Yes I see what you mean, it'd be interesting to see what the exact rules around type checking are; especially when dealing with []. The string type for example, always give you a string when indexed, even when incorrect:

  var s = 'foo';
  var l = s['length'];
  // type of l is `string`

I also agree about non-nullable types, and I've found even with TypeScript, having to make sure your vars are defined and not-null is still a pain.