Mapped Types (Partial/Required/etc)

type User = { id: string; name: string; email: string };

// Deep readonly (recursive)
type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K];
};

// Mutable inverse of Readonly
type Mutable<T> = { -readonly [K in keyof T]: T[K] };

// Keep only the keys whose value matches a type
type KeysOfType<T, V> = { [K in keyof T]: T[K] extends V ? K : never }[keyof T];

type StringKeys = KeysOfType<User, string>;     // 'id' | 'name' | 'email'

// Make some keys optional (the others stay required)
type PartialBy<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;

type NewUser = PartialBy<User, 'id'>;
// { name: string; email: string; id?: string }