Yes. As a matter of fact, I'm currently working on a large JS codebase that is a decade old. We switched over all the code to TypeScript in one go, and we are gradually fixing the type errors over time. VSCode does an excellent job of showing you type annotations and type errors inline, so the typing is very useful even if you have thousands of errors elsewhere in the project and never even run the type checker globally.
In fact, out of the dozen or so TS codebases I've worked on, I've rarely seen one completely free of type errors. And yet no team has ever doubted the value of the type system. It's a very pragmatic approach which works very well for many teams in my experience.
