Question

在下面给出的对象为book的第一个代码中，开头book.year设置为2013。当我分配book.year = 2015并再次尝试通过执行book.year检索其值时，我仍然得到2013而不是2015。我在哪里做错了？

代码如下：

var book = {};
Object.defineProperties(book, {
    _yearOrigin: {
        value: 2013
    },
    edition: {
        value: "1st"
    },
    year: {
        get: function(){return this._yearOrigin},

        set: function(newValue){
            //assigning this._yearOrigin
            this._yearOrigin = newValue; 

            //carrying the operation for evaluating the `subscript` to add in this.edition
            //diff = difference in year
            var diff = String(newValue - 2013); 
            var diffLast2ndChar = diff.charAt(diff.length - 2);
            var diffLastChar = diff.charAt(diff.length - 1);
            var subscript = "";

            if (diff.length > 1 && diffLast2ndChar == "1") {
                subscript = "th"; 
            } else {
                subscript = diffLastChar == "1"
                                ? "st"
                                : diffLastChar == "2"
                                    ? "nd"
                                    : diffLastChar == "3"
                                        ? "rd"
                                        : "th" ;
            }
            //--end of subscript evaluation

            //assigment operation of this.edition
            var rawEdition = Number(this.edition.charAt(0)) + Number(diff);
            this.edition = String(rawEdition) + subscript;
        }
    }
});

>>> book.year = 2015
>>>book.year //output is 2013 , but expected output is 2015

然而，相反，在尝试检索book2.year的值时，在下面给出的另一个等效代码段中，在分配book2.year = 2013后，按预期提供输出2013

var book2 = {
    _year: 2004,
    edition: 1
};
Object.defineProperty(book2, "year", {
    get: function(){
        return this._year;
    },
    set: function(newValue){
        if (newValue > 2004) {
            this._year = newValue;
            this.edition += newValue - 2004;
        }
    }
});

book2.year = 2005;
console.log(book2.year); //2005 (Now this time the output is expected unlike in the previous code shown above)

Answer 1

我忽略了“当使用Object.Property/Object.properties定义对象属性时，默认writable到false，即定义的值无法更改（非可写）“评论部分的人提到了这一点。

当我添加writable = true，如下所示，然后问题得到解决，预期结果弹出

_yearOrigin: {
    value: 2013,
    writable: true
},

Object.defineProperties（） - 在定义属性后为其赋值

1 个答案: