Javascript is Typescript. Typescript ⊇ Javascript.
Especially if you already use Babel (or any other JS preprocessor, like uglify or concat) and have a build pipeline ready, you can start using Typescript today.
Don't miss out!
PS: It will throw a lot of errors on a conventional codebase, but compile despite them. That's the point; install the compiler today, slowly start fixing the errors as you go. It's all about gradual improvement and optional typing.