You can write quite crazy expressions abusing Boolean operators and he successfully keeps the static type.

You can also have different method overloads signatures as long as you have a more general implementation that does the dynamic checks at run-time manually. Most of the time you can use Union types anyway.

The big collection of definition files for almos any JS library out there is a living proof of how is possible to describe statically almost any JS API.

Let's face it, dynamic languages are about laziness, not about 'unconstraibed creativity'