Mapped Types

type Nullable<T> = {
  [K in keyof T]: T[K] | null;
};

interface User {
  id: number;
  name: string;
}

type NullableUser = Nullable<User>;
// { id: number | null; name: string | null }

type DeepPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K];
};

interface Config {
  server: { host: string; port: number };
  database: { url: string };
}

type PartialConfig = DeepPartial<Config>;
// All nested properties are optional

type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K];
};

type RequiredKeys<T> = {
  [K in keyof T]-?: {} extends Pick<T, K> ? never : K;
}[keyof T];

interface Form {
  name: string;
  email: string;
  age?: number;
}

type Required = RequiredKeys<Form>; // "name" | "email"