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