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);
Union types are present in the master branch of the TS compiler. The compiler also uses instanceof and typeof === ... to reduce the range of types inside a branch, similar to Flow.
tl;dr: they want to support both of those features, the question is what syntax to use and how to introduce those features into the existing ecosystem.
