LibJS: Disallow changing the prototype of non-extensible objects

Object::set_prototype() now returns a boolean indicating success.
Setting the prototype to an identical object is always considered
successful, even if the object is non-extensible.

1 files changed, 4 insertions, 1 deletions

diff --git a/Libraries/LibJS/Runtime/ObjectConstructor.cpp b/Libraries/LibJS/Runtime/ObjectConstructor.cpp
index 23b247d6b7..a4882889b4 100644
--- a/Libraries/LibJS/Runtime/ObjectConstructor.cpp
+++ b/Libraries/LibJS/Runtime/ObjectConstructor.cpp
@@ -112,7 +112,10 @@ Value ObjectConstructor::set_prototype_of(Interpreter& interpreter)
         interpreter.throw_exception<TypeError>("Prototype must be null or object");
         return {};
     }
-    object->set_prototype(prototype);
+    if (!object->set_prototype(prototype)) {
+        interpreter.throw_exception<TypeError>("Can't set prototype of non-extensible object");
+        return {};
+    }
     return object;
 }