prototype property is created when a function is declared.

For instance:

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

Person.prototype property is created internally once you declare above function. Many properties can be added to the Person.prototype which are shared by Person instances created using new Person().

// adds a new method age to the Person.prototype Object.
Person.prototype.age = function(){return date-dob}; 

It is worth noting that Person.prototype is an Object literal by default (it can be changed as required).

Every instance created using new Person() has a __proto__ property which points to the Person.prototype. This is the chain that is used to traverse to find a property of a particular object.

var person1 = new Person(somedate);
var person2 = new Person(somedate);

creates 2 instances of Person, these 2 objects can call age method of Person.prototype as person1.age, person2.age.

In the above picture from your question, you can see that Foo is a Function Object and therefore it has a __proto__ link to the Function.prototype which in turn is an instance of Object and has a __proto__ link to Object.prototype. The proto link ends here with __proto__ in the Object.prototype pointing to null.

Any object can have access to all the properties in its proto chain as linked by __proto__ , thus forming the basis for prototypal inheritance.

__proto__ is not a standard way of accessing the prototype chain, the standard but similar approach is to use Object.getPrototypeOf(obj).

Below code for instanceof operator gives a better understanding:

object instanceof Class operator returns true when an object is an instance of a Class, more specifically if Class.prototype is found in the proto chain of that object then the object is an instance of that Class.

function instanceOf(Func){
  var obj = this;
  while(obj !== null){
    if(Object.getPrototypeOf(obj) === Func.prototype)
      return true;
    obj = Object.getPrototypeOf(obj);
  }
  return false;
}      

The above method can be called as: instanceOf.call(object, Class) which return true if object is instance of Class.

__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;
( new Foo ).prototype === undefined;

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

__proto__ is an internal property of an object, pointing to its prototype. Current standards provide an equivalent Object.getPrototypeOf(obj) method, though the 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.

my understanding is: __proto__ and prototype are all served for the prototype chain technique . the difference is functions named with underscore(like __proto__) are not aim for developers invoked explicitly at all. in other words, they are just for some mechanisms like inherit etc. they are 'back-end'. but functions named without underscore are designed for invoked explicitly, they are 'front-end'.

A nice way to think of it is...

prototype is used by constructor functions. It should've really been called something like, "prototypeToInstall", since that's what it is.

and __proto__ is that "installed prototype" on an object (that was created/installed upon the object from said constructor() function)

Prototype VS. __proto__ VS. [[Prototype]]

When creating a function, a property object called prototype is being created automatically (you didn't create it yourself) and is being attached to the function object (the constructor).
Note: This new prototype object also points to, or has an internal-private link to, the native JavaScript Object.

Example:

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;
}

If you create a new object out of Foo using the new keyword, you are basically creating (among other things) a new object that has an internal or private link to the function Foo's prototype we discussed earlier:

var b = new Foo();

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

To be more specific, __proto__ is actually a getter function that belong to the native JavaScript Object. It returns the internal-private prototype linkage of whatever the this binding is (returns the [[Prototype]] of b):

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

It is worth noting that starting of ECMAScript5, you can also use the getPrototypeOf method to get the internal private linkage:

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

To make it a little bit clear in addition to above great answers:

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

var eve = new Person("Eve");

eve.__proto__ == Person.prototype //true

eve.prototype  //undefined

Instances have __proto__, classes have prototype.

To put it simply:

> var a = 1
undefined
> a.__proto__
[Number: 0]
> Number.prototype
[Number: 0]
> Number.prototype === a.__proto__
true

This allows you to attach properties to X.prototype AFTER objects of type X has been instantiated, and they will still get access to those new properties through the __proto__ reference which the Javascript-engine uses to walk up the prototype chain.

Prototype or Object.prototype is a property of an object literal. It represents the Object prototype object which you can override to add more properties or methods further along the prototype chain.

__proto__ is an accessor property (get and set function) that exposes the internal prototype of an object thru which it is accessed.

References:

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

2. http://www.w3schools.com/js/js_object_prototypes.asp

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

Another good way to understand it:

var foo = {}

/* 
foo.constructor is Object, so foo.constructor.prototype is actually 
Object.prototype; Object.prototype in return is what foo.__proto__ links to. 
*/
console.log(foo.constructor.prototype === foo.__proto__);
// this proves what the above comment proclaims: Both statements evaluate to true.
console.log(foo.__proto__ === Object.prototype);
console.log(foo.constructor.prototype === Object.prototype);

Only after IE11 __proto__ is supported. Before that version, such as IE9, you could use the constructor to get the __proto__.

!!!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 %)

DEFINITIONS

(number inside the parenthesis () is a 'link' to the code that is written below)

prototype - an object that consists of:
=> functions (3) of this particular ConstructorFunction.prototype(5) that are accessible by each object (4) created or to-be-created through this constructor function (1)
=> the constructor function itself (1)
=> __proto__ of this particular object (prototype object)

__proto__ (dandor proto?) - a link BETWEEN any object (2) created through a particular constructor function (1), AND the prototype object's properties (5) of that constructor THAT allows each created object (2) to have access to the prototype's functions and methods (4) (__proto__ is by default included in every single object in JS)

CODE CLARIFICATION

1.

    function Person (name, age) {
        this.name = name;
        this.age = age;  

    } 

2.

    var John = new Person(‘John’, 37);
    // John is an object

3.

    Person.prototype.getOlder = function() {
        this.age++;
    }
    // getOlder is a key that has a value of the function

4.

    John.getOlder();

5.

    Person.prototype;

Every function you create has a property called prototype, and it starts off its life as an empty object. This property is of no use until you use this function as constructor function i.e. with the 'new' keyword.

This is often confused with the __proto__ property of an object. Some might get confused and except that the prototype property of an object might get them the proto of an object. But this is not case. prototype is used to get the __proto__ of an object created from a function constructor.

In the above example:

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

var eve = new Person("Eve");

console.log(eve.__proto__ == Person.prototype) // true
// this is exactly what prototype does, made Person.prototype equal to eve.__proto__

I hope it makes sense.

To explain let us create a function

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

When JavaScript executes this code, it adds prototype property to a, prototype property is an object with two properties to it:

1. constructor
2. __proto__

So when we do

a.prototype it returns

     constructor: a  // function definition
    __proto__: Object

Now as you can see constructor is nothing but the function a itself and __proto__ points to the root level Object of JavaScript.

Let us see what happens when we use a function with new key word.

var b = new a ('JavaScript');

When JavaScript executes this code it does 4 things:

1. It creates a new object, an empty object // {}
2. It creates __proto__ on b and makes it point to a.prototype so b.__proto__ === a.prototype
3. It executes a.prototype.constructor (which is definition of function a ) with the newly created object (created in step#1) as its context (this), hence the name property passed as 'JavaScript' (which is added to this) gets added to newly created object.
4. It returns newly created object in (created in step#1) so var b gets assigned to newly created object.

Now if we add a.prototype.car = "BMW" and do b.car, the output "BMW" appears.

this is because when JavaScript executed this code it searched for car property on b, it did not find then JavaScript used b.__proto__ (which was made to point to 'a.prototype' in step#2) and finds car property so return "BMW".

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.

In JavaScript, a function can be used as a constructor. That means we can create objects out of them using the new keyword. Every constructor function comes with a built-in object chained with them. This built-in object is called a prototype. Instances of a constructor function use __proto__ to access the prototype property of its constructor function.

1. 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

2. 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).

3. 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.

4. 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.

5. 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.

6. 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.

I'll try a 4th grade explanation:

Things are very simple. A prototype is an example of how something should be built. So:

• I'm a function and I build new objects similar to my prototype

• I'm an object and I was built using my __proto__ as an example

proof:

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

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

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
    };
}

Explanatory example:

function Dog(){}
Dog.prototype.bark = "woof"

let myPuppie = new Dog()

now, myPupppie has __proto__ property which points to Dog.prototype.

> myPuppie.__proto__
>> {bark: "woof", constructor: ƒ}

but myPuppie does NOT have a prototype property.

> myPuppie.prototype
>> undefined

So, __proto__ of mypuppie is the reference to the .prototype property of constructor function that was used to instantiate this object (and the current myPuppie object has "delegates to" relationship to this __proto__ object), while .prototype property of myPuppie is simply absent (since we did not set it).

Good explanation by MPJ here: proto vs prototype - Object Creation in JavaScript

__proto__ is the base to construct prototype and a constructor function eg: function human(){} has prototype which is shared via __proto__ in the new instance of the constructor function. A more detailed read here

(function(){ 
      let a = function(){console.log(this.b)};
      a.prototype.b = 1;
      a.__proto__.b = 2;
      let q = new a();
      console.log(a.b);
      console.log(q.b) 
    })()

Try this code to understand

'use strict'
function A() {}
var a = new A();
class B extends A {}
var b = new B();
console.log('====='); // =====
console.log(B.__proto__ === A); // true
console.log(B.prototype.__proto__ === A.prototype); // true
console.log(b.__proto__ === B.prototype); // true
console.log(a.__proto__ === A.prototype); // true
console.log(A.__proto__ === Function.__proto__); // true
console.log(Object.__proto__ === Function.__proto__); // true
console.log(Object.prototype === Function.__proto__.__proto__); // true
console.log(Object.prototype.__proto__ === null); // true

In JavaScript, Every object(function is object too!) has a __proto__ property, the property is reference to its prototype.

When we use the new operator with a constructor to create a new object, the new object's __proto__ property will be set with constructor's prototype property, then the constructor will be call by the new object, in that process "this" will be a reference to the new object in the constructor scope, finally return the new object.

Constructor's prototype is __proto__ property, Constructor's prototype property is work with the new operator.

Constructor must be a function, but function not always is constructor even if it has prototype property.

Prototype chain actually is object's __proto__ property to reference its prototype, and the prototype's __proto__ property to reference the prototype's prototype, and so on, until to reference Object's prototype's __proto__ property which is reference to null.

For example:

console.log(a.constructor === A); // true
// "a" don't have constructor,
// so it reference to A.prototype by its ``__proto__`` property,
// and found constructor is reference to A

[[Prototype]] and __proto__ property actually is same thing.

We can use Object's getPrototypeOf method to get something's prototype.

console.log(Object.getPrototypeOf(a) === a.__proto__); // true

Any function we written can be use to create an object with the new operator, so anyone of those functions can be a constructor.

Summary:

The __proto__ property of an object is a property that maps to the prototype of the constructor function of the object. In other words:

instance.__proto__ === constructor.prototype // true

This is used to form the prototype chain of an object. The prototype chain is a lookup mechanism for properties on an object. If an object's property is accessed, JavaScript will first look on the object itself. If the property isn't found there, it will climb all the way up to protochain until it is found (or not)

Example:

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

Person.prototype.age = 25;

const willem = new Person('Willem');

console.log(willem.__proto__ === Person.prototype); // the __proto__ property on the instance refers to the prototype of the constructor

console.log(willem.age); // 25 doesn't find it at willem object but is present at prototype
console.log(willem.__proto__.age); // now we are directly accessing the prototype of the Person function 

Our first log results to true, this is because as mentioned the __proto__ property of the instance created by the constructor refers to the prototype property of the constructor. Remember, in JavaScript, functions are also Objects. Objects can have properties, and a default property of any function is one property named prototype.

Then, when this function is utilized as a constructor function, the object instantiated from it will receive a property called __proto__. And this __proto__ property refers to the prototype property of the constructor function (which by default every function has).

Why is this useful?

JavaScript has a mechanism when looking up properties on Objects which is called 'prototypal inheritance', here is what it basically does:

• First, it's checked if the property is located on the Object itself. If so, this property is returned.
• If the property is not located on the object itself, it will 'climb up the protochain'. It basically looks at the object referred to by the __proto__ property. There, it checks if the property is available on the object referred to by __proto__.
• If the property isn't located on the __proto__ object, it will climb up the __proto__ chain, all the way up to Object object.
• If it cannot find the property anywhere on the object and its prototype chain, it will return undefined.

For example:

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

let mySelf = new Person('Willem');

console.log(mySelf.__proto__ === Person.prototype);

console.log(mySelf.__proto__.__proto__ === Object.prototype);

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`

As this rightly stated

__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;
( new Foo ).prototype === undefined;

We can further note that __proto__ property of an object created using function constructor points towards the memory location pointed towards by prototype property of that respective constructor.

If we change the memory location of prototype of constructor function, __proto__ of derived object will still continue to point towards the original address space. Therefore to make common property available down the inheritance chain, always append property to constructor function prototype, instead of re-initializing it (which would change its memory address).

Consider the following example:

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

There is only one object that is used for protypal chaining. This object obviously has a name and a value: __proto__ is its name, and prototype is its value. That's all.

to make it even easier to grasp, look at the diagram on the top of this post (Diagram by dmitry soshnikov), you'll never find __proto__ points to something else other than prototype as its value.

The gist is this: __proto__ is the name that references the prototypal object, and prototype is the actual prototypal object.

It's like saying:

let x = {name: 'john'};

x is the object name (pointer), and {name: 'john'} is the actual object (data value).

NOTE: this just a massively simplified hint on how they are related on a high level.

Update: Here is a simple concrete javascript example for better illustration:

let x = new String("testing") // Or any other javascript object you want to create

Object.getPrototypeOf(x) === x.__proto__; // true

This means that when Object.getPrototypeOf(x) gets us the actual value of x (which is its prototype), is exactly what the __proto__ of x is pointing to. Therefore __proto__ is indeed pointing to the prototype of x. Thus __proto__ references x (pointer of x), and prototype is the value of x (its prototype).

I hope it's a bit clear now.

I've made for myself a small drawing that represents the following code snippet:

var Cat = function() {}
var tom = new Cat()

I have a classical OO background, so it was helpful to represent the hierarchy in this manner. To help you read this diagram, treat the rectangles in the image as JavaScript objects. And yes, functions are also objects. ;)

Objects in JavaScript have properties and __proto__ is just one of them.

The idea behind this property is to point to the ancestor object in the (inheritance) hierarchy.

The root object in JavaScript is Object.prototype and all other objects are descendants of this one. The __proto__ property of the root object is null, which represents the end of inheritance chain.

You'll notice that prototype is a property of functions. Cat is a function, but also Function and Object are (native) functions. tom is not a function, thus it does not have this property.

The idea behind this property is to point to an object which will be used in the construction, i.e. when you call the new operator on that function.

Note that prototype objects (yellow rectangles) have another property called constructor which points back to the respective function object. For brevity reasons this was not depicted.

Indeed, when we create the tom object with new Cat(), the created object will have the __proto__ property set to the prototype object of the constructor function.

In the end, let us play with this diagram a bit. The following statements are true:

• tom.__proto__ property points to the same object as Cat.prototype.

• Cat.__proto__ points to the Function.prototype object, just like Function.__proto__ and Object.__proto__ do.

• Cat.prototype.__proto__ and tom.__proto__.__proto__ point to the same object and that is Object.prototype.

Cheers!

so many good answers exist for this question, but for recap and compact form of answer that have good details I add the following:

the first thing that we must consider is when JS was invented, computers have very low memory, so if we need a process for creating new object types, we must consider memory performance.

so they located methods that object created from that specific object type need, on the separate part of memory instead of every time we create a new object, store methods besides the object. so if we reinvent the new operator and constructor function concept with JS's new features we have these steps:

1. and empty object. (that will be the final result of instantiation of object type)

let empty={}

2. we already know that for memory performance reasons all the methods that are needed for instances of an object type are located on the constructor function's prototype property. (functions are also objects so they can have properties) so we reference the empty object's __protp__ to the location where those methods exist. (we consider the function that we use conceptually as the constructor, named constructor.

empty.__proto__ = constructor.prototype

3. we must initialize object type values. in JS function are disconnected from objects. with dot notation or methods like bind call apply that function objects have we must tell "what is the this context of the function".

let newFunc = constructor.bind(empty)

4. now we have a new function that has an empty object as this context. after execution of this function. the empty object will be filled, and the result of instantiation of type object will be this empty object if defined constructor function doesn't return(as if that will be the result of the process)

so as you see __proto__ is a property of objects that refers to other objects(in JS functions are also object) prototype object property which consisted of properties that use across instances of a specific object type.

as you can guess from the phrase, functions are objects, functions also have __proto__ property so they can refer to other object's prototype properties. this is how prototype inheritance is implemented.