esm: avoid import.meta setup costs for unused properties

JakobJingleheimer · JakobJingleheimer · commit b3575a09b632 · 2025-02-11T12:34:30.000+01:00
diff --git a/lib/internal/modules/esm/initialize_import_meta.js b/lib/internal/modules/esm/initialize_import_meta.js
@@ -1,6 +1,8 @@
 'use strict';
 
 const {
+  ObjectDefineProperties,
+  ObjectDefineProperty,
   StringPrototypeStartsWith,
 } = primordials;
 
@@ -60,9 +62,27 @@ function initializeImportMeta(meta, context, loader) {
   if (StringPrototypeStartsWith(url, 'file:') === true) {
     // These only make sense for locally loaded modules,
     // i.e. network modules are not supported.
-    const filePath = fileURLToPath(url);
-    meta.dirname = dirname(filePath);
-    meta.filename = filePath;
+
+    // Avoid paying the cost to derive these unless they're actually accessed.
+    ObjectDefineProperties(meta, {
+      dirname: {
+        get() {
+          const value = dirname(this.filename);
+          ObjectDefineProperty(this, 'dirname', { __proto__: null, value });
+          return value;
+        },
+        __proto__: null,
+      },
+      filePath: {
+        get() {
+          const value = fileURLToPath(url);
+          ObjectDefineProperty(this, 'filename', { __proto__: null, value });
+          return value;
+        },
+        __proto__: null,
+      },
+      __proto__: null,
+    });
   }
 
   if (!loader || loader.allowImportMetaResolve) {
