luhn.js

var Luhn = function(number) {
  // console.log("Doing Luhn check on " + number);
  this.checkDigit = number % 10;
  this.addends = [];
  this.checksum = 0;

  // for (var i = 1; number > 0; i++) {
  // var i = 1;
  var double = false;
  while (number > 0) {
    // Extract the last digit
    var digit = number % 10;

    // Double even-counted digits
    if (double) {
      digit *= 2;

      // If the result >= 10, subtract 9
      if (digit >= 10) {
        digit -= 9;
      }
    }
    // console.log(digit);
    this.checksum += digit;
    this.addends.unshift(digit);
    number = Math.floor(number / 10);
    double = !double;
    // i++;
  }

  this.valid = this.checksum % 10 == 0;
}

Luhn.create = function(number) {
  number *= 10;
  var luhnResult = new Luhn(number);
  if (luhnResult.valid) {
    return number;
  }
  var checksum = luhnResult.checksum;

  var addend = 10 - (checksum % 10);
  return number + addend;
};

module.exports = Luhn;