Mapped types

Mapped types help build types that are based off another type. A mapped type is a generic type which uses in keyof to iterate through the keys of one type to create another.

They build off of [[20230403122631-index-signature-ts]].

In the following snippet, in keyof uses keyof to generate a union of all keys in Type. These are then assigned as properties (e.g. keys in a key/value pair) inside of OptionsFlags.

type OptionsFlags<Type> = {
  [Property in keyof Type]: boolean;
};

type FeatureFlags = {
  darkMode: () => void;
  newUserProfile: () => void;
};

type FeatureOptions = OptionsFlags<FeatureFlags>;
//   ^ = type FeatureOptions = {
//       darkMode: boolean;
//       newUserProfile: boolean;
//   }

[[20210331123346-keyof-typescript]] [[20210610110122-indexed-access-types]]

[[js]] [[ts]]