A simple OOish JavaScript demonstration

characters.js

function Character(name, health, strength) {
  this.name = name;
  this.health = health;
  this.strength = strength;
}

Character.prototype.attack = function attack(target) {
  if (this.health > 0) {
      var damage = this.strength;
      console.log(this.name + " is attacking " + target.name + " and deals " + damage + " damage points");
      target.health -= damage;
      if (target.health > 0) {
          console.log(target.name + " has " + target.health + " health points left");
      } else {
          target.health = 0;
          console.log(target.name + " has died !");
      }
  } else {
      console.log(this.name + " cannot attack: he's dead...");
  }
};

function Player(name, health, strength) {
  Character.call(this, name, health, strength);
  this.xp = 0;
}

Player.prototype = Object.create(Character.prototype);

Player.prototype.describe = function () {
    return [
      this.name,
      "has",
      this.health,
      "health points and",
      this.strength,
      "strength, with",
      this.xp,
      "XP Points"
    ].join(" ");
};

Player.prototype.attack = function playerAttack() {
  if (this.health > 0) {
      var damage = this.strength;
      console.log(this.name + " is attacking " + target.name + " and deals " + damage + " damage points");
      target.health -= damage;
      if (target.health > 0) {
          console.log(target.name + " has " + target.health + " health points left");
      } else {
          target.health = 0;
          console.log(target.name + " has died !");
      }
  } else if (target.health === 0) {
    console.log(this.name + " killed " + target.name + " and wins " +
        monster.value + " XP points");
    this.xp += target.value;
  } else {
      console.log(this.name + " cannot attack: he's dead...");
  }
};

function NPC(name, health, strength, race, value) {
    Character.call(this, name, health, strength);
    this.race = race;
    this.value = value;
}

NPC.prototype = Object.create(Character.prototype);

NPC.prototype.describe = function npmDescribe() {
  return [
      this.name,
      "has",
      this.health,
      "health points and",
      this.strength,
      "strength. It's a",
      this.race,
      "worth",
      this.value,
      "XP points"
  ].join(" ");
};