events: use spread function param in emit

With recent changes in V8, it is now as performant or faster to use
spread parameter within EventEmitter.prototype.emit, especially
in cases where looping over arguments is required.

events/ee-emit.js n=2000000               4.40 %    *** 1.505543e-06
events/ee-emit-1-arg.js n=2000000         2.16 %    *** 2.434584e-10
events/ee-emit-2-args.js n=2000000        1.05 %     ** 0.001764852
events/ee-emit-3-args.js n=2000000        2.18 %    *** 3.234954e-08
events/ee-emit-6-args.js n=2000000       17.17 %    *** 1.298702e-103
events/ee-emit-10-args.js n=2000000      17.14 %    *** 1.144958e-97

This has a knock-on effect for modules that use events extensively,
such as http2:

http2/headers.js nheaders=0 n=1000        2.10 %    *** 6.792106e-11

PR-URL: nodejs/node#16212
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>

lib/events.js

@@ -162,23 +162,22 @@ function emitMany(handler, isFn, self, args) {
   }
 }
 
-EventEmitter.prototype.emit = function emit(type) {
-  var er, handler, len, args, i, events, domain;
-  var needDomainExit = false;
-  var doError = (type === 'error');
+EventEmitter.prototype.emit = function emit(type, ...args) {
+  let doError = (type === 'error');
 
-  events = this._events;
+  const events = this._events;
   if (events !== undefined)
     doError = (doError && events.error === undefined);
   else if (!doError)
     return false;
 
-  domain = this.domain;
+  const domain = this.domain;
 
   // If there is no 'error' event listener then throw.
   if (doError) {
-    if (arguments.length > 1)
-      er = arguments[1];
+    let er;
+    if (args.length > 0)
+      er = args[0];
     if (domain !== null && domain !== undefined) {
       if (!er) {
         const errors = lazyErrors();
@@ -202,37 +201,32 @@ EventEmitter.prototype.emit = function emit(type) {
     return false;
   }
 
-  handler = events[type];
+  const handler = events[type];
 
   if (handler === undefined)
     return false;
 
+  let needDomainExit = false;
   if (domain !== null && domain !== undefined && this !== process) {
     domain.enter();
     needDomainExit = true;
   }
 
-  var isFn = typeof handler === 'function';
-  len = arguments.length;
-  switch (len) {
-    // fast cases
-    case 1:
+  const isFn = typeof handler === 'function';
+  switch (args.length) {
+    case 0:
       emitNone(handler, isFn, this);
       break;
+    case 1:
+      emitOne(handler, isFn, this, args[0]);
+      break;
     case 2:
-      emitOne(handler, isFn, this, arguments[1]);
+      emitTwo(handler, isFn, this, args[0], args[1]);
       break;
     case 3:
-      emitTwo(handler, isFn, this, arguments[1], arguments[2]);
-      break;
-    case 4:
-      emitThree(handler, isFn, this, arguments[1], arguments[2], arguments[3]);
+      emitThree(handler, isFn, this, args[0], args[1], args[2]);
       break;
-    // slower
     default:
-      args = new Array(len - 1);
-      for (i = 1; i < len; i++)
-        args[i - 1] = arguments[i];
       emitMany(handler, isFn, this, args);
   }