events: improve once() performance

This commit takes advantage of the performance improvements V8 has
made to function.bind() in V8 5.4 and uses it to avoid constant
recompilation/reoptimization of the wrapper closure used in once().
This change results in ~27% performance increase for once().

PR-URL: https://github.com/nodejs/node/pull/10445
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

benchmark/events/ee-once.js

@@ -0,0 +1,20 @@
+'use strict';
+var common = require('../common.js');
+var EventEmitter = require('events').EventEmitter;
+
+var bench = common.createBenchmark(main, {n: [2e7]});
+
+function main(conf) {
+  var n = conf.n | 0;
+
+  var ee = new EventEmitter();
+
+  function listener() {}
+
+  bench.start();
+  for (var i = 0; i < n; i += 1) {
+    ee.once('dummy', listener);
+    ee.emit('dummy');
+  }
+  bench.end(n);
+}

lib/events.js

@@ -283,17 +283,20 @@ EventEmitter.prototype.prependListener =
       return _addListener(this, type, listener, true);
     };
 
-function _onceWrap(target, type, listener) {
-  var fired = false;
-  function g() {
-    target.removeListener(type, g);
-    if (!fired) {
-      fired = true;
-      listener.apply(target, arguments);
-    }
+function onceWrapper() {
+  this.target.removeListener(this.type, this.wrapFn);
+  if (!this.fired) {
+    this.fired = true;
+    this.listener.apply(this.target, arguments);
   }
-  g.listener = listener;
-  return g;
+}
+
+function _onceWrap(target, type, listener) {
+  var state = { fired: false, wrapFn: undefined, target, type, listener };
+  var wrapped = onceWrapper.bind(state);
+  wrapped.listener = listener;
+  state.wrapFn = wrapped;
+  return wrapped;
 }
 
 EventEmitter.prototype.once = function once(type, listener) {