求教一个js面试题

Question

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为什么输出undefined呢？我以为是‘aaa’，我的理解是闭包内部函数里的this指向全局的widow