javascript - 为什么这个对象变量表达式不起作用？

Question

有人可以解释一下为什么 hotel.availableRooms 是 NaN 吗？我正在尝试学习 JavaScript 中的对象。谢谢!

var hotel = {
  name: 'The Shining',
  totalRooms: 238,
  bookedRooms: 0,
  availableRooms: this.totalRooms - this.bookedRooms,
  roomTypes: ['single', 'double', 'haunted'],
  bookRoom: function() {
    this.availableRooms -= 1;
    this.bookedRooms += 1;
  }
};


hotel.bookRoom();

console.log(hotel.totalRooms);
console.log(hotel.bookedRooms);
console.log(hotel.availableRooms);

Answer 1

当表达式 availableRooms: this.totalRooms - this.bookedRooms, 被计算时，this 不是 hotel 而是您正在运行的任何上下文代码。

var hotel = {
  name: 'The Shining',
  totalRooms: 238,
  bookedRooms: 0,
  roomTypes: ['single', 'double', 'haunted'],
  bookRoom: function() {
    this.availableRooms -= 1;
    this.bookedRooms += 1;
  }
}

hotel.availableRooms = hotel.totalRooms - hotel.bookedRooms

或者

  Object.defineProperty(hotel, 'availableRooms', {
      get: function() {
         return this.totalRooms - this.bookedRooms
      }
  })