Again i got stuck in prototype chaining?

[dmjpro]

Hey Experts!
I want to know the prototype chaining? How does it take part in Inheritance in JavaScript?

[Dormilich]

put in simple words, prototyping is the Javascript inheritance.

more info on google

[dmjpro]

I think it would be a best place to know how prototype chaining works ?

[Dormilich]

you mean an example?

[dmjpro]

See ....
I have this example.

function Super(){
this.a = 100;
}

function Sub(){
Super.apply(this,arguments);
}

function test(){
Sub.prototype = new Super();
Super.prototype.x = 200;
Sub s = new Sub();
alert(s.a); alert(s.x);
}

Actually I got a sample code ...i changed here in two places.
There it was .... Sub.prototype.c onstructor = Sub and Super.prototype .constructor.ap ply..
Actually what would be the benifit if i left the code as it was?
Could you explain?

[Dormilich]

Actually I got a sample code ...i changed here in two places.
There it was .... Sub.prototype.c onstructor = Sub

this re-sets the constructor function to Sub() (otherwise the Super() function would be called at: var obj = new Sub;)

and Super.prototype .constructor.ap ply..

this calls Super's constructor function (which may not necessarily be named Super()) with the context of "this" set to your actual Sub object.

Actually what would be the benifit if i left the code as it was?

probably less typing and less re-usability (though that may make hardly any difference in this case)

[dmjpro]

Still it's not clear.
Still it's calling Sub function on calling new Sub()
And what you told abut apply, i didn't get you ;(

[Dormilich]

Still it's calling Sub function on calling new Sub()

how do you know that?

And what you told abut apply, i didn't get you ;(

you know what the apply() function is for?

[dmjpro]

If i add something into Sub..

function Sub(){
this.b = 200;
.....
}

If I alert s.b then it's coming. i am getting the Sub's properties as well as of Super.

Yeah i know .. actually what apply does here .. it simply copies the Super's properties and put it into Sub object. I don't know whether it is correct or not ? ;)

[Dormilich]

Yeah i know .. actually what apply does here .. it simply copies the Super's properties and put it into Sub object. I don't know whether it is correct or not ? ;)

nearly, apply() runs the specified function in a different context (= your own definition of "this" inside the called function) (it does not copy anything) i.e. here it creates the property "a" but the property is attached to the Sub object not the window object (window is the global scope/object that is used, if no context is specified (like if you would simply call Super() (without the new keyword)))

see apply() – MDC

[dmjpro]

That means the properties of Super set into Sub.
Then what's the difference ... Super.prototype .constructor.ap ply and Super.apply? ;)

[Dormilich]

in your case there isn't.

[dmjpro]

Then please give me an example ?
An one more thing .. where do i need to reset the constructor again ? Please give also an example of it .. ;)

[Dormilich]

An one more thing .. where do i need to reset the constructor again ?

you'll need the correct constructor function if you want to use inherited methods. It seems not a problem when creating instances of an object, rather than using them.

example taken from http://phrogz.net/JS/Classes/OOPinJS2.html

function Mammal(name){ 
	this.name=name;
	this.offspring=[];
} 
Mammal.prototype.haveABaby=function(){ 
	var newBaby=new [B]this.constructor[/B]("Baby "+this.name);
	this.offspring.push(newBaby);
	return newBaby;
} 
Mammal.prototype.toString=function(){ 
	return '[Mammal "'+this.name+'"]';
} 


Cat.prototype = new Mammal();        // Here's where the inheritance occurs 
/*
  comment out this line and compare the output to the original one
*/
[U]Cat.prototype.constructor=Cat;[/U]       // Otherwise instances of Cat would have a constructor of Mammal 
function Cat(name){ 
	this.name=name;
} 
Cat.prototype.toString=function(){ 
	return '[Cat "'+this.name+'"]';
} 


var myPet = new Cat('Felix');
myPet.haveABaby();                    // calls a method inherited from Mammal 

alert(myPet.offspring[0]);            // results in '[Cat "Baby Felix"]'

[dmjpro]

And what about Apply?
Give me an example where those two make differences? ;)