events: improve on() performance

Author: mscdex

benchmark/events/ee-add-remove.js

@@ -2,24 +2,26 @@
 var common = require('../common.js');
 var events = require('events');
 
-var bench = common.createBenchmark(main, {n: [25e4]});
+var bench = common.createBenchmark(main, {n: [25e4], listeners: [10]});
 
 function main(conf) {
   var n = conf.n | 0;
+  var listeners = conf.listeners | 0;
 
   var ee = new events.EventEmitter();
-  var listeners = [];
+  ee.setMaxListeners(listeners + 1);
+  var fns = [];
 
   var k;
-  for (k = 0; k < 10; k += 1)
-    listeners.push(function() {});
+  for (k = 0; k < listeners; k += 1)
+    fns.push(function() { return 0; });
 
   bench.start();
   for (var i = 0; i < n; i += 1) {
-    for (k = listeners.length; --k >= 0; /* empty */)
-      ee.on('dummy', listeners[k]);
-    for (k = listeners.length; --k >= 0; /* empty */)
-      ee.removeListener('dummy', listeners[k]);
+    for (k = listeners; --k >= 0; /* empty */)
+      ee.on('dummy', fns[k]);
+    for (k = listeners; --k >= 0; /* empty */)
+      ee.removeListener('dummy', fns[k]);
   }
   bench.end(n);
 }

lib/events.js

@@ -86,9 +86,10 @@ EventEmitter.prototype.setMaxListeners = function setMaxListeners(n) {
 };
 
 function $getMaxListeners(that) {
-  if (that._maxListeners === undefined)
+  const maxListeners = that._maxListeners;
+  if (maxListeners === undefined)
     return EventEmitter.defaultMaxListeners;
-  return that._maxListeners;
+  return maxListeners;
 }
 
 EventEmitter.prototype.getMaxListeners = function getMaxListeners() {
@@ -230,7 +231,6 @@ EventEmitter.prototype.emit = function emit(type) {
 };
 
 function _addListener(target, type, listener, prepend) {
-  var m;
   var events;
   var existing;
 
@@ -275,7 +275,7 @@ function _addListener(target, type, listener, prepend) {
 
     // Check for listener leak
     if (!existing.warned) {
-      m = $getMaxListeners(target);
+      const m = $getMaxListeners(target);
       if (m && m > 0 && existing.length > m) {
         existing.warned = true;
         const w = new Error('Possible EventEmitter memory leak detected. ' +