function-types

`ReturnType<T>`

Infers the return type of a function type T using `infer R`. Widely used to derive types from existing functions without redeclaring them.

Definition

type ReturnType<T extends (...args: any) => any> = T extends (...args: any) => infer R ? R : any

Usage patterns

Pattern	Purpose
ReturnType<typeof fn>	Get a value's function return type via typeof
Awaited<ReturnType<T>>	Get the resolved type for async functions
ReturnType<T> extends Promise<infer R> ? R : ReturnType<T>	Manual async unwrap (pre-Awaited)
infer R	The inference site that names the return type

function greet() { return 'hi' as const; }
type G = ReturnType<typeof greet>;

Result: 'hi'

async function load() { return { ok: true }; }
type Loaded = Awaited<ReturnType<typeof load>>;

Unwraps the Promise to { ok: boolean }

type Handler = (e: Event) => void;
type R = ReturnType<Handler>;

Result: void

type BadReturn = ReturnType<string>; // Error

T must be a function type — non-function inputs error at compile time

For overloaded functions, ReturnType only picks the last overload signature. For async functions it gives Promise — wrap with Awaited if you want X.