Keys of Type

Extract all the keys of a certain type T from object O.

type KeysOfType<O, T> = {
  [K in keyof O]: O[K] extends T ? K : never;
}[keyof O & string];

Use case

Suppose we have a type User like so:

type User = {
  id: string;
  name: string;
  isAdmin: boolean;
  isEnabled: boolean;
  hasVerifiedEmail: boolean;
};

And we want to extract all the boolean fields from User and put them in a new type called UserFlags. We can do so by:

type UserFlags = KeysOfType<User, boolean>;
//   ^? type UserFlags = "isAdmin" | "isEnabled" | "hasVerifiedEmail"