在下面給出的第一個代碼中,對象爲book
,開頭的book.year
設置爲2013
。當我分配book.year = 2015
並再次嘗試通過執行book.year
來檢索其值時,我仍然得到2013
而不是2015
。我在哪裏做錯了?Object.defineProperties() - 在定義屬性後爲屬性賦予新值
的代碼如下給出:
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,在另一方面,在當嘗試檢索的book2.year
的值,低於給定的分配book2.year = 2013
後另一當量一段代碼,給出預期
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)
在第一部分要設置你的'_yearOrigin'到'2013'的值作爲常數值。由於您只給出[value](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperties)參數,因此將其視爲不可變的值。在第二個版本中,您定義了一個對象的屬性,默認值爲「2004」。有一個很大的差異 – Icepickle
由'defineProperties()'與'value'創建的屬性,比如'_yearOrigin',默認爲非['可寫入]](https://developer.mozilla.org/en- US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperties)(不可變)。 –
@Ipickle @Jonathan我該怎麼辦?我是否需要指定'writable:true'或其他? – Neel