never and exhaustiveness checking

Yesterday, in one sentence

Interview question

never is the type for a value that can never exist. It represents an impossibility.

type Impossible = string & number;
// never, because no value can be both a string and a number

You also get never from narrowingNarrowing. Once type guardsType Guard have ruled out every type in a union typeUnion Type, there is nothing left and the type becomes never.

function describe(id: string | number) {
  if (typeof id === "string") return id;
  if (typeof id === "number") return id;

  return id; // id has type never, all other types have been ruled out
}

An exhaustiveness check uses this to make TypeScript error when a discriminated unionDiscriminated Union gains a member you have not handled. It works because you can only assign a value to a never once narrowing has ruled everything else out.

type Character = { kind: "hero" } | { kind: "villain" };

function describe(character: Character) {
  if (character.kind === "hero") return "saves people";
  if (character.kind === "villain") return "causes trouble";

  const unhandled: never = character; // ok, character is never here
  return unhandled;
}

Add a third member and the assignment stops compiling.

type Character = { kind: "hero" } | { kind: "villain" } | { kind: "antihero" };

function describe(character: Character) {
  if (character.kind === "hero") return "saves people";
  if (character.kind === "villain") return "causes trouble";

  const unhandled: never = character;
  // ❌ character is not never here, "antihero" has not been handled
  return unhandled;
}

That error is the point. Without it, an antihero would fall past both checks and nothing would tell you.

Check yourself

type Rank = "alpha" | "beta" | "omega"

function classify(rank: Rank) {
  if (rank === "alpha") return
  if (rank === "beta") return
  if (rank === "omega") return

  rank   // ← here
}