events: improve arrayClone performance

PR-URL: https://github.com/nodejs/node/pull/33774
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>

1 files changed, 13 insertions, 7 deletions

diff --git a/lib/events.js b/lib/events.js
index a5af5ff930..06fed54c61 100644
--- a/lib/events.js
+++ b/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) {