ES6 Monster

monster.js

(function(target) {
  let _health = Symbol();
  let _speed = Symbol();
  
  class Monster {
    constructor(name, health, speed) {
      this.name = name;
      this[_health] = health;
      this[_speed] = speed;
    }
    
    get speed() {
      return this[_speed];
    }
    
    get health() {
      return this[_health];
    }
    
    attack(monsterToAttack) {
      this.takeDamage(1);
      monsterToAttack.takeDamage(this.speed);
    }
    
    takeDamage(damageToTake) {
      this[_health] = this[_health] - damageToTake;
      
      if(this[_health] > 0) {
        console.log(`${this.name} is still alive`);
      }
      else {
      	console.log(`${this.name} has died`);
      }
    }
  }
  
  target.Monster = Monster;
})(window);

var quickJames = new Monster('James', 10, 10);
var bigFred = new Monster('Fred', 100, 1);

quickJames.attack(bigFred);