const config = Object.freeze({
  apiUrl: 'https://api.example.com',
  timeout: 5000
});

config.apiUrl = 'https://evil.com'; // ❌ Fails silently (strict mode: throws)
config.newProp = 'test';            // ❌ Cannot add
delete config.timeout;              // ❌ Cannot delete
Object.isFrozen(config);            // true

const config = Object.freeze({
  settings: { theme: 'dark' }
});

config.settings.theme = 'light'; // ✅ Allowed — nested object is not frozen

function deepFreeze(obj) {
  Object.keys(obj).forEach(key => {
    if (typeof obj[key] === 'object' && obj[key] !== null) {
      deepFreeze(obj[key]);
    }
  });
  return Object.freeze(obj);
}