currency.js

String.prototype.reverse = function() {
  return this.split("").reverse().join("");
}

Number.prototype.formatAsCurrency = function() {
  return this.toString()
             .replace(/^(\d)$/, "00$1") // pad 1 digit
             .replace(/^(\d{2})$/, "0$1") // pad 2 digits
             .reverse()
             .replace(/(\d{2}(?=\d))/, "$1.") // place decimal point
             .replace(/(\d{3}(?=\d))/g, "$1,") // place commas
             .reverse();
}

(1).formatAsCurrency();  //=> "0.01"
(12).formatAsCurrency();  //=> "0.12"
(123).formatAsCurrency();  //=> "1.23"
(12345678).formatAsCurrency();  //=> "123,456.78"