[Moo] question about Object.reset

[Romansky]

I am trying to understand what is Object.delete doing, here's the
relevant code:
#######################
Object.reset = function(object, key){

        if (key == null){
                for (var p in object) Object.reset(object, p);
                return object;
        }

        delete object[key];

        switch ($type(object[key])){
                case 'object':
                        var F = function(){};
                        F.prototype = object[key];
                        var i = new F;
                        object[key] = Object.reset(i);
                break;
                case 'array': object[key] = $unlink(object[key]); break;
        }

        return object;

};
#######################

I dont understand how after :
delete object[key];
you can do this:
switch ($type(object[key])){ ....

Isnt the object[key] already unset by the delete??
How is it working??