Worth noting that this is not just for your actual code. You can also typecheck (and, just as importantly, refactor) JSX-based views like React's. You can't do that on stringly-typed templates like jQuery.tmpl, Knockout templates, or similar.

If you've ever had to debug a misbehaving Knockout template just to find it was referencing a misspelled object property, you can appreciate how much cleaner TSX templating is.