Luhn Checksum

HTML

<input type="text" id="number" value="5661588891014675">
<input type="button" id="check" value="Check">
<div id="result_div"></div>

JavaScript

$("#check").click(function() {
	var num;
  num = $("#number").val();
  $("#result_div").append("<p>" + num + ":" + luhn_checksum(num) + "</p>");
});

function luhn_checksum(card_number) {
  var a, s, i, x;
  a = card_number.split("").reverse();
  s = 0;
  for (i = 0; i < a.length; i++) {
    x = a[i] * (1 + i % 2);
    s += x > 9 ? x - 9 : x;
  }
  return s % 10 == 0;
}