X'inhu l-assertion `as const`?

Question

Accepted Answer

`as const` hu **const assertion** li jgħid lit-TypeScript li jinferixxi t-**tip (literal) l-aktar speċifiku, deeply readonly** għal valur, minflok li jossuh.

```ts
// without as const — widened, mutable
let a = "up";            // type: string
const obj = { x: 1 };     // type: { x: number }
const arr = [1, 2];       // type: number[]

// with as const — narrow literals, readonly
const b = "up" as const;          // type: "up"
const obj2 = { x: 1 } as const;    // type: { readonly x: 1 }
const arr2 = [1, 2] as const;      // type: readonly [1, 2] (a tuple!)
```

Agħmel tliet affarijiet: ma jossax xejn (iħalli valuri literali), jagħmel kollox `readonly`, u jbiddel arrays f'**tuples**.

## Għaliex hija utli tant

**Derivazzjoni ta' union type minn lista ta' valuri (sors waħid tal-verità):**

```ts
const ROLES = ["admin", "editor", "viewer"] as const;
type Role = typeof ROLES[number]; // "admin" | "editor" | "viewer"
// add a role to the array → the union updates automatically
```

**Preservazzjoni ta' literal types għal config/state:**

```ts
const action = { type: "INCREMENT", by: 1 } as const;
// action.type is "INCREMENT" (a literal), so it works in discriminated unions
// without as const it'd be `string`, breaking narrowing
```

## Għaliex hija importanti

`as const` tobloq bejn valuri runtime u tipji preċiżi.

Tippermettilek tiddefinixxi data darba (array/object) u tiderivja tip literal eżatt minnu — invaluabel għal action types, route maps, config, u kwalunkwe discriminated-union tag li għandu jibqa' literal minflok li jossa għal `string`.