如何从原型链中获取包含非可枚举属性的对象键

Question

现在这是我对Javascript对象的理解......     var a = {};

Object.defineProperties(a, {
    one: {enumerable: true, value: 'one'},
    two: {enumerable: false, value: 'two'},
});
a.__proto__ = {three : 'three' };

Object.keys(a); // ["one"]
Object.getOwnPropertyNames(a); // ["one", "two"]
for (var i in a) console.log(i); // one  three

所以

Object.keys(a); 

返回对象自己的密钥，这些密钥也是可枚举的。

Object.getOwnPropertyNames(a); // ["one", "two"]

返回对象自己的密钥，包括不可枚举的密钥。

for (var i in a) 

返回对象及其原型链，只返回可枚举的键

那么如何从原型链中获取不可枚举的密钥？

代码::

var  o = {a:1};

Object.defineProperty(o,'b',{configurable:true, enumerable :false, value:2,writable: true });

o.__proto__ = {c:3};

Object.defineProperty(o.__proto__,'d',{configurable:true, enumerable :false, value:4,writable: true });

Object.keys(o); // ["a"] -only self nonenum props.
Object.getOwnPropertyNames(o); // ["a", "b"] -only self including enum props.
for (var i in o) console.log(i) // a c  -only enum props including from proto.