javascript - 如何使用 Object.defineProperties？

Question

大家好，最近看了Professional.JavaScript.for.Web.Developers.3rd.Edition。这是我从中学到的代码。但是，输出与我阅读的书不同。当我运行下面的代码时，book.edition 是 1，book._year 是 2004 年，book.year 是 2004 年。发生了什么？什么是我的代码有问题吗？

var book = {};

Object.defineProperties(book, {
  _year: {
    value: 2004
  },
  edition: {
    value: 1
  },

  year: {
    get: function() {
      return this._year;
    },

    set: function(newValue) {
      if (newValue > 2004) {
        this._year = newValue;
        this.edition += newValue - 2004;
      }
    }
  }
});

book.year = 2005;
console.log(book.edition);
console.log(book._year);
console.log(book.year);

Answer 1

对象的 _year 和 edition 属性应定义为可写。否则在 year's setter 中重新定义它们是没有用的。

var book = {};

Object.defineProperties(book, {
  _year: {
    value: 2004,
    writable:true
  },
  edition: {
    value: 1,
    writable:true
  },

  year: {
    
    get: function() {
      return this._year;
    },

    set: function(newValue) {
      
      if (newValue > 2004) {
        this._year = newValue;
        this.edition += newValue - 2004;
      }
    }
  }
});


console.log(book.edition);
console.log(book.year);
book.year=2005;
console.log(book.edition);
console.log(book.year);