Async ও Promise (টাইপ সহ)

function wait(ms: number): Promise<void> {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

function fetchSettings(): Promise<{ dark: boolean }> {
  return Promise.resolve({ dark: true });
}

fetchSettings().then((settings) => {
  console.log(settings.dark);   // boolean
});

async function loadUser(id: string): Promise<string> {
  const response = await fetch("/api/users/" + id);
  const data = (await response.json()) as { name: string };
  return data.name;
}

// async ফাংশন সবসময় Promise ফিরত দেয়
const user: Promise<string> = loadUser("u-1");

async function safeLoad(id: string): Promise<{ ok: boolean; error?: string }> {
  try {
    const response = await fetch("/api/users/" + id);
    if (!response.ok) throw new Error("সার্ভার ত্রুটি");
    return { ok: true };
  } catch (error) {
    const message = error instanceof Error ? error.message : "অজানা ত্রুটি";
    return { ok: false, error: message };
  }
}

interface User {
  id: number;
  name: string;
  email: string;
}

interface ApiResponse<T> {
  data: T;
  message?: string;
}

async function getUsers(): Promise<ApiResponse<User[]>> {
  const response = await fetch("/api/users");
  if (!response.ok) {
    throw new Error("ডেটা পাওয়া যায়নি");
  }
  return (await response.json()) as ApiResponse<User[]>;
}

const users = await getUsers();
users.data.forEach((user) => console.log(user.name));

// Parallel — দুটো আরেকটা fetch একসাথে
const [userA, userB] = await Promise.all([
  getUsers(),
  getUsers(),
]);

// formatError-এ known type-কেবল
type ErrorType = { code: number; message: string };
function formatError(err: unknown): string {
  if (err instanceof Error) return err.message;
  if (typeof err === "object" && err !== null && "code" in err) {
    return (err as ErrorType).message;
  }
  return "অজানা ত্রুটি";
}