How to fix The return type of an async function or method must be the global Promise<T> type in TypeScript

async function getUsername(): string {  // ❌ Error: TS1058
  const user = await fetchUser();
  return user.name;
}

// ✅ Correct - Named function
async function getUsername(): Promise<string> {
  const user = await fetchUser();
  return user.name;
}

// ✅ Correct - Arrow function
const getUsername = async (): Promise<string> => {
  const user = await fetchUser();
  return user.name;
};

// ✅ Correct - Class method
class UserService {
  async getUsername(): Promise<string> {
    const user = await fetchUser();
    return user.name;
  }
}

// ❌ Error
async function logUser(): undefined {
  const user = await fetchUser();
  console.log(user.name);
}

// ✅ Correct
async function logUser(): Promise<void> {
  const user = await fetchUser();
  console.log(user.name);
}

// ✅ Return a union
async function fetchData(): Promise<string | null> {
  try {
    return await api.getData();
  } catch {
    return null;
  }
}

// ✅ Return an object
async function getUser(): Promise<{ id: string; name: string }> {
  const response = await fetch('/api/user');
  return response.json();
}

// ✅ Return an array
async function getUsers(): Promise<User[]> {
  const response = await fetch('/api/users');
  return response.json();
}

npx tsc --noEmit

npm run dev