Mapped Types & `keyof`

keyof Operator

type Person = { name: string; age: number; };
type PersonKeys = keyof Person; // "name" | "age"

Mapped Types – Transform each property

type ReadonlyPerson = { readonly [K in keyof Person]: Person[K] };
// equivalent to: { readonly name: string; readonly age: number; }

Partial & Required

type PartialPerson = Partial<Person>; // all props optional
type RequiredPerson = Required<PartialPerson>; // back to required

Record Utility

type RoleMap = Record<string, "admin" | "user" | "guest">;
const roles: RoleMap = { alice: "admin", bob: "user" };