Lesson 24 — Exhaustive checks with never

Make the compiler find missing cases

never is the type of a value that cannot exist. In a switch over a discriminated union, once every case is handled, the leftover value in default has type never. If someone adds a new shape and forgets a case, assigning it to a never variable becomes a compile error that points straight at the gap.

default: {
  const unreachable: never = s; // error if a case is missing
  throw new Error("Unhandled");
}

Why it matters: the compiler, not your users, tells you about the case you forgot.