Apa sing ditindakake dening `as const` assertion?

Question

Accepted Answer

`as const` yaiku **const assertion** sing ngandika marang TypeScript supaya nggawe **tipe sing paling spesifik (literal), deeply readonly** kanggo nilai, ananing ora ngadal-adal.

```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!)
```

Ngono dadi telu bab: ora ngadal-adal apa-apa (tetep nilai literal), nggawe kabeh `readonly`, lan ngobah array dadi **tuple**.

## Kenapa paling guna

**Nggawe tipe union saka daftar nilai (sumber kasunyatan tunggal):**

```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
```

**Ngeling-eleng tipe literal kanggo 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
```

## Apa pentinge

`as const` nglungsurang nilai runtime lan tipe sing akurat.

Kulonuwun bisa nemtokake data sapisan (array/obyek) lan nggawe tipe literal sing tepat saka iku — penting banget kanggo tipe aksi, peta rute, config, lan discriminated-union tag apa-apa sing kudu tetep literal ananing ora ngadal-adal dadi `string`.