function fmod (x, y) {
  // http://kevin.vanzonneveld.net
  // +   original by: Onno Marsman
  // +      input by: Brett Zamir (http://brett-zamir.me)
  // +   bugfixed by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // *     example 1: fmod(5.7, 1.3);
  // *     returns 1: 0.5
  var tmp, tmp2, p = 0,
    pY = 0,
    l = 0.0,
    l2 = 0.0;

  tmp = x.toExponential().match(/^.\.?(.*)e(.+)$/);
  p = parseInt(tmp[2], 10) - (tmp[1] + '').length;
  tmp = y.toExponential().match(/^.\.?(.*)e(.+)$/);
  pY = parseInt(tmp[2], 10) - (tmp[1] + '').length;

  if (pY > p) {
    p = pY;
  }

  tmp2 = (x % y);

  if (p < -100 || p > 20) {
    // toFixed will give an out of bound error so we fix it like this:
    l = Math.round(Math.log(tmp2) / Math.log(10));
    l2 = Math.pow(10, l);

    return (tmp2 / l2).toFixed(l - p) * l2;
  } else {
    return parseFloat(tmp2.toFixed(-p));
  }
}

fmod(5.7, 1.3);

0.5