a concise way of writing L-systems in Javascript. this rule set is the dragon curve

l-system.js

var rules = {
  X: "X+YF+",
  Y: "-FX-Y"
};

function rewrite(str) {
  var s = "";
  for (var i = 0; i < str.length; i++) {
    if (rules[str[i]] !== undefined) {
      s += rules[str[i]];
    } else {
      s += str[i];
    }
  }
  return s;
}

// call n times
rewrite(rewrite(rewrite("FX")));