TS bolts on a straightforward nominative type system without type unions (or non-nullable types), so it can't handle a variable typed as `number | string`, it'll immediately drop down to `any`. That is, flow aims to remain useful in the face of more JS idioms. It won't make a difference between nullable and non-nullable either, so AFAIK

    function length(x) {
      return x.length;
    }

    length(null);

can never be a compile-time error in typescript.