After checking "id" in value on a non-null object, what has been proved?

After checking "id" in value on a non-null object, what has been proved?

1. The property is found on the object or its prototype chain, but its value and ownership are not proved.
2. id is an own, nonempty string property.
3. The complete object now matches every field of the target interface.
4. The check removes id from the object after the branch.