type AsyncReturnType<T extends (...args: any) => Promise<any>> =
  T extends (...args: any) => Promise<infer R> ? R : never;

async function fetchUserData(id: string) {
  const response = await fetch(`/api/users/${id}`);
  const data: { id: string; name: string; email: string } = await response.json();
  return data;
}

async function processData(input: number): Promise<string> {
  return Promise.resolve(input.toString());
}

// Example Usage:
type UserDataType = AsyncReturnType<typeof fetchUserData>;
// UserDataType will be: { id: string; name: string; email: string }

type ProcessResultType = AsyncReturnType<typeof processData>;
// ProcessResultType will be: string

// Type 'UserDataType' is assignable to variable 'user'
const user: UserDataType = {
  id: "user-123",
  name: "Alice",
  email: "alice@example.com"
};

// Type 'ProcessResultType' is assignable to variable 'result'
const result: ProcessResultType = "42";

// Function returning non-Promise is not assignable
// function syncFunc(): string { return 'hello'; }
// type SyncResult = AsyncReturnType<typeof syncFunc>; // Error: Type 'typeof syncFunc' does not satisfy the constraint '(...args: any) => Promise<any>'.