[[DefineOwnProperty]] should always return true/false (or throw an exception), never undefined.

src/v8natives.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 686 matching lines @@
 
   var current = ConvertDescriptorArrayToDescriptor(current_or_access);
   var extensible = %IsExtensible(ToObject(obj));
 
   // Error handling according to spec.
   // Step 3
   if (IS_UNDEFINED(current) && !extensible) {
     if (should_throw) {
       throw MakeTypeError("define_disallowed", [p]);
     } else {
-      return;
+      return false;
     }
   }
 
   if (!IS_UNDEFINED(current)) {
     // Step 5 and 6
     if ((IsGenericDescriptor(desc) ||
          IsDataDescriptor(desc) == IsDataDescriptor(current)) &&
         (!desc.hasEnumerable() ||
          SameValue(desc.isEnumerable(), current.isEnumerable())) &&
         (!desc.hasConfigurable() ||
          SameValue(desc.isConfigurable(), current.isConfigurable())) &&
         (!desc.hasWritable() ||
          SameValue(desc.isWritable(), current.isWritable())) &&
         (!desc.hasValue() ||
          SameValue(desc.getValue(), current.getValue())) &&
         (!desc.hasGetter() ||
          SameValue(desc.getGet(), current.getGet())) &&
         (!desc.hasSetter() ||
          SameValue(desc.getSet(), current.getSet()))) {
       return true;
     }
     if (!current.isConfigurable()) {
       // Step 7
       if (desc.isConfigurable() ||
           (desc.hasEnumerable() &&
            desc.isEnumerable() != current.isEnumerable())) {
         if (should_throw) {
           throw MakeTypeError("redefine_disallowed", [p]);
         } else {
-          return;
+          return false;
         }
       }
       // Step 8
       if (!IsGenericDescriptor(desc)) {
         // Step 9a
         if (IsDataDescriptor(current) != IsDataDescriptor(desc)) {
           if (should_throw) {
             throw MakeTypeError("redefine_disallowed", [p]);
           } else {
-            return;
+            return false;
           }
         }
         // Step 10a
         if (IsDataDescriptor(current) && IsDataDescriptor(desc)) {
           if (!current.isWritable() && desc.isWritable()) {
             if (should_throw) {
               throw MakeTypeError("redefine_disallowed", [p]);
             } else {
-              return;
+              return false;
             }
           }
           if (!current.isWritable() && desc.hasValue() &&
               !SameValue(desc.getValue(), current.getValue())) {
             if (should_throw) {
               throw MakeTypeError("redefine_disallowed", [p]);
             } else {
-              return;
+              return false;
             }
           }
         }
         // Step 11
         if (IsAccessorDescriptor(desc) && IsAccessorDescriptor(current)) {
           if (desc.hasSetter() && !SameValue(desc.getSet(), current.getSet())) {
             if (should_throw) {
               throw MakeTypeError("redefine_disallowed", [p]);
             } else {
-              return;
+              return false;
             }
           }
           if (desc.hasGetter() && !SameValue(desc.getGet(),current.getGet())) {
             if (should_throw) {
               throw MakeTypeError("redefine_disallowed", [p]);
             } else {
-              return;
+              return false;
             }
           }
         }
       }
     }
   }
 
   // Send flags - enumerable and configurable are common - writable is
   // only send to the data descriptor.
   // Take special care if enumerable and configurable is not defined on
@@ skipping 82 matching lines @@
   }
 
   // Step 4 - Special handling for array index.
   var index = ToUint32(p);
   if (index == ToNumber(p) && index != 4294967295) {
     if ((index >= length && !length_desc.isWritable()) ||
         !DefineObjectProperty(obj, p, desc, true)) {
       if (should_throw) {
         throw MakeTypeError("define_disallowed", [p]);
       } else {
-        return;
+        return false;
       }
     }
     if (index >= length) {
       // TODO(mstarzinger): We should actually set the value of the property
       // descriptor here and pass it to DefineObjectProperty(). Take a look at
       // ES5 section 15.4.5.1, step 4.e.i and 4.e.ii for details.
       obj.length = index + 1;
     }
     return true;
   }
@@ skipping 711 matching lines @@
 
 function SetUpFunction() {
   %CheckIsBootstrapping();
   InstallFunctions($Function.prototype, DONT_ENUM, $Array(
     "bind", FunctionBind,
     "toString", FunctionToString
   ));
 }
 
 SetUpFunction();