Extending objects (Dojo)

Black Ninja is killing Bob

HTML

<div id='result'></div>

CSS

#result {
    margin-top:  10px;
    color: navy;
    font-weight:  bold;
    font-size: 11px;
    font-family:  Helvetica, Tahoma, Arial;
}

JavaScript

var printData = function(obj){
    dojo.create('p', {
        'innerHTML': obj.name + ' - energy: ' + obj.energy + ', alive: ' + obj.isAlive
    }, 'result');
};
// --------------------------------
dojo.declare("Human", null, {
    constructor: function(kwArgs) {
        dojo.mixin(this, kwArgs);
    },
    isAlive: true,
    energy: 1,
    eat: function(){
        this.energy++;
    }
});
// -------------------------------
dojo.declare("Ninja", Human, {
    // constructor inheritance is automatic unless Manual constructor chaining is used
    energy: 100,
    attack: function(target){
        this.energy = this.energy - 5;
        target.isAlive = false;
    }
});

var bob = new Human({name:'Bob', age:25});
var blackNinja  = new Ninja({side:'evil', name:'Nin Tendo', age:'unknown'});

printData(bob);
printData(blackNinja);

blackNinja.attack(bob);
printData(bob);
printData(blackNinja);