How does __proto__ differ from constructor.prototype?

Question

Hi, if I have:

function Gadget(name, color)
{
   this.name = name;
   this.color = color;
}

Gadget.prototype.rating = 3

var newtoy = new Gadget("webcam", "black")

newtoy.constructor.prototype.constructor.prototype.constructor.prototype

it will returns always the object with rating = 3

but if I do

newtoy.__proto__.__proto__.__proto__

here the chain end up with null

So why do that difference?

And in IE how can I check the null if there is not a __proto__ property?

Thanks

Answer 1

constructor is a pre-defined [[DontEnum]] property of the object pointed to by the prototype property of a function object and will initially point to the function object itself.

__proto__ is equivalent to the internal [[Prototype]] property of an object, ie it's actual prototype.

When you create an object via use of the new operator, it's internal [[Prototype]] property will be set to the object pointed to by the constructor function's prototype property.

This means that .constructor will evaluate to .__proto__.constructor, ie the constructor function used to create the object, and as we have leared, the protoype property of this function was used to set the object's [[Prototype]].

It follows that .constructor.prototype.constructor is identical to .constructor (as long as these properties haven't been overwritten); see here for a more detailed explanation.

If __proto__ is available, you can walk the actual prototype chain of the object. There's no way to do this in plain ECMAScript3 because JavaScript wasn't designed for deep inheritance hierarchies.