeListener) {
         if (arguments.length === 0) {
-          this._events = {};
+          this._events = Object.create(null);
           this._eventsCount = 0;
         } else if (events[type]) {
           if (--this._eventsCount === 0)
-            this._events = {};
+            this._events = Object.create(null);
           else
             delete events[type];
         }
@@ -355,7 +355,7 @@ EventEmitter.prototype.removeAllListeners =
           this.removeAllListeners(key);
         }
         this.removeAllListeners('removeListener');
-        this._events = {};
+        this._events = Object.create(null);
         this._eventsCount = 0;
         return this;
       }

test/parallel/test-event-emitter-listener-count.js

@@ -1,18 +1,34 @@
 'use strict';
 
-const common = require('../common');
+require('../common');
 const assert = require('assert');
 const EventEmitter = require('events');
-
 const emitter = new EventEmitter();
-emitter.on('foo', function() {});
-emitter.on('foo', function() {});
-emitter.on('baz', function() {});
+
+function noop() {}
+
+emitter.on('foo', noop);
+emitter.on('foo', noop);
+emitter.on('baz', noop);
 // Allow any type
-emitter.on(123, function() {});
+emitter.on(123, noop);
 
 assert.strictEqual(EventEmitter.listenerCount(emitter, 'foo'), 2);
 assert.strictEqual(emitter.listenerCount('foo'), 2);
 assert.strictEqual(emitter.listenerCount('bar'), 0);
 assert.strictEqual(emitter.listenerCount('baz'), 1);
 assert.strictEqual(emitter.listenerCount(123), 1);
+
+// The inherited properties should not be counted towards the actual
+// listeners count
+assert.strictEqual(EventEmitter.listenerCount(emitter, 'toString'), 0);
+
+// when we add a new listener with the name of an inherited property, it should
+// accept it
+emitter.on('toString', noop);
+assert.strictEqual(EventEmitter.listenerCount(emitter, 'toString'), 1);
+
+// after removing a listener with the name of an inherited property, the count
+// should reduce by one
+emitter.removeListener('toString', noop);
+assert.strictEqual(EventEmitter.listenerCount(emitter, 'toString'), 0);

test/parallel/test-event-emitter-listeners-side-effects.js

@@ -23,7 +23,7 @@ assert(fl.length === 1);
 assert(fl[0] === assert.fail);
 
 e.listeners('bar');
-assert(!e._events.hasOwnProperty('bar'));
+assert(!e._events['bar']);
 
 e.on('foo', assert.ok);
 fl = e.listeners('foo');