Gist from mistakes.io

index.js

function roll(die) {
  var result = 0, a = /(\d+)d(\d+)(?:([\+\-\*\/])(\d+))?/.exec(die);
  var times = a[1], sides = a[2], op = a[3], mod = +a[4];
  result = roll.raw(times, sides);
  if(op && mod) result = roll.ops[op.replace(/x/g, '*')](result, mod);
  return result
}
roll.raw = function(times, sides, total) {
  total = total || 0;
  total += Math.ceil(Math.random() * sides);
  return times <= 1 ? total : roll.raw(times - 1, sides, total);
}
roll.ops = function(ops) {
  var o = {};
  ops.forEach(function(op) {
    o[op] = new Function('a, b', 'return a ' + op + ' b;')
  })
  return o;
}('+-*/'.split(''));

function Point(x, y) {
  this.x = x;
  this.y = y;
}
Point.prototype.dist = function dist(p) {
  return Math.sqrt(Math.pow(Math.abs(this.x - p.x), 2)
    + Math.pow(Math.abs(this.y - p.y), 2));
}
Point.prototype.toString = function() {
  return '(' + this.x + ',' + this.y + ')'
}

function Creature (name, hp, weapon, pos) {
  this.name = name;
  this.weapon = weapon;
  this.hp = this.maxhp = hp;
  this.pos = pos;
  this.dead = false;
}

Creature.prototype.hit = function(target, times) {
  if(this.pos.dist(target.pos) <= this.weapon.reach) {
    target.hurt(roll(this.weapon.dam));
  }
};
Creature.prototype.hurt = function(amt) {
  this.hp -= amt;
  if(this.hp <= 0) {
    this.die()
  }
};
Creature.prototype.die = function() {
  this.dead = true;
};

var bob = new Creature('bob', 10, new Weapon('sword', '1d6', 1.5), new Point(1, 1));
var orc = new Creature('orc', 15, new Weapon('dagger', '1d4', 1), 1.5, new Point(1, 1));

bob.hit(orc);
bob;
orc;