Apa itu tipe never lan carane ngaktifake pemeriksaan exhaustiveness?

Question

Accepted Answer

`never` yaiku tipe saka nilai-nilai sing **ora bakal ana** — fungsi sing ora bakal bali (manthuk utawa loop selamanya), utawa cabang sing sudah kabukti dening compiler ora mungkin. Iki tipe *kosong*: ora ana nilai sing bisa ditugasake menyang `never` (kajaba `never` dhewe).

```ts
function fail(msg: string): never { throw new Error(msg); } // never returns
function loop(): never { while (true) {} }
```

## Mulane never iku migunani: exhaustiveness checking

Kadhangkala sampeyan sempit union menyang ora ana, TypeScript ngetik nilai sisa minangka `never`. Sampeyan manfaatake iki kanggo meksa nangani saben kasus:

```ts
type Shape =
  | { kind: "circle"; r: number }
  | { kind: "square"; side: number };

function area(s: Shape): number {
  switch (s.kind) {
    case "circle": return Math.PI * s.r ** 2;
    case "square": return s.side ** 2;
    default:
      // if all cases are handled, s is `never` here — assignment OK
      const _exhaustive: never = s;
      return _exhaustive;
  }
}
```

Mbesuk yen wong nambah `{ kind: "triangle"; ... }` menyang `Shape` nanging lali kasus, `s` ing default yaiku tipe triangle (ora `never`), dadi `const _exhaustive: never = s` **gagal dikompilasi** — nuduhke sampeyan persis switch sing kudu dianyari.

## never vs void

```ts
function a(): void {}   // returns normally, just no value
function b(): never { throw "x"; } // never returns at all
```

## Mulane iki penting

`never` ngganti "apa aku nangani saben kasus?" saka harapan runtime dadi jaminan wektu kompilasi.

Iki kaunci kanggo nangani discriminated union kanthi aman, sing maju-tekan — nambah varian lan compiler nulis setiap papan sing kudu sampeyan anyari.