javascript原型：为什么这不起作用？(javascript prototyping: why doesn't this work?)

 我正在尝试下面的代码： 
 
var classA = function() {};
classA.prototype = { x: 4, y: 6 };
classA.prototype.prototype = { z: 10 };
var foo = new classA();
alert(foo.z);
 
 为什么警报恢复为未定义？ JavaScript不应该遵循原型链来找到z？ 

I'm looking at trying the following code:
 
var classA = function() {};
classA.prototype = { x: 4, y: 6 };
classA.prototype.prototype = { z: 10 };
var foo = new classA();
alert(foo.z);
 
Why does the alert come back as undefined? Shouldn't javascript follow the prototype chain to find z?

原文：https://stackoverflow.com/questions/5794240