Mapped Types

A mapped type builds a new type by transforming every property of an existing one — how Partial, Readonly, and similar utility types are actually implemented.

type Optional<T> = {
    [K in keyof T]?: T[K];
};

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

type OptionalUser = Optional<User>; // { name?: string; age?: number }

keyof T gets a union of all of T's property names; [K in ...] iterates over them, producing a new property for each.