The only way I can think of off the top of my head where this would be an issue is when calling JSON.stringify on the object, or a similar function that operate

The only way I can think of off the top of my head where this would be an issue is when calling JSON.stringify on the object, or a similar function that operates on all available keys. But for the most part, why would you write a function like that if you only wanted to operate on certain keys? Of course, you'd run into the same issue in regular Javascript by expecting a base class and receiving a child class of that base.