TypeScript Utility Types

Overview

TypeScript ships built-in utility types that transform existing types. They reduce duplication and keep DTOs, form state, and API layers in sync with core interfaces.

Syntax / Usage

interface User {
  id: string
  name: string
  email: string
  role: 'admin' | 'user'
}

type PartialUser = Partial<User>           // all optional
type RequiredUser = Required<PartialUser>  // all required
type UserPreview = Pick<User, 'id' | 'name'>
type UserWithoutEmail = Omit<User, 'email'>
type ReadonlyUser = Readonly<User>

type RoleMap = Record<'admin' | 'user', string>

type CreateUser = Omit<User, 'id'>
type UpdateUser = Partial<CreateUser>

type GreetFn = (name: string) => string
type GreetReturn = ReturnType<GreetFn>  // string
type GreetParams = Parameters<GreetFn> // [string]

Examples

Form state from entity:

type UserForm = Partial<Pick<User, 'name' | 'email'>> & {
  password: string
}

Extract promise result:

type FetchUserResult = Awaited<ReturnType<typeof fetchUser>>

Non-nullable fields:

type NonNullableUser = {
  [K in keyof User]: NonNullable<User[K]>
}

Common Mistakes

• Partial everywhere when only some fields should be optional—use Pick + Partial
• Omit removing fields still required downstream
• Deep partial needs a custom mapped type—Partial is shallow
• Over-nesting utility types hurts readability—alias intermediate types

See Also

interfaces generics unions type-guards