短小精悍的 js 银行卡校验(luhn 算法)

网上搜索的看着都蛋疼。实现的太复杂，故精简如下:

function checkBankCard(cardNo) {
  var tmp = true,
    total = 0
  for (var i = cardNo.length; i > 0; i--) {
    var num = cardNo.substring(i, i - 1)
    if (((tmp = !tmp), tmp)) num = num * 2
    var gw = num % 10
    total += gw + (num - gw) / 10
  }
  return total % 10 == 0
}