expect-type

expect-type is a devtool library that enables compile-time assertions for TypeScript types, allowing developers to write tests that verify types remain strict and don't silently broaden over time. It's designed for TypeScript library authors and teams maintaining complex type systems who need to prevent type regressions during refactoring or feature additions. Its interesting value lies in bringing the rigor of unit testing to the type level, catching overly-permissive types before they cause runtime surprises.