perf(node:events): optimize `emit(...)` function (#5485)

1 files changed, 49 insertions, 5 deletions

diff --git a/src/js/node/events.js b/src/js/node/events.js
index bd79dd3ae..4af75f8e2 100644
--- a/src/js/node/events.js
+++ b/src/js/node/events.js
@@ -92,9 +92,30 @@ const emitWithoutRejectionCapture = function emit(type, ...args) {
   if (events === undefined) return false;
   var handlers = events[type];
   if (handlers === undefined) return false;
-
-  for (var handler of [...handlers]) {
-    handler.apply(this, args);
+  // Clone handlers array if necessary since handlers can be added/removed during the loop.
+  // Cloning is skipped for performance reasons in the case of exactly one attached handler
+  // since array length changes have no side-effects in a for-loop of length 1.
+  const maybeClonedHandlers = handlers.length > 1 ? handlers.slice() : handlers;
+  for (let i = 0, { length } = maybeClonedHandlers; i < length; i++) {
+    const handler = maybeClonedHandlers[i];
+    // For performance reasons Function.call(...) is used whenever possible.
+    switch (args.length) {
+      case 0:
+        handler.call(this);
+        break;
+      case 1:
+        handler.call(this, args[0]);
+        break;
+      case 2:
+        handler.call(this, args[0], args[1]);
+        break;
+      case 3:
+        handler.call(this, args[0], args[1], args[2]);
+        break;
+      default:
+        handler.apply(this, args);
+        break;
+    }
   }
   return true;
 };
@@ -107,8 +128,31 @@ const emitWithRejectionCapture = function emit(type, ...args) {
   if (events === undefined) return false;
   var handlers = events[type];
   if (handlers === undefined) return false;
-  for (var handler of [...handlers]) {
-    var result = handler.apply(this, args);
+  // Clone handlers array if necessary since handlers can be added/removed during the loop.
+  // Cloning is skipped for performance reasons in the case of exactly one attached handler
+  // since array length changes have no side-effects in a for-loop of length 1.
+  const maybeClonedHandlers = handlers.length > 1 ? handlers.slice() : handlers;
+  for (let i = 0, { length } = maybeClonedHandlers; i < length; i++) {
+    const handler = maybeClonedHandlers[i];
+    let result;
+    // For performance reasons Function.call(...) is used whenever possible.
+    switch (args.length) {
+      case 0:
+        result = handler.call(this);
+        break;
+      case 1:
+        result = handler.call(this, args[0]);
+        break;
+      case 2:
+        result = handler.call(this, args[0], args[1]);
+        break;
+      case 3:
+        result = handler.call(this, args[0], args[1], args[2]);
+        break;
+      default:
+        result = handler.apply(this, args);
+        break;
+    }
     if (result !== undefined && $isPromise(result)) {
       addCatch(this, result, type, args);
     }