Instance creation part 2


My previous post shows how instance creation works with new operator.So in this post we will try to deduce what really goes on.
We will start with the basic and try and go deep


var constructorFn = function(){this.a=1};
var obj = new constructorFn();
obj instanceof constructorFn // true

Nothing unusual here.When the new keyword is encountered ,the “this” is said to the current object itself and the key value pair is set on the object.

Lets now modify our code a bit


var constructorFn = function(){
this.a =1 ;
}
constructorFn.prototype.key1 ="value1";
constructorFn.prototype.key2 ="value2";

on printing obj we see

constructorFn {a: 1, key1: "value1", key2: "value2"}

So what exactly did happen ? We already understand how a came into the picture.
To understand how the other 2 property came in ,look at the below piece of code

Object.getPrototypeOf(obj);
//displays
Object {key1: "value1", key2: "value2"}

So the algorithm of obj = new fn() goes as follows
1.Execute fn.
2.If “this” is encountered,set its value to the current object
3.If a primitive value is returned then ignore that and set obj to point to the current object
4.If a complex object is returned then obj should ignore the current object and instead be set to the returned value.
5.obj[[prototype]] should now point to fn[[prototype]]

An important thing to understand is that __proto__ and prototype is not same.In the above example


( new Foo ).__proto__ === Foo.prototype ;//true
( new Foo ).prototype === undefined ;//true

__proto__ is the actual object that is used in the lookup chain to resolve methods, etc.Prototype is a property belonging only to functions. Used in case the function happens to be used as a constructor

PS:__proto__ is deprecated and Object.getPrototypeOf should be used.

So the last question remains as to what is the “constructor”.constructor is nothing but the function that constructed the object.

obj.__proto__ === obj.constructor.prototype //returns true
Object.getPrototypeOf(obj) === obj.constructor.prototype //returns true

Why was the last line necessary ? So that we understand that “prototype” is for functions and Object.getPrototypeOf() is for instances.

PPS: Object.getPrototypeOf(obj) === Object.getPrototypeOf(obj.constructor)//returns false .

The reason being getPrototypeOf internally checks the __proto__ property as that is how the chain is traversed and not prototype and hence constructorFn.key1 will be undefined.

Advertisements

One response to “Instance creation part 2

  1. Pingback: Instance Creation Part 3 | Using Abhik.Mitra.myThoughts;

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s