events: improve arrayClone performance

benchmark/events/ee-emit.js

@@ -10,6 +10,7 @@ const bench = common.createBenchmark(main, {
 
 function main({ n, argc, listeners }) {
  const ee = new EventEmitter();
+ ee.setMaxListeners(listeners + 1);
 
  for (let k = 0; k < listeners; k += 1)
  ee.on('dummy', () => {});

benchmark/events/ee-listeners.js

@@ -2,20 +2,34 @@
 const common = require('../common.js');
 const EventEmitter = require('events').EventEmitter;
 
-const bench = common.createBenchmark(main, { n: [5e6] });
+const bench = common.createBenchmark(main, {
+ n: [5e6],
+ listeners: [5, 50],
+ raw: ['true', 'false']
+});
 
-function main({ n }) {
+function main({ n, listeners, raw }) {
  const ee = new EventEmitter();
+ ee.setMaxListeners(listeners * 2 + 1);
 
- for (let k = 0; k < 5; k += 1) {
+ for (let k = 0; k < listeners; k += 1) {
  ee.on('dummy0', () => {});
  ee.on('dummy1', () => {});
  }
 
- bench.start();
- for (let i = 0; i < n; i += 1) {
- const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
- ee.listeners(dummy);
+ if (raw === 'true') {
+ bench.start();
+ for (let i = 0; i < n; i += 1) {
+ const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
+ ee.rawListeners(dummy);
+ }
+ bench.end(n);
+ } else {
+ bench.start();
+ for (let i = 0; i < n; i += 1) {
+ const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
+ ee.listeners(dummy);
+ }
+ bench.end(n);
  }
- bench.end(n);
 }

lib/events.js

@@ -322,7 +322,7 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
  }
  } else {
  const len = handler.length;
- const listeners = arrayClone(handler, len);
+ const listeners = arrayClone(handler);
  for (let i = 0; i < len; ++i) {
  const result = ReflectApply(listeners[i], this, args);
 
@@ -560,7 +560,7 @@ function _listeners(target, type, unwrap) {
  return unwrap ? [evlistener.listener || evlistener] : [evlistener];
 
  return unwrap ?
- unwrapListeners(evlistener) : arrayClone(evlistener, evlistener.length);
+ unwrapListeners(evlistener) : arrayClone(evlistener);
 }
 
 EventEmitter.prototype.listeners = function listeners(type) {
@@ -599,11 +599,17 @@ EventEmitter.prototype.eventNames = function eventNames() {
  return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
 };
 
-function arrayClone(arr, n) {
- const copy = new Array(n);
- for (let i = 0; i < n; ++i)
- copy[i] = arr[i];
- return copy;
+function arrayClone(arr) {
+ // At least since V8 8.3, this implementation is faster than the previous
+ // which always used a simple for-loop
+ switch (arr.length) {
+ case 2: return [arr[0], arr[1]];
+ case 3: return [arr[0], arr[1], arr[2]];
+ case 4: return [arr[0], arr[1], arr[2], arr[3]];
+ case 5: return [arr[0], arr[1], arr[2], arr[3], arr[4]];
+ case 6: return [arr[0], arr[1], arr[2], arr[3], arr[4], arr[5]];
+ }
+ return arr.slice();
 }
 
 function unwrapListeners(arr) {