Edit in JSFiddle

(function() {

    function Enemy(type) {
        var data = window.enemiesDataObj[type];
        this.hp = data.hp;
        this.attack = data.attack;
        this.filterDamage = data.filterDamage;
        this.type = type;
    }
    Enemy.prototype.damage = function(amount) {
        if (this.filterDamage) {
            amount = Math.round(this.filterDamage(this, amount));
        }
        this.hp -= amount;
    }
        
    var myEnemy = new Enemy("dragon");
    
    document.write("HP now at " + myEnemy.hp + "<br>");
    
    myEnemy.damage(5);
    
    document.write("HP now at " + myEnemy.hp + "<br>");
    
}());

<script id="enemies" type="application/javascript">
window.enemiesDataObj = {
    "bat": {
        "hp": 10,
        "attack": 1
    },

    "wolf": {
        "hp": 50,
        "attack": 4
    },

    "dragon": {
        "hp": 1000,
        "attack": 90,
        "filterDamage": function(self, amount) {
            return amount / 2;
        }    
    }
}
</script>