Apa sing ditindakake dening operator keyof lan typeof ing konteks tipe?

Question

Accepted Answer

Loro operator iki ngidini sampeyan deduce tipe saka tipe lan nilai sing ana — fondasi saka pemrograman tingkat-tipe.

## keyof — gabungan kunci-kunci tipe obyek

```ts
interface User { id: number; name: string; }
type UserKey = keyof User; // "id" | "name"
```

## typeof — tipe saka nilai

```ts
const config = { host: "localhost", port: 3000 };
type Config = typeof config; // { host: string; port: number }
```

`typeof` (ing posisi tipe) nangkep tipe sing diandal-andal saka nilai runtime, supaya sampeyan ora kudu nulis tipe mula-mula.

## Digabung: property getter sing aman tipe

```ts
function getProp<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}
const user = { id: 1, name: "Ann" };
getProp(user, "name"); // returns string
getProp(user, "age");  // ❌ Error: "age" is not a key of user
```

Dene `K extends keyof T` mbatesi `key` menyang kunci nyata, lan `T[K]` (tipe *indexed access*) menehi balik tipe nilai sing persis kanggo kunci kasebut. Typo dadi compile errors.

## Deduce gabungan saka obyek const

```ts
const Roles = { Admin: "admin", User: "user" } as const;
type Role = typeof Roles[keyof typeof Roles]; // "admin" | "user"
```

## Mengapa iki penting

`keyof` lan `typeof` nyambung dunia nilai lan dunia tipe.

Kai nguwasi property access sing aman tipe, deduce tipe saka config/konstanta (sumber kebenaran tunggal), lan minangka blok bangun kanggo tipe mapped lan conditional.

Kai cara sampeyan ngalap tangan-njaga definisi tipe sejajar.