An Object's __proto__ property references the same object as its internal [[Prototype]] (often referred to as "the prototype"), which may be an object or, as in the default case of Object.prototype.__proto__, null . This property is an abstraction error, because a property with the same name, but some other value, could be defined on the object too. If there is a need to reference an object's prototype, the preferred method is to use Object.getPrototypeOf.

A __proto__ pseudo property has been included in §B.3.1 of the draft ECMAScript ed. 6 specification (note that the specification codifies what is already in implementations and what websites may currently rely on).

var proto = obj.__proto__;

一个对象的__proto__ 属性和自己的内部属性[[Prototype]]指向一个相同的值 (通常称这个值为原型),原型的值可以是一个对象值也可以是null(比如说Object.prototype.__proto__的值就是null).该属性可能会引发一些错误,因为用户可能会不知道该属性的特殊性,而给它赋值,从而改变了这个对象的原型. 如果需要访问一个对象的原型,应该使用方法Object.getPrototypeOf.

__proto__ 属性已经被添加在了ES6草案 §B.3.1中.

不要认为__proto__和prototype相等。

Description

When an object is created, its __proto__ property is set to reference the same object as its internal [[Prototype]] (i.e. its constructor's prototype object).  Assigning a new value to __proto__ also changes the value of the internal [[Prototype]] property, except where the object is non–extensible.

To understand how prototypes are used for inheritance, see the MDN article Inheritance and the prototype chain.

当一个对象被创建时,它的 __proto__ 属性和内部属性[[Prototype]]指向了相同的对象 (也就是它的构造函数的prototype属性).改变__proto__ 属性的值同时也会改变内部属性[[Prototype]]的值,除非该对象是不可扩展的.

想要知道如何使用原型来实现继承,查看MDN文章继承和原型链.

Example

In the following, a new instance of Employee is created, then tested to show that its __proto__ is the same object as its constructor's prototype.

// 声明一个函数作为构造函数function Employee() {

  /* 初始化实例 */

}

// 创建一个Employee实例

var fred = new Employee();

// 测试相等性

fred.__proto__ === Employee.prototype; // true

这是, fred 继承了 Employee, 但是如果给fred.__proto__ 赋另外一个对象值,则会改变它的继承对象:

// Assign a new object to __proto__

fred.__proto__ = Object.prototype;

现在,fred不在继承于Employee.prototype, 而是直接继承了Object.prototype, 也就丢失了所有从Employee.prototype继承来的属性.

可是,这只适用于可扩展的 对象,一个不可扩展的对象的 __proto__ 属性是不可变的:

var obj = {};

Object.preventExtensions(obj);

obj.__proto__ = {}; // throws a TypeError

Note that even Object.prototype's __proto__ property can be redefined as long as the chain leads to null:

var b = {};

Object.prototype.__proto__ = {

    hi: function () {alert('hi');},

    __proto__: null

};

b.hi();

If Object.prototype's __proto__ had not been set to null, or had not been set to another object whose prototype chain did not eventually lead explicitly to null, a "cyclic __proto__ value" TypeError would result since the chain must eventually lead to null (as it normally does on Object.prototype).

参考：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/proto

上面的

bject.prototype.__proto__不能改成

bject.prototype.prototype.
为什么？
我们可以：

function func(){};
alert(typeof Object.prototype);//Object,不是Function
alert(typeof func); //Function。

可以看出Object.prototype是一个object，没有prototype属性.alert( Object.prototype.prototype);显示undefined。

(从这里我们可以看出Object 是 一个function，typeof Object 为function。)

prototype is a property of a Function object. It is the prototype of objects constructed by that function.

只有函数才有prototype属性，对象没有。

我们可以看*上的一个问题：

This figure again shows that every object has a prototype. Constructor function Foo also has its own__proto__ which is Function.prototype, and which in turn also references via its __proto__property again to the Object.prototype. Thus, repeat, Foo.prototype is just an explicit property of Foo which refers to the prototype of b and c objects.

var b =new Foo(20);var c =new Foo(30);

What are the __proto__ and the prototype properties?

（要仔细理解这幅图 typeof Object=='function' 为true，说明Object类型为function。

Foo.prototype.__proto__ === Object.prototype 为true。

（个人理解：上面的b->Foo.prototype-->Object.prototype组成了一条链，但b没有在自己中找到相应的属性和方法时，就会向上去寻找 。我们可以这么理解，继承与prototype无关，而与__proto__有关。？我们在这里简单地说下。每个对象都会在其内部初始化一个属性，就是__proto__，当我们访问一个对象的属性时，如果这个对象内部不存在这个属性，那么他就会去__proto__里找这个属性，这个__proto__又会有自己的__proto__，于是就这样一直找下去，也就是我们平时所说的原型链的概念。

参考：http://www.cnblogs.com/youxin/archive/2013/03/08/2950751.html

可以看到，Function.prototype是函数Foo的__proto__。我们只要在Function.prototype增加了一个方法，所有的函数都可以调用这个方法，如《javascript精粹》中的一个例子：

Function.prototype.method=function(name,func){

    if(!this.prototype[name]){

        this.prototype[name]=func;

    }

    return this;

};

 Foo.method("say2",function(){alert("say2");});

//和上面的话作用一样:Foo.prototype.say2=function(){alert("say2");};

var c=new Foo();

c.say2();

）

下面的这段代码是我编的。

function Foo(){}
var b=new Foo();
alert(Foo.prototype==Foo.__proto__); //false
alert( Foo.__proto__); //function Empty(){}
alert(Foo.prototype); //[object object]

alert(Foo.prototype.constructor); //function Foo(){}

与上图对应的代码：

// a constructor function

function Foo(y) {

  // which may create objects

  // by specified pattern: they have after

  // creation own "y" property

  this.y = y;

}

// also "Foo.prototype" stores reference

// to the prototype of newly created objects,

// so we may use it to define shared/inherited

// properties or methods, so the same as in

// previous example we have:

// inherited property "x"

Foo.prototype.x = 10;

// and inherited method "calculate"

Foo.prototype.calculate = function (z) {

  return this.x + this.y + z;

};

// now create our "b" and "c"

// objects using "pattern" Foo

var b = new Foo(20);

var c = new Foo(30);

// call the inherited method

b.calculate(30); //

c.calculate(40); //

// let's show that we reference

// properties we expect

console.log(

  b.__proto__ === Foo.prototype, // true

  c.__proto__ === Foo.prototype, // true

  // also "Foo.prototype" automatically creates

  // a special property "constructor", which is a

  // reference to the constructor function itself;

  // instances "b" and "c" may found it via

  // delegation and use to check their constructor

  b.constructor === Foo, // true

  c.constructor === Foo, // true

  Foo.prototype.constructor === Foo // true

  b.calculate === b.__proto__.calculate, // true

  b.__proto__.calculate === Foo.prototype.calculate // true

);

具体参考：http://dmitrysoshnikov.com/ecmascript/javascript-the-core/

答案1：

__proto__ is internal property of an object, pointing to its prototype. Current standards provide an equivalent Object.getPrototypeOf(O) method, though de facto standard __proto__ is quicker.

You can find instanceof relationships by comparing a function's prototype to an object's__proto__ chain, and you can break these relationships by changing prototype.

function Point(x, y) {

    this.x = x;

    this.y = y;

}

var myPoint = new Point();

// the following are all true

myPoint.__proto__ == Point.prototype

myPoint.__proto__.__proto__ == Object.prototype

myPoint instanceof Point;

myPoint instanceof Object;

Here Point is a constructor function, it builds an object (data structure) procedurally. myPoint is an object constructed by Point() so Point.prototype gets saved to myPoint.__proto__ at that time.

答案2：

__proto__ is the actual object that is used in the lookup chain to resolve methods, etc. prototype is the object that is used to build __proto__ when you create an object with new:

(new Foo).__proto__ ===Foo.prototype

(newFoo).prototype ===undefined

转自：http://*.com/questions/9959727/what-is-the-difference-between-proto-and-prototype-in-javascript

 看以前写的：

Javascript原型链和原型的一个误区