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TypeScript Tutorial · Generics and Type Manipulation · Lesson 17 of 25

TypeScript Utility Types

Partial, Required, Readonly, Pick, Omit, Record, ReturnType and friends.

Built-in type transformations

TypeScript ships with generic utility types that derive new types from existing ones:

Utility Result
Partial<T> all properties optional
Required<T> all properties required
Readonly<T> all properties readonly
Pick<T, K> only properties K
Omit<T, K> all properties except K
Record<K, V> object with keys K and values V
ReturnType<F> / Parameters<F> a function's return / parameter types
Awaited<T> the value a promise resolves to
NonNullable<T> removes null and undefined
Example 1Try it yourself
interface User {
  id: number;
  name: string;
  email: string;
  isAdmin: boolean;
}

type UserUpdate = Partial<Omit<User, "id">>;      // everything but id, all optional
type PublicUser = Pick<User, "id" | "name">;
type UsersById = Record<number, PublicUser>;

function updateUser(user: User, changes: UserUpdate): User {
  return { ...user, ...changes };
}

const ada: User = { id: 1, name: "Ada", email: "[email protected]", isAdmin: false };
const promoted = updateUser(ada, { isAdmin: true });
const directory: UsersById = { [ada.id]: { id: ada.id, name: ada.name } };

function makeUser() { return { id: 2, name: "Lin" }; }
type Made = ReturnType<typeof makeUser>;          // { id: number; name: string }
const lin: Made = makeUser();

console.log(promoted.isAdmin, directory[1], lin);

Output

true { id: 1, name: 'Ada' } { id: 2, name: 'Lin' }

Implement Pick and Omit yourself in the Pick & Omit problem.

Test yourself

Which utility type makes every property optional?

  1. Optional<T>
  2. Partial<T>
  3. Maybe<T>
  4. Pick<T>
Show answer

Answer: Partial<T>

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