Advanced TypeScript

Conditional Types

Conditional types choose one type or another based on assignability. They are the foundation for many TypeScript utility types and library-level type transformations.

Conditional types are type-level branches
infer extracts part of a type
Naked type parameters distribute over unions
They are best for reusable type helpers
Type cleverness has maintenance cost

Extract awaited value

type AwaitedValue<T> = T extends Promise<infer U> ? U : T

type A = AwaitedValue<Promise<string>> // string
type B = AwaitedValue<number>          // number

`infer U` captures the value inside Promise<U>.

Distributive conditional type

type OnlyStrings<T> = T extends string ? T : never

type Result = OnlyStrings<string | number | "ok">
// string | "ok" simplifies to string

The conditional is applied to each union member.

Conditional type pieces
Piece	Meaning
T extends U	Assignability test
? A : B	Type-level branch
infer X	Capture part of matched type
never	Remove impossible union branch
[T] extends [U]	Prevent distributive behavior