這段代碼中，為什么year聲明的時候前面加了下劃線，用的時候卻沒加，我嘗試將聲明處的下劃線去掉，結果程序執行不了，求解釋。var book = {_year:2004,edition:1};Object.defineProperty(book,"year",{get:function(){return this._year;},set:function(newValue){if(newValue>2004){this._year = newValue;this.edition += newValue - 2004;}}});book.year = 2005;alert(book.edition);