__proto__ VS. prototype in JavaScript

Question

This figure again shows that every object has a prototype. Constructor function Foo also has its own __proto__ which is Function.prototype, a

Answer 1

I happen to be learning prototype from You Don't Know JS: this & Object Prototypes, which is a wonderful book to understand the design underneath and clarify so many misconceptions (that's why I'm trying to avoid using inheritance and things like instanceof).

But I have the same question as people asked here. Several answers are really helpful and enlightening. I'd also love to share my understandings.

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What is a prototype?

Objects in JavaScript have an internal property, denoted in the specification as[[Prototype]], which is simply a reference to another object. Almost all objects are given a non-nullvalue for this property, at the time of their creation.

How to get an object's prototype?

via __proto__or Object.getPrototypeOf

var a = { name: "wendi" };
a.__proto__ === Object.prototype // true
Object.getPrototypeOf(a) === Object.prototype // true

function Foo() {};
var b = new Foo();
b.__proto__ === Foo.prototype
b.__proto__.__proto__ === Object.prototype

What is the prototype ?

prototype is an object automatically created as a special property of a function, which is used to establish the delegation (inheritance) chain, aka prototype chain.

When we create a function a, prototype is automatically created as a special property on a and saves the function code on as the constructor on prototype.

function Foo() {};
Foo.prototype // Object {constructor: function}
Foo.prototype.constructor === Foo // true

I'd love to consider this property as the place to store the properties (including methods) of a function object. That's also the reason why utility functions in JS are defined like Array.prototype.forEach() , Function.prototype.bind(), Object.prototype.toString().

Why to emphasize the property of a function?

{}.prototype // undefined;
(function(){}).prototype // Object {constructor: function}

// The example above shows object does not have the prototype property.
// But we have Object.prototype, which implies an interesting fact that
typeof Object === "function"
var obj = new Object();

So, Arary, Function, Objectare all functions. I should admit that this refreshes my impression on JS. I know functions are first-class citizen in JS but it seems that it is built on functions.

What's the difference between __proto__ and prototype?

__proto__a reference works on every object to refer to its [[Prototype]]property.

prototype is an object automatically created as a special property of a function, which is used to store the properties (including methods) of a function object.

With these two, we could mentally map out the prototype chain. Like this picture illustrates:

function Foo() {}
var b = new Foo();

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

Answer 2

This is a very important question relevant to anyone who wants to understand prototypical inheritance. From what I understand, prototype is assigned by default when an object is created with new from a function because Function has prototype object by definition:

function protofoo(){
}
var protofoo1 = new protofoo();
console.log(protofoo.prototype.toString()); //[object Object]

When we create an ordinary object without new, ie explicitly from a function, it doesn't have prototype but it has an empty proto which can be assigned a prototype.

var foo={
  check: 10
};
console.log(foo.__proto__); // empty
console.log(bar.prototype); //  TypeError
foo.__proto__ = protofoo1; // assigned
console.log(foo.__proto__); //protofoo

We can use Object.create to link an object explicitly.

// we can create `bar` and link it to `foo`
var bar = Object.create( foo );
bar.fooprops= "We checking prototypes";
console.log(bar.__proto__); // "foo"
console.log(bar.fooprops); // "We checking prototypes"
console.log(bar.check); // 10 is delegated to `foo`

Answer 3

I know, I am late but let me try to simplify it.

Let us say there is a function

    function Foo(message){

         this.message = message ; 
     };

     console.log(Foo.prototype);

Foo function will have a prototype object linked. So,Whenever we create a function in JavaScript, it always has a prototype object linked to it.

Now let us go ahead and create two objects using the function Foo.

    var a = new Foo("a");
    var b = new Foo("b");
    console.log(a.message);
    console.log(b.message);

1. Now we have two objects, object a and object b. Both are created using constructor Foo. Keep in mind constructor is just a word here.
2. Object a and b both have a copy of message property.
3. These two objects a and b are linked to prototype object of constructor Foo.
4. On objects a and b, we can access Foo prototype using __proto__ property in all browsers and in IE we can use Object.getPrototypeOf(a) or Object.getPrototypeOf(b)

Now, Foo.prototype, a.__proto__, and b.__proto__ all denotes same object.

    b.__proto__ === Object.getPrototypeOf(a);
    a.__proto__ ===  Foo.prototype;
    a.constructor.prototype  === a.__proto__;

all of above would return true.

As we know, in JavaScript properties can be added dynamically. We can add property to object

    Foo.prototype.Greet = function(){

         console.log(this.message);
    }
    a.Greet();//a
    b.Greet();//b
    a.constructor.prototype.Greet();//undefined 

As you see we added Greet() method in Foo.prototype but it is accessible in a and b or any other object which is constructed using Foo.

While executing a.Greet(), JavaScript will first search Greet in object a on property list. On not finding , it will go up in __proto__ chain of a. Since a.__proto__ and Foo.prototype is same object, JavaScript will find Greet() method and execute it.

I hope, now prototype and __proto__ is simplified a bit.

Answer 4

!!!THIS IS THE BEST EXPLANATION IN THE WORLD!!!!!

var q = {}
var prototype = {prop: 11}

q.prop // undefined
q.__proto__ = prototype
q.prop // 11

in function constructors javascript engine call this q.__proto__ = prototype automatically when we write new Class, and in to the __proto__ prop set Class.prototype

function Class(){}
Class.prototype = {prop: 999} // set prototype as we need, before call new

var q = new Class() // q.__proto__ = Class.prototype
q.prop // 999

Enjoy %)

Answer 5

prototype

prototype is a property of a Function. It is the blueprint for creating objects by using that (constructor) function with new keyword.

__proto__

__proto__ is used in the lookup chain to resolve methods, properties. when an object is created (using constructor function with new keyword), __proto__ is set to (Constructor) Function.prototype

function Robot(name) {
    this.name = name;
}
var robot = new Robot();

// the following are true   
robot.__proto__ == Robot.prototype
robot.__proto__.__proto__ == Object.prototype

Here is my (imaginary) explanation to clear the confusion:

Imagine there is an imaginary class (blueprint/coockie cutter) associated with function. That imaginary class is used to instantiate objects. prototype is the extention mechanism (extention method in C#, or Swift Extension) to add things to that imaginary class.

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

The above can be imagined as:

// imaginary class
class Robot extends Object{

    static prototype = Robot.class  
    // Robot.prototype is the way to add things to Robot class
    // since Robot extends Object, therefore Robot.prototype.__proto__ == Object.prototype

    var __proto__;

    var name = "";

    // constructor
    function Robot(name) {

        this.__proto__ = prototype;
        prototype = undefined;

        this.name = name;
    }

} 

So,

var robot = new Robot();

robot.__proto__ == Robot.prototype
robot.prototype == undefined
robot.__proto__.__proto__ == Object.prototype

Now adding method to the prototype of Robot:

Robot.prototype.move(x, y) = function(x, y){ Robot.position.x = x; Robot.position.y = y};
// Robot.prototype.move(x, y) ===(imagining)===> Robot.class.move(x, y)

The above can be imagined as extension of Robot class:

// Swift way of extention
extension Robot{
    function move(x, y){    
        Robot.position.x = x; Robot.position.y = y
    }
}

Which in turn,

// imaginary class
class Robot{

    static prototype = Robot.class // Robot.prototype way to extend Robot class
    var __proto__;

    var name = "";

    // constructor
    function Robot(name) {

        this.__proto__ = prototype;
        prototype = undefined;

        this.name = name;
    }

    // added by prototype (as like C# extension method)
    function move(x, y){ 
        Robot.position.x = x; Robot.position.y = y
    };
}

Answer 6

What about using __proto__ for static methods?

function Foo(name){
  this.name = name
  Foo.__proto__.collection.push(this)
  Foo.__proto__.count++

}

Foo.__proto__.count=0
Foo.__proto__.collection=[]

var bar = new Foo('bar')
var baz = new Foo('baz')

Foo.count;//2
Foo.collection // [{...}, {...}]
bar.count // undefined