Mapped Types to Transform Object Shapes

You need to derive a new type from an existing one (make all fields optional, readonly, or transform value types) without manually duplicating the interface.

Use mapped types with 'keyof' to iterate over a type's keys and transform them.

// Make all properties nullable
type Nullable<T> = { [K in keyof T]: T[K] | null };

// Make all properties async getters
type AsyncGetters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => Promise<T[K]>;
};

interface User { name: string; age: number; }
type AsyncUser = AsyncGetters<User>;
// { getName: () => Promise<string>; getAge: () => Promise<number>; }

// Filter keys by value type
type PickByValue<T, V> = {
  [K in keyof T as T[K] extends V ? K : never]: T[K];
};
type StringFields = PickByValue<User, string>; // { name: string }

Mapped types iterate over a union of keys (usually 'keyof T') and produce a new object type. Key remapping with 'as' enables filtering and renaming keys.