You can't really use sum types as they are found in Haskell in Javascript. With Haskell option types you do a pattern matching and create a new binding for the non-null value but in Javascript you don't do that - you keep using the same object that you tested against null.

    case mx of Just x -> f(x)

vs

    if (x != null){ f(x) }

What you can do in a Javascript-like language is use union and intersection types. However, they can get a bit complicated (specially if you allow unions of non-primitive types) and the extra flexibility can confuse the type inference a bit so I can understand them restricting things to the common case of handling null.