lib: don't penalize setTimeout() common case

The common case is where setTimeout() is called with two arguments,
the callback and the timeout.  Specifying optional arguments in the
parameter list forces common case calls to go through an arguments
adaptor stack frame.

PR-URL: https://github.com/iojs/io.js/pull/1221
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

lib/timers.js

@@ -173,51 +173,41 @@ exports.active = function(item) {
  */
 
 
-exports.setTimeout = function(callback, after, arg1, arg2, arg3) {
-  var timer, i, args;
-  var len = arguments.length;
-
+exports.setTimeout = function(callback, after) {
   after *= 1; // coalesce to number or NaN
 
   if (!(after >= 1 && after <= TIMEOUT_MAX)) {
     after = 1; // schedule on next tick, follows browser behaviour
   }
 
-  timer = new Timeout(after);
-
-  switch (len) {
+  var timer = new Timeout(after);
+  var length = arguments.length;
+  var ontimeout = callback;
+  switch (length) {
     // fast cases
     case 0:
     case 1:
     case 2:
-      timer._onTimeout = callback;
       break;
     case 3:
-      timer._onTimeout = function() {
-        callback.call(timer, arg1);
-      };
+      ontimeout = callback.bind(timer, arguments[2]);
       break;
     case 4:
-      timer._onTimeout = function() {
-        callback.call(timer, arg1, arg2);
-      };
+      ontimeout = callback.bind(timer, arguments[2], arguments[3]);
       break;
     case 5:
-      timer._onTimeout = function() {
-        callback.call(timer, arg1, arg2, arg3);
-      };
+      ontimeout =
+          callback.bind(timer, arguments[2], arguments[3], arguments[4]);
       break;
     // slow case
     default:
-      args = new Array(len - 2);
-      for (i = 2; i < len; i++)
+      var args = new Array(length - 2);
+      for (var i = 2; i < length; i++)
         args[i - 2] = arguments[i];
-
-      timer._onTimeout = function() {
-        callback.apply(timer, args);
-      };
+      ontimeout = callback.apply.bind(callback, timer, args);
       break;
   }
+  timer._onTimeout = ontimeout;
 
   if (process.domain) timer.domain = process.domain;