JS Practice — Built-in Objects

Utility Types — Readonly<T>

📖 Usage Example

type Config = Readonly<{ host: string; port: number }>
const c: Config = { host: 'localhost', port: 3000 }
// c.host = 'other'  → Error: cannot assign to read-only property

Readonly<T> makes every property of T immutable at the TypeScript type level.

Utility Types: Readonly<T>

Readonly<T> marks all properties as non-writable at the type level.

Challenge: Implement createReadonly<T extends object>(obj: T): Readonly<T> that prevents mutations using Object.freeze.

const ro = createReadonly({ x: 1, y: 2 })
ro.x  // → 1  (but writing would throw in strict mode)

💡 Hints (1)

• Object.freeze prevents property mutation at runtime.