#infer

TypeScriptadvanced

Conditional types + infer to extract a function's return type

type AsyncReturnType<Fn extends (...args: never[]) => unknown> = Fn extends (
  ...args: never[]
) => Promise<infer R>
  ? R
  : Fn extends (...args: never[]) => infer R

Build a return-type extractor that also unwraps the Promise, so async and sync functions land on the same resolved type.

TypeScriptadvanced

Template literal types for typed route strings

type ParamNames<Path extends string> = Path extends `${string}:${infer Param}/${infer Rest}`
  ? Param | ParamNames<Rest>
  : Path extends `${string}:${infer Param}`
    ? Param
    : never;

Extract :param names out of a route string at the type level, so a route's caller must supply exactly the params that route needs.