@fast-check/expect-type
Make sure your types are the ones you expect (similar to tsd)
Easy to use
This package performs compilation time checks only. Running the check functions at runtime is a no-op.
import { expectType, expectTypeAssignable } from '@fast-check/expect-type';
// your code or you own imports
expectType<number>()(f(1, 2), 'expect the output of f when passed 1 and 2 to be number');
expectTypeAssignable<number>()(f(1, 2), 'expect the output of f when passed 1 and 2 to be assignable to number');