nodejs
diff --git a/‎lib/internal/webidl.js‎
Lines changed: 45 additions & 11 deletions b/‎lib/internal/webidl.js‎
Lines changed: 45 additions & 11 deletions
@@ -20,6 +20,7 @@ const {
   NumberIsNaN,
   NumberMAX_SAFE_INTEGER,
   NumberMIN_SAFE_INTEGER,
+  ObjectPrototypeHasOwnProperty,
   ObjectPrototypeIsPrototypeOf,
   SafeArrayIterator,
   SafeSet,
@@ -699,16 +700,47 @@ function createDictionaryConverter(
   const dictionaries = ArrayIsArray(members[0]) ? members : [members];
   const sortedDictionaries = [];
 
+  function ownMember(member, key) {
+    return ObjectPrototypeHasOwnProperty(member, key) ? member[key] : undefined;
+  }
+
+  // Dictionaries with no defaults and no required members skip steps
+  // 4.1.5/4.1.6 entirely, keeping the absent-member path free.
+  let anyMissingMemberHandling = false;
+
   // Web IDL dictionary conversion steps 3-4 process inherited dictionaries
   // from least-derived to most-derived and sort only within each dictionary.
   // Callers with inheritance pass one member array per dictionary level.
   for (let i = 0; i < dictionaries.length; i++) {
-    ArrayPrototypePush(
-      sortedDictionaries,
-      ArrayPrototypeToSorted(dictionaries[i], compareMembers),
+    const sortedMembers = ArrayPrototypeToSorted(
+      dictionaries[i],
+      compareMembers,
     );
+    // Definition sites spell out only the members they need, so reading the
+    // optional ones below would resolve through %Object.prototype%.
+    // Re-materialize each descriptor once with every key present, copied from
+    // own properties only. The ordinary prototype is deliberate: nothing
+    // consults it now, and detaching it measurably slows these reads down.
+    for (let j = 0; j < sortedMembers.length; j++) {
+      const member = sortedMembers[j];
+      const defaultValue = ownMember(member, 'defaultValue');
+      const required = ownMember(member, 'required');
+      if (typeof defaultValue === 'function' || required) {
+        anyMissingMemberHandling = true;
+      }
+      sortedMembers[j] = {
+        key: ownMember(member, 'key'),
+        converter: ownMember(member, 'converter'),
+        defaultValue,
+        required,
+        validator: ownMember(member, 'validator'),
+      };
+    }
+    ArrayPrototypePush(sortedDictionaries, sortedMembers);
   }
 
+  const hasMissingMemberHandling = anyMissingMemberHandling;
+
   return function(jsDict, options = kEmptyObject) {
     // Step 1: reject non-object, non-null, non-undefined values.
     if (jsDict != null && type(jsDict) !== 'Object') {
@@ -747,14 +779,16 @@ function createDictionaryConverter(
           member.validator?.(idlMemberValue, jsDict);
           // Step 4.1.4.2: set idlDict[key] to the IDL value.
           idlDict[key] = idlMemberValue;
-        } else if (typeof member.defaultValue === 'function') {
-          // Step 4.1.5: store the member default value.
-          idlDict[key] = member.defaultValue();
-        } else if (member.required) {
-          // Step 4.1.6: required missing members throw.
-          throw makeException(
-            missingDictionaryMemberMessage(dictionaryName, key),
-            makeOptions(options, options.context, 'ERR_MISSING_OPTION'));
+        } else if (hasMissingMemberHandling) {
+          if (typeof member.defaultValue === 'function') {
+            // Step 4.1.5: store the member default value.
+            idlDict[key] = member.defaultValue();
+          } else if (member.required) {
+            // Step 4.1.6: required missing members throw.
+            throw makeException(
+              missingDictionaryMemberMessage(dictionaryName, key),
+              makeOptions(options, options.context, 'ERR_MISSING_OPTION'));
+          }
         }
       }
     }