Unnecessarily clever number formatting function

formatNumber.js

// 1234567890 => "1,234,567,890"
function formatNumber(n) {
  var a = [], s = String(n), i = l = s.length;
  while (i--) a.push(s[i]), i && (l - i) % 3 === 0 && a.push(",");
  return a.reverse().join("");
}