The major Javascript engines already have the concept of a type system that applies at runtime. Their JITs will learn the 'shapes' of objects that commonly go through hot-path functions and will JIT against those with appropriate bailout paths to slower dynamic implementations in case a value with an unexpected 'shape' ends up being used instead.

There's a lot of lore you pick up with Javascript when you start getting into serious optimization with it; and one of the first things you learn in that area is to avoid changing the shapes of your objects because it invalidates JIT assumptions and results in your code running slower -- even though it's 100% valid Javascript.