Lesson 20 +10 XP

Conditional Utility Types (Exclude, Extract, ReturnType)

Conditional & Union Utility Types

TypeScript provides utilities to operate on unions, functions, and instance types.

Union & Function Utilities Overview

Utility Type	Usage	Description
`Exclude<Union, ExcludedMembers>`	Union filtering	Removes specified types from a union.
`Extract<Union, ExtractedMembers>`	Union selection	Keeps only types assignable to specified members.
`NonNullable<T>`	Null check	Strips `null` and `undefined` from `T`.
`ReturnType<Fn>`	Function inspect	Extracts the return type of a function signature.
`Parameters<Fn>`	Function inspect	Obtains parameter types of a function as a tuple.

Code Examples

type T0 = Exclude<"a" | "b" | "c", "a">; // "b" | "c"
type T1 = Extract<string | number | boolean, string | number>; // string | number

type T2 = NonNullable<string | number | null | undefined>; // string | number

function getUser() {
  return { id: 101, username: "dev_user" };
}

type UserResult = ReturnType<typeof getUser>; // { id: number; username: string; }

TL;DR

Exclude strips union types; Extract keeps matching union types.
NonNullable removes null and undefined.
ReturnType<typeof fn> captures function return types automatically.