__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

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.