I wouldn't exactly call what TypeScript has the best type system out there. It's deliberately unsound, doesn't enforce full static type-checking, _and_ doesn't provide for runtime checks (there are third-party libraries, but if you're bringing up third parties one might as well argue that JavaScript has a great type system because TypeScript exists).

A lot of it is due to the project's overwhelming priority being as easy to interop with JS as possible, but I've found that quite a few of TypeScript's fancier features are sugar that's entirely up to the developer to enforce.