Read an env var that must name one of an allowed set.
Unset returns undefined, so the caller decides what a missing lever means. Set-but-invalid throws instead: silently dropping a typo runs a configuration nobody asked for.
undefined
Read an env var that must name one of an allowed set.
Unset returns
undefined, so the caller decides what a missing lever means. Set-but-invalid throws instead: silently dropping a typo runs a configuration nobody asked for.