Format currency putting in commas every 3 digits and a $ sign at the front. Input a number, returns a string.

format-currency.js

function formatCurrency (num) {
  aDigits = num.toFixed(2).split(".");
  aDigits[0] = aDigits[0].split("").reverse().join("").replace(/(\d{3})(?=\d)/g, "$1,").split("").reverse().join("");
  return "$" + aDigits.join(".");
}