为什么可以检索原型但在JavaScript中未定义__proto__? [英] Why the prototype can be retrieved but the __proto__ is undefined in JavaScript?

问题描述

现在我正在学习JavaScript prototype和__proto__，并找到几个有用的链接

__proto__VS. JavaScript原型

__proto__与Constructor.prototype有何不同?

我可以在Chrome下的以下代码中获取对象f的__proto__值.

var Foo = function() {}
var f = new Foo();
f.__proto__
> Foo {} 

但是，将Foo.prototype.__proto__设置为null后，__proto__的值是undefined.

var Foo = function() {}
Foo.prototype = {name: 'cat', age: 10};
Foo.prototype.__proto__ = null;
var f = new Foo();
f.__proto__
> undefined 

但是我可以得到f.name的值，即cat.这是我的理解，因为值f.name是可检索的，所以对象f的__proto__应该指向Foo.prototype.为什么f.__proto__的值是undefined?

解决方案

obj.__proto__未定义如果原型链不包含Object.prototype :

这按指定方式工作. ES6 __proto__是在Object.prototype上定义的吸气剂.对于在其原型链中没有该对象的对象，它是不可访问的(就像没有hasOwnProperty一样).您需要改用Object.getPrototypeOf.

Now I am learning JavaScript prototype and __proto__, and find several useful links

__proto__ VS. prototype in JavaScript

How does __proto__ differ from constructor.prototype?

I can get the value of __proto__ of object f in the following codes under Chrome.

var Foo = function() {}
var f = new Foo();
f.__proto__
> Foo {} 

However, after setting the Foo.prototype.__proto__ to null, the value of __proto__ is undefined.

var Foo = function() {}
Foo.prototype = {name: 'cat', age: 10};
Foo.prototype.__proto__ = null;
var f = new Foo();
f.__proto__
> undefined 

But I can get the value of f.name, which is cat. Here is my understanding, since the value f.name can be retrievable, the __proto__ of object f should point to Foo.prototype. Why the value of f.__proto__ is undefined?

解决方案

According to the ES2015 spec, __proto__ is an accessor property that is inherited from Object.prototype.

Since your prototype chain for the instance f is rooted in null, rather than Object.prototype, the f object does not inherit any properties from Object.prototype, including Object.prototype.__proto__.

The object still knows its prototype internally (through [[Prototype]] internal slot), but it does not inherit the __proto__ accessor property for getting this value. You can still access it through Object.getPrototypeOf(f), though.

See also the resolution on the Chromium issue "obj.__proto__ is undefined if prototype chain does not contain Object.prototype":

This is working as specified. ES6 __proto__ is a getter defined on Object.prototype. For an object that doesn't have that in its prototype chain it is not accessible (just like, say, hasOwnProperty isn't). You need to use Object.getPrototypeOf instead.

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