var title = "aaa";var myclass = function(){}myclass.title = "bbb";myclass.prototype.getTitle = function(){    function doGetTitle(){        return this.title;    }    return doGetTitle.call(this);}var obj = new myclass();console.log(obj.getTitle());console為什么輸出undefined呢？我以為是‘aaa’，我的理解是閉包內部函數里的this指向全局的widow