Luhn algorithm - generate / validate

Luhn.js

var Luhn = {
  // length of our number (check digit included)
  length: 10,
  pow2: [0, 2, 4, 6, 8, 1, 3, 5, 7, 9],
  // compute check digit
  checksum: function (x) {
    var sum = 0;
    var n;
    var odd = false;
    for (var i = x.length - 1; i >= 0; --i) {
      n = parseInt(x.charAt(i), 10);
      sum += odd ? n : this.pow2[n];
      odd = !odd;
    }
    return sum;
  },
  // validate a number
  check: function (x) {
    var clean = String(x).replace(/[^\d]/g, '');
    if (clean.length !== this.length) {
      return false;
    }
    var code = clean.substring(0, clean.length - 1);
    var check = parseInt(clean.substring(clean.length - 1), 10);
    var checksum = this.checksum(code);
    return (checksum + check) % 10 === 0;
  },
  // create a random valid number (check digit included at the end)
  // prefix : if given, our final number will start with prefix (must be a number)
  generate: function (prefix) {
    var prefixLength = prefix ? Math.log(prefix) * Math.LOG10E + 1 | 0 : 0;
    var suffixLength = this.length - 2 - prefixLength;
    var x = Math.pow(10, suffixLength) + Math.floor(Math.random() * 9 * (Math.pow(10, suffixLength)));    
    x = prefix ? prefix + '' + x : '' + x;
    var checksum = this.checksum(x) % 10;
    return checksum === 0 ? x + '0' : x + (10 - checksum);
  }
};
Luhn.check(Luhn.generate());
Luhn.check(Luhn.generate(4972));