__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

[[Prototype]] :

[[Prototype]] is an internal hidden property of objects in JS and it is a reference to another object. Every object at the time of creation receives a non-null value for [[Prototype]]. Remember [[Get]] operation is invoked when we reference a property on an object like, myObject.a. If the object itself has a property, a on it then that property will be used.

let myObject= {
    a: 2
};

console.log(myObject.a);            // 2

But if the object itself directly does not have the requested property then [[Get]] operation will proceed to follow the [[Prototype]] link of the object. This process will continue until either a matching property name is found or the [[Prototype]] chain ends(at the built-in Object.prototype). If no matching property is found then undefined will be returned. Object.create(specifiedObject) creates an object with the [[Prototype]] linkage to the specified object.

let anotherObject= {
    a: 2
};

// create an object linked to anotherObject
let myObject= Object.create(anotherObject);
console.log(myObject.a);                // 2

Both for..in loop and in operator use [[Prototype]] chain lookup process. So if we use for..in loop to iterate over the properties of an object then all the enumerable properties which can be reached via that object's [[Prototype]] chain also will be enumerated along with the enumerable properties of the object itself. And when using in operator to test for the existence of a property on an object then in operator will check all the properties via [[Prototype]] linkage of the object regardless of their enumerability.

// for..in loop uses [[Prototype]] chain lookup process
let anotherObject= {
    a: 2
};

let myObject= Object.create(anotherObject);

for(let k in myObject) {
    console.log("found: " + k);            // found: a
}

// in operator uses [[Prototype]] chain lookup process
console.log("a" in myObject);              // true

.prototype :

.prototype is a property of functions in JS and it refers to an object having constructor property which stores all the properties(and methods) of the function object.

let foo= function(){}

console.log(foo.prototype);        
// returns {constructor: f} object which now contains all the default properties

foo.id= "Walter White";

foo.job= "teacher";

console.log(foo.prototype);       
// returns {constructor: f} object which now contains all the default properties and 2 more properties that we added to the fn object
/*
{constructor: f}
    constructor: f()
        id: "Walter White"
        job: "teacher"
        arguments: null
        caller: null
        length: 0
        name: "foo"
        prototype: {constructor: f}
        __proto__: f()
        [[FunctionLocation]]: VM789:1
        [[Scopes]]: Scopes[2]
    __proto__: Object

*/

But normal objects in JS does not have .prototype property. We know Object.prototype is the root object of all the objects in JS. So clearly Object is a function i.e. typeof Object === "function" . That means we also can create object from the Object function like, let myObj= new Object( ). Similarly Array, Function are also functions so we can use Array.prototype, Function.prototype to store all the generic properties of arrays and functions. So we can say JS is built on functions.

{}.prototype;                            // SyntaxError: Unexpected token '.'

(function(){}).prototype;                // {constructor: f}

Also using new operator if we create objects from a function then internal hidden [[Prototype]] property of those newly created objects will point to the object referenced by the .prototype property of the original function. In the below code, we have created an object, a from a fn, Letter and added 2 properties one to the fn object and another to the prototype object of the fn. Now if we try to access both of the properties on the newly created object, a then we only will be able to access the property added to the prototype object of the function. This is because the prototype object of the function is now on the [[Prototype]] chain of the newly created object, a.

let Letter= function(){}

let a= new Letter();

Letter.from= "Albuquerque";

Letter.prototype.to= "New Hampshire";

console.log(a.from);                // undefined

console.log(a.to);                  // New Hampshire

.__proto__:

.__proto__ is a property of objects in JS and it references the another object in the [[Prototype]] chain. We know [[Prototype]] is an internal hidden property of objects in JS and it references another object in the [[Prototype]] chain. We can get or set the object referred by the internal [[Prototype]] property in 2 ways

1. Object.getPrototypeOf(obj) / Object.setPrototypeOf(obj)

2. obj.__proto__

We can traverse the [[Prototype]] chain using: .__proto__.__proto__. . . Along with .constructor, .toString( ), .isPrototypeOf( ) our dunder proto property (__proto__) actually exists on the built-in Object.prototype root object, but available on any particular object. Our .__proto__ is actually a getter/setter. Implementation of .__proto__ in Object.prototype is as below :

Object.defineProperty(Object.prototype, "__proto__", {
    get: function() {
        return Object.getPrototypeOf(this);
    },
    set: function(o) {
        Object.setPrototypeOf(this, o);
        return o;
    }
});

To retrieve the value of obj.__proto__ is like calling, obj.__proto__() which actually returns the calling of the getter fn, Object.getPrototypeOf(obj) which exists on Object.prototype object. Although .__proto__ is a settable property but we should not change [[Prototype]] of an already existing object because of performance issues.

Using new operator if we create objects from a function then internal hidden [[Prototype]] property of those newly created objects will point to the object referenced by the .prototype property of the original function. Using .__proto__ property we can access the other object referenced by internal hidden [[Prototype]] property of the object. But __proto__ is not the same as [[Prototype]] rather a getter/setter for it. Consider below code :

let Letter= function() {}

let a= new Letter();

let b= new Letter();

let z= new Letter();

// output in console
a.__proto__ === Letter.prototype;               // true

b.__proto__ === Letter.prototype;               // true

z.__proto__ === Letter.prototype;               // true

Letter.__proto__ === Function.prototype;        // true

Function.prototype.__proto__ === Object.prototype;        // true

Letter.prototype.__proto__ === Object.prototype;          // true