Very happy to hear TypeScript support is included! I wouldn't like to work on a large app without it.
interface A {
x: number;
}
let a: A = {x: 3}
let b: {x: number | string} = a;
b.x = "unsound";
let x: number = a.x; // unsound
a.x.toFixed(0); // WTF is it?