Ts Async Guide

Use Promise.all for independent concurrent operations

Correct

async function loadDashboard(userId: string): Promise<Dashboard> {
  const [user, orders, notifications] = await Promise.all([
    fetchUser(userId),
    fetchOrders(userId),
    fetchNotifications(userId),
  ]);
  return { user, orders, notifications };
}

Anti-Pattern

// BAD: sequential awaits when operations are independent — 3x slower
async function loadDashboard(userId: string): Promise<Dashboard> {
  const user = await fetchUser(userId);
  const orders = await fetchOrders(userId);
  const notifications = await fetchNotifications(userId);
  return { user, orders, notifications };
}

Use Promise.allSettled when partial failure is acceptable

Correct

const results = await Promise.allSettled([
  sendEmail(user.email),
  sendPush(user.deviceToken),
  sendSms(user.phone),
]);

const failures = results.filter(
  (r): r is PromiseRejectedResult => r.status === 'rejected',
);
if (failures.length > 0) {
  logger.warn('Some notifications failed', { failures });
}

Type async function return values explicitly

Correct

// Explicit Promise<T> return type — callers know exactly what to expect
export async function getConfig(): Promise<AppConfig> {
  const raw = await fs.readFile('config.json', 'utf-8');
  return JSON.parse(raw) as AppConfig;
}

Avoid async functions that do not await anything

Anti-Pattern

// BAD: async keyword is unnecessary — adds overhead wrapping in Promise
async function add(a: number, b: number): Promise<number> {
  return a + b;
}

// GOOD: return plain value or wrap explicitly if needed
function add(a: number, b: number): number {
  return a + b;
}