events: improve once() performance

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) {