将函数的继承封装成如下函数：
function extend(child, parent){
var f = function(){}
f.prototype = parent.prototype
child.prototype = new f()
child.prototype.constructor = child
child.uber = parent.prototype
}
function Creature(){}
function Person(){}
extend(Person, Creature)
请教一下，这是访问Person.uber属性，为什么是undefined？
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child.uber = parent.prototype = {}