luhn.js

!function() {

var Luhn = {};

Luhn.valid = function(value) {
  value = '' + value;
  return this.generate(value.slice(- value.length, -1)) === value;
};

Luhn.generate = (function() {
  var table = (function() {
    var t = {};

    for (var i = 0, j = 9 * 2; i <= j; i ++)
      t[i] = i > 9 ? i - 9 : i;

    return t;
  })();

  var re = /^[0-9]+$/;

  return function(n) {
    if (!re.test(n))
      return null;

    n = '' + n;

    var value = (n + '0')
      .split('')
      .reverse()
      .reduce(function(previous, current, index) {
        return previous + table[current * (1 * ((index & 1) + 1))];
      }, 0);

    return n + ((10 - (value % 10)) % 10);
  }
})();

this.Luhn = Luhn;

}();