JavaScript & Prototype

What is prototype?

Prototype objects are just simple JavaScript objects and may have their own prototypes. If a prototype has a non-null reference to its prototype, and so on, this is called the prototype chain.


ECMAScript deals with objects, which are an abstract form of object-orientated language. They are primitives but can be converted to objects when needed.


Property __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:

function Foo() {}

var foo = new Foo();


  foo.__proto__ === Foo.prototype, // true
  foo.prototype === undefined // true


So prototype is not available on the instances themselves (or other objects), but only on the constructor functions.

prototype is only available on functions since they are derived from Function, and Object but in anything else it is not. However, __proto__ is available on every object.

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


__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 Foo(x, y) {
  this.x = x;
  this.y = y;

var foo = new Foo();

// the following are all true

  foo.__proto__ === Foo.prototype,
  foo.__proto__.__proto__ === Object.prototype,
  foo instanceof Foo,
  foo instanceof Object


Here Foo is a constructor function. It builds an object (data structure) procedurally. foo is an object constructed by Foo() so Foo.prototype gets saved to foo.__proto__ at that time.

function A(){
  this.x = 10;
  this.calc = function(z) {
    return this.x + this.y + z;

function B(){}
B.prototype = new A();
B.prototype.y = 20;

function C(){}
C.prototype = new A();
C.prototype.y = 30;

var b = new B();
var c = new C();

b.calc(30); // 60
c.calc(30); // 70

What is the difference between __proto__ and prototype properties?

The rule is simple: if a property or a method is not found in the object itself (i.e. the object has no such an own property), then there is an attempt to find this property/method in the prototype chain.

If the property is not found in the prototype, then a prototype of the prototype is considered, and so on, i.e. the whole prototype chain (absolutely the same is made in class-based inheritance, when resolving an inherited method — there we go through the class chain). The first found property/method with the same name is used. Thus, a found property is called inherited property. If the property is not found after the whole prototype chain lookup, then undefined value is returned.

If a prototype is not specified for an object explicitly, then the default value for __proto__ is taken – Object.prototype.

Object Object.prototype itself also has a __proto__, which is the final link of a chain and is set to null.

In JavaScript, every function is actually a Function object. And Function is one of the Fundamental objects, along with Object.

function A() {} = 1;

function B() {}
B.prototype = Object.create(A.prototype);
/* or, B.prototype = new A();*/ = 2;

var b = new B();
  /* b's contructor is A */
  b.constructor === A, // true
  b.constructor === B, // false
  /* B's contructor is Function */
  b.constructor === Function, // false
  B.constructor === Function, // true
  b instanceof B // true

var a = new A();

  a.__proto__ === b.__proto__.__proto__ // true

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