events: stricter prop & variable checks for perf

Replace truthy/falsey checks of _events and _events[type] with
comparisons to undefined for better performance:

events/ee-add-remove.js n=250000    5.30 %    *** 4.260028e-07
events/ee-emit.js n=2000000         4.18 %    *** 1.026649e-05

This has a knock-on effect on modules that use lots of events, e.g.:

http2/headers.js nheaders=0 n=1000  2.60 %    *** 0.000298338

PR-URL: #16212
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

lib/events.js

@@ -78,7 +78,8 @@ EventEmitter.init = function() {
     }
   }
 
-  if (!this._events || this._events === Object.getPrototypeOf(this)._events) {
+  if (this._events === undefined ||
+      this._events === Object.getPrototypeOf(this)._events) {
     this._events = Object.create(null);
     this._eventsCount = 0;
   }
@@ -170,8 +171,8 @@ EventEmitter.prototype.emit = function emit(type) {
   var doError = (type === 'error');
 
   events = this._events;
-  if (events)
-    doError = (doError && events.error == null);
+  if (events !== undefined)
+    doError = (doError && events.error === undefined);
   else if (!doError)
     return false;
 
@@ -181,7 +182,7 @@ EventEmitter.prototype.emit = function emit(type) {
   if (doError) {
     if (arguments.length > 1)
       er = arguments[1];
-    if (domain) {
+    if (domain !== null && domain !== undefined) {
       if (!er) {
         const errors = lazyErrors();
         er = new errors.Error('ERR_UNHANDLED_ERROR');
@@ -206,10 +207,10 @@ EventEmitter.prototype.emit = function emit(type) {
 
   handler = events[type];
 
-  if (!handler)
+  if (handler === undefined)
     return false;
 
-  if (domain && this !== process) {
+  if (domain !== null && domain !== undefined && this !== process) {
     domain.enter();
     needDomainExit = true;
   }
@@ -255,13 +256,13 @@ function _addListener(target, type, listener, prepend) {
   }
 
   events = target._events;
-  if (!events) {
+  if (events === undefined) {
     events = target._events = Object.create(null);
     target._eventsCount = 0;
   } else {
     // To avoid recursion in the case that type === "newListener"! Before
     // adding it to the listeners, first emit "newListener".
-    if (events.newListener) {
+    if (events.newListener !== undefined) {
       target.emit('newListener', type,
                   listener.listener ? listener.listener : listener);
 
@@ -272,7 +273,7 @@ function _addListener(target, type, listener, prepend) {
     existing = events[type];
   }
 
-  if (!existing) {
+  if (existing === undefined) {
     // Optimize the case of one listener. Don't need the extra array object.
     existing = events[type] = listener;
     ++target._eventsCount;
@@ -384,11 +385,11 @@ EventEmitter.prototype.removeListener =
       }
 
       events = this._events;
-      if (!events)
+      if (events === undefined)
         return this;
 
       list = events[type];
-      if (!list)
+      if (list === undefined)
         return this;
 
       if (list === listener || list.listener === listener) {
@@ -424,7 +425,7 @@ EventEmitter.prototype.removeListener =
         if (list.length === 1)
           events[type] = list[0];
 
-        if (events.removeListener)
+        if (events.removeListener !== undefined)
           this.emit('removeListener', type, originalListener || listener);
       }
 
@@ -436,15 +437,15 @@ EventEmitter.prototype.removeAllListeners =
       var listeners, events, i;
 
       events = this._events;
-      if (!events)
+      if (events === undefined)
         return this;
 
       // not listening for removeListener, no need to emit
-      if (!events.removeListener) {
+      if (events.removeListener === undefined) {
         if (arguments.length === 0) {
           this._events = Object.create(null);
           this._eventsCount = 0;
-        } else if (events[type]) {
+        } else if (events[type] !== undefined) {
           if (--this._eventsCount === 0)
             this._events = Object.create(null);
           else
@@ -472,7 +473,7 @@ EventEmitter.prototype.removeAllListeners =
 
       if (typeof listeners === 'function') {
         this.removeListener(type, listeners);
-      } else if (listeners) {
+      } else if (listeners !== undefined) {
         // LIFO order
         for (i = listeners.length - 1; i >= 0; i--) {
           this.removeListener(type, listeners[i]);
@@ -487,11 +488,11 @@ EventEmitter.prototype.listeners = function listeners(type) {
   var ret;
   var events = this._events;
 
-  if (!events)
+  if (events === undefined)
     ret = [];
   else {
     evlistener = events[type];
-    if (!evlistener)
+    if (evlistener === undefined)
       ret = [];
     else if (typeof evlistener === 'function')
       ret = [evlistener.listener || evlistener];
@@ -514,12 +515,12 @@ EventEmitter.prototype.listenerCount = listenerCount;
 function listenerCount(type) {
   const events = this._events;
 
-  if (events) {
+  if (events !== undefined) {
     const evlistener = events[type];
 
     if (typeof evlistener === 'function') {
       return 1;
-    } else if (evlistener) {
+    } else if (evlistener !== undefined) {
       return evlistener.length;
     }
   }