Utility Types

TypeScript's standard library (lib.es5.d.ts) defines utility types with the operators of this section: Partial is a mapped type, Exclude a conditional type, NoInfer a compiler intrinsic. Learn them before writing your own:

The most used utility types and the TypeScript version that added them
Utility	Result	Since
Partial<T>, Required<T>	All keys optional, required	2.1, 2.8
Readonly<T>	Top-level keys readonly	2.1
Record<K, V>	Keys K, values V	2.1
Pick<T, K>, Omit<T, K>	Keep keys K, drop keys K	2.1, 3.5
Exclude<U, M>	Union minus members of M	2.8
NonNullable<T>	Drops null, undefined	2.8
ReturnType<F>	A function's return type	2.8
Parameters<F>	A function's parameter tuple	3.1
Awaited<T>	Unwrapped promise result	4.5
NoInfer<T>	Blocks inference here	5.4

Deriving request types from one modelCSS

interface User { id: number; name: string; email: string; role: "admin" | "member" }
type NewUser = Omit<User, "id">;                         // a signup form's payload
type UserPatch = Partial<Pick<User, "name" | "email">>;  // { name?: string; email?: string }
const roleLabels: Record<User["role"], string> = { admin: "Administrator", member: "Member" };
declare function updateUser(id: number, patch: UserPatch): Promise<User>;
type Updated = Awaited<ReturnType<typeof updateUser>>;   // User
function choose<T extends string>(sizes: T[], fallback: NoInfer<T>): T {
  return sizes[0] ?? fallback;
}
const draft: NewUser = { id: 7, name: "Ada", email: "ada@example.com", role: "admin" };
updateUser(1, { role: "admin" });
choose(["sm", "md", "lg"], "xl");

Output

users.ts(10,26): error TS2353: Object literal may only specify known properties, and 'id' does
  not exist in type 'NewUser'.
users.ts(11,17): error TS2353: Object literal may only specify known properties, and 'role'
  does not exist in type 'Partial<Pick<User, "email" | "name">>'.
users.ts(12,28): error TS2345: Argument of type '"xl"' is not assignable to parameter of type
  '"lg" | "md" | "sm"'.

Without NoInfer, TypeScript would also infer T from "xl" and accept it. Record<User["role"], string> forces a label for every role, so adding "guest" to User breaks the build until roleLabels is updated.

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