exhaustive is a function to help statically check that you explicitly handle all possible values.
See its documentation for a longer explanation. TypeScript's never type is the novel thing that makes it work.
We could copy & paste the function definition into a "utils" file in each project, but why not define it centrally and reuse it? That's why it's been put on this module registry:
import exhaustive from "jsr:@duncan/exhaustive"