// Destructuring
const { name, email } = user;
const [first, ...rest] = items;

// Spread operator
const merged = { ...defaults, ...options };
const combined = [...array1, ...array2];

// Template literals
const message = `Hello, ${name}!`;

// Optional chaining and nullish coalescing
const city = user?.address?.city ?? 'Unknown';

// Async/await
async function fetchData(url) {
  const response = await fetch(url);
  if (!response.ok) throw new Error('Fetch failed');
  return response.json();
}

// Promise.all for parallel
const results = await Promise.all([
  fetchData(url1),
  fetchData(url2),
]);

// Error handling
try {
  const data = await fetchData(url);
} catch (error) {
  console.error('Failed:', error.message);
}

// Map, filter, reduce
const names = users.map(u => u.name);
const active = users.filter(u => u.active);
const total = items.reduce((sum, i) => sum + i.price, 0);

// Find and includes
const user = users.find(u => u.id === id);
const hasAdmin = users.some(u => u.role === 'admin');

class UserService {
  #db; // Private field

  constructor(database) {
    this.#db = database;
  }

  async findById(id) {
    return this.#db.users.find(u => u.id === id);
  }
}