Just changing the file extension from js to ts will often bring out bugs in your code. The type inference will often catch things. Functions that are declared multiple types or that have duplicate parameters, and a lot of other stuff will be caught.
Actually, it's better. If the core of your application has non trivial business logic, it's a good idea to write it in typed way and write ui and other support stuff with partially typed code.
Fair enough and I think with strict null type checking, where null/undefined isn't part of any anymore things would still get much better than with plain JS.