Javascript is a prototype-based language. It resolve property using prototype chains.

Prototype chains (aka prototype inheritance)

The proto property

proto is the internal property of an object, it points to the prototype object of the object and is used for property look-up. Every object has a proto perperty and its relevant prototype object. But it maybe null, which means the object has no prototype object.

function as constructor

In JavaScript you create an instance of a function like this:

function Foo() {
  this.kind = ‘foo’
}

var foo = new Foo(); 
foo.kind //=> ‘foo’

Behind this scenes it is like doing something like this:

function Foo() {
  var this = {}; // this is not valid, just for illustration
  this.__proto__ = Foo.prototype;

  this.kind = ‘foo’

  return this;
}

But keep in mind that the implicit ‘this’ is only assigned to a new object when using ‘new’. If you forget ‘new’ keyword then ‘this’ will be the global object.

the ‘function prototype’

Every function in JavaScript has a special property called ‘prototype’. To avoid confusion, we call it “function prototype”.

Basically, this object is provided at run-time by Javascript engine. This object has a property called “constructor” which points to the function itself.

It is used to** build __proto__** when you create an object with new keyword.

Javascript object model

Now we illustrate the complete relations for prototype: