readonly: 쓰기 권한을 타입에서 거둬들이기

// Property 에 readonly.
interface User {
  readonly id: number;
  name: string;
}

const u: User = { id: 1, name: 'Pippa' };
u.name = 'Pippa2';                  // ✅ name 은 mutable
u.id = 999;                         // ❌ Cannot assign to 'id' because it is a read-only property

// readonly 배열 — mutation method 제거됨.
const frozen: readonly number[] = [1, 2, 3];
frozen.push(4);                     // ❌ Property 'push' does not exist on type 'readonly number[]'
frozen[0] = 99;                     // ❌ Index signature is readonly

// readonly 가 shallow:
interface Wrapped {
  readonly inner: { name: string };
}
const w: Wrapped = { inner: { name: 'Pippa' } };
w.inner = { name: 'Other' };        // ❌ outer 가 readonly
w.inner.name = 'Mutated';           // ✅ inner 는 readonly 아님 — wrapper 만

// `as const` 와 Readonly<T> — utility 둘.

// `as const` — 모든 property readonly, 모든 값 literal 로 narrow.
const route = {
  path: '/api/chat',
  method: 'POST',
} as const;
// route 는 { readonly path: '/api/chat'; readonly method: 'POST' }
// readonly AND literal 타입으로 narrow 됨.

route.path = '/api/foo';            // ❌ readonly

// Readonly<T> — utility-type 버전, 기존 타입에 opt-in 더 쉬움.
interface MutUser { id: number; name: string }
type FrozenUser = Readonly<MutUser>;
// FrozenUser 는 { readonly id: number; readonly name: string }

// 둘 다 shallow. Deep 원하면 만들:
type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K];
};