Lesson 20 — Mapped types

Loop over properties at the type level

A mapped type builds a new type by going through every key of another type: { [K in keyof T]: … }. You can add ? to make every property optional, readonly to lock them, or change each property's type. Partial, Readonly and Record are written exactly this way.

type Flags<T> = { [K in keyof T]: boolean };
type Settings = { dark: string; sound: string };
type SettingsOn = Flags<Settings>; // { dark: boolean; sound: boolean }

Why it matters: you describe a rule once and TypeScript applies it to every property, now and in the future.