获取具有多个子对象的JavaScript对象的特定值

const obj = {
  user: {
    name: 'Alice',
    age: 30,
    address: {
      city: 'Wonderland',
      zip: '12345'
    }
  },
  preferences: {
    theme: 'dark',
    notifications: true
  }
};

const userName = obj.user.name; // 使用点表示法
console.log(userName); // 输出: Alice

const userName = obj['user']['name']; // 使用方括号表示法
console.log(userName); // 输出: Alice

const userCity = obj.user.address.city;
console.log(userCity); // 输出: Wonderland

const userCity = obj?.user?.address?.city;
console.log(userCity); // 如果任何属性不存在，输出将是: undefined

for (const key in obj) {
  if (obj.hasOwnProperty(key)) {
    console.log(`${key}:`, obj[key]);
  }
}

// 或者使用 Object.keys() 和 forEach
Object.keys(obj).forEach(key => {
  console.log(`${key}:`, obj[key]);
});