正如这句话所说

__proto__是用于查找链的实际对象 原型是用来构建的对象 当你用new创建一个对象时: (新的Foo)。__proto__ === Foo.prototype; (新的Foo)。原型=== undefined;

我们可以进一步注意到，使用函数构造函数创建的对象的__proto__属性指向相应构造函数的prototype属性所指向的内存位置。

如果我们改变构造函数的prototype的内存位置，派生对象的__proto__仍将继续指向原始地址空间。因此，要使公共属性沿着继承链向下可用，总是将属性附加到构造函数函数原型，而不是重新初始化它(这会改变它的内存地址)。

考虑下面的例子:

function Human(){
    this.speed = 25;
}

var himansh = new Human();

Human.prototype.showSpeed = function(){
    return this.speed;
}

himansh.__proto__ === Human.prototype;  //true
himansh.showSpeed();    //25

//now re-initialzing the Human.prototype aka changing its memory location
Human.prototype = {lhs: 2, rhs:3}

//himansh.__proto__ will still continue to point towards the same original memory location. 

himansh.__proto__ === Human.prototype;  //false
himansh.showSpeed();    //25

在JavaScript中，函数可以用作构造函数。这意味着我们可以使用new关键字从它们中创建对象。每个构造函数都带有一个内置对象。这个内置对象称为原型。构造函数的实例使用__proto__来访问其构造函数的prototype属性。

First we created a constructor: function Foo(){}. To be clear, Foo is just another function. But we can create an object from it with the new keyword. That's why we call it the constructor function Every function has a unique property which is called the prototype property. So, Constructor function Foo has a prototype property which points to its prototype, which is Foo.prototype (see image). Constructor functions are themselves a function which is an instance of a system constructor called the [[Function]] constructor. So we can say that function Foo is constructed by a [[Function]] constructor. So, __proto__ of our Foo function will point to the prototype of its constructor, which is Function.prototype. Function.prototype is itself is nothing but an object which is constructed from another system constructor called [[Object]]. So, [[Object]] is the constructor of Function.prototype. So, we can say Function.prototype is an instance of [[Object]]. So __proto__ of Function.prototype points to Object.prototype. Object.prototype is the last man standing in the prototype chain. I mean it has not been constructed. It's already there in the system. So its __proto__ points to null. Now we come to instances of Foo. When we create an instance using new Foo(), it creates a new object which is an instance of Foo. That means Foo is the constructor of these instances. Here we created two instances (x and y). __proto__ of x and y thus points to Foo.prototype.

除了以上这些精彩的答案外，我还想说得更清楚一点:

function Person(name){
    this.name = name
 }; 

var eve = new Person("Eve");

eve.__proto__ == Person.prototype //true

eve.prototype  //undefined

实例有__proto__，类有prototype。

Prototype VS. __proto__ VS. [[Prototype]]

在创建函数时，会自动创建一个名为prototype的属性对象(不是您自己创建的)，并将其附加到函数对象(构造函数)。注意:这个新的原型对象也指向本机JavaScript对象，或者有一个内部私有链接。

例子:

function Foo () {
    this.name = 'John Doe';
}

// Foo has an object property called prototype.
// prototype was created automatically when we declared the function Foo.
Foo.hasOwnProperty('prototype'); // true

// Now, we can assign properties and methods to it:
Foo.prototype.myName = function () {
    return 'My name is ' + this.name;
}

如果你使用new关键字从Foo中创建一个新对象，你基本上是在创建一个新对象，它有一个内部或私有的链接到我们之前讨论过的函数Foo的原型:

var b = new Foo();

b.[[Prototype]] === Foo.prototype  // true

到该函数对象的私有链接称为双括号prototype或[[prototype]]。许多浏览器为我们提供了一个公共链接，称为__proto__!

更具体地说，__proto__实际上是一个属于原生JavaScript对象的getter函数。它返回this绑定的内部私有原型链接(返回b的[[prototype]]):

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

值得注意的是，ECMAScript5的启动，你也可以使用getPrototypeOf方法来获得内部的私有链接:

Object.getPrototypeOf(b) === b.__proto__ // true

注意:这个答案并不打算涵盖创建新对象或新构造函数的整个过程，而是帮助更好地理解__proto__、prototype和[[prototype]]以及它是如何工作的。

一种很好的思考方式是……

Prototype用于构造函数。它真的应该被称为“prototypeToInstall”，因为它就是这样的。

__proto__是对象上的“已安装原型”(由上述构造函数()函数创建/安装在对象上)

我试试四年级的解释:

事情很简单。原型是如何构建某样东西的例子。所以:

我是一个函数，我构建与原型相似的新对象 我是一个对象，我是用我的__proto__作为例子来构建的

证明:

function Foo() { }

var bar = new Foo()

// `bar` is constructed from how Foo knows to construct objects
bar.__proto__ === Foo.prototype // => true

// bar is an instance - it does not know how to create objects
bar.prototype // => undefined