Nini ni aina ya never na jinsi gani inavyoweza kuwezesha exhaustiveness checks?

Question

Accepted Answer

`never` ni aina ya thamani ambazo **hazizatokei kamwe** — kazi inayokataa bila kurudi (inarusha au kurudia sana), au matawi ambayo kujaza nambari imekubali kuwa haiwezekani. Ni aina *tupu*: hakuna thamani inayoweza kuwekwa kwa `never` (isipokuwa `never` yenyewe).

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

## Kwa nini never ni muhimu: exhaustiveness checking

Unapoingia zaidi ungo hadi kwa chochote, TypeScript inaandika thamani iliyobaki kama `never`. Unatumia hii kulazimisha kukabiliana na kila kesi:

```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;
  }
}
```

Sasa kama mtu yeyote anaongeza `{ kind: "triangle"; ... }` kwa `Shape` lakini husahau kesi, `s` katika chaguo-msingi ni aina ya pembetatu (si `never`), kwa hiyo `const _exhaustive: never = s` **inashindwa kukusanya** — ikikuonyesha mahali haswa ambapo switch inahitaji kusasishwa.

## never vs void

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

## Kwa nini hii inamaanisha

`never` inabadilisha "je, nilikabiliana na kila kesi?" kutoka tumaini la wakati wa uendeshaji hadi kuhakikisha wa wakati wa ukusanyaji.

Ni ufunguo wa salama, na inayolindwa na baadaye kukabiliana na vyama vilivyotofautiana — ongeza tofauti na kujaza kuhesabu mahali yote ambayo lazima usasishe.