> Types are a safeguard, they rule out certain errors

I have migrated to TypeScript just about a year ago and it's my third try to migrate to TS from JS during the last decade and finally a successful one. While TS went a long road since the first versions which were incredibly hostile, my rewrite of a large codebase from js to ts revealed exactly zero type-related bugs.

eons ago, I migrated a frontend to Typescript and caught a lot of type-related bugs[1]. It was a 5kLoC, fast-moving productized prototype written by a team of 5. I won't ever do dynamic-typed plain Javascript in a team ever again, type-checker is superior to human code-reviews when it comes to catching potential bugs. Then again I prefer codebase stability of clever code or "expressiveness"

1. 20% were type-coercion bugs, 30% were non-boolean values being passed to boolean-named fields (with some overlap with the former). Linters have come a long way, but compile-time type-checking is better in almost every way.

Static types are not _that_ useful to catch bugs, if only because type related bugs tend to surface very quickly, especially in strongly typed language like Python. So a good CI suite is usually enough to catch them, but you do need good coverage. Even if they make it to prod, they won't survive long...

Static types are IMHO more useful for speed, maintenance/refactoring of large projects, and code completion in IDEs. But a codebase in production is unlikely to have much type related bugs...

Catching bugs in CI is orders of magnitude slower and more expensive than catching them as type errors while writing the code.

> Even if they make it to prod, they won't survive long...

I don't think that's an acceptable way to treat your users. If something is trivial to prevent, do it.

Static type checking greatly affects code design. That’s why converting from JS to TS doesn’t give you full benefits of the type system – the code doesn’t lean into types to model invariants.

This is such a limited view on types.

The concept of them being more than compiler bookkeeping, but as propositions about program behavior and invariant encoding is more than 50 years old at this point.