Well, in React's case if you use PropTypes, the rewrite is mostly mechanical, and can be automated via things like https://github.com/lyft/react-javascript-to-t

Well, in React's case if you use PropTypes, the rewrite is mostly mechanical, and can be automated via things like https://github.com/lyft/react-javascript-to-typescript-transform https://github.com/lyft/react-javascript-to-typescript-trans... But many tools, flow, Python, TS allow you to incrementally add types to an existing codebase. That way refactoring isn't some herculean effort, but something you can do for new features, or gradually over time.