There are problems in their approach: In the first case the behavior is quirky because they are passing an untyped object into a function. If the object is typ

There are problems in their approach: In the first case the behavior is quirky because they are passing an untyped object into a function. If the object is typed the behavior is consistent. The second example is about classes. Classes are created as easy extension objects via inheritance and poly-instantiation. Those ideas are friendly concepts for many developers but they increase complexity so I intentionally avoid classes and use a custom ESLint rule to enforce such. In the third case they can solve for complexity with a simple refactor. The two interfaces are structurally identical so they should be a single interface with either a union on property values (static literals) or by defining a property again a union of other interfaces called by reference.