luhn10() implementation

creditcard.js

function luhn10(cc) {
  let num = normalize(cc);

  let sum = 0;
  let dbl = true;
  let checkDigit = num.slice(-1);

  for (let i = num.length - 2; i >= 0; i--) {
    if (dbl) {
      let val = +num.charAt(i) * 2;
      if (val > 9) {
        val = Math.floor(val / 10) + (val % 10);
      }
      sum += +val;
      dbl = false;
    } else {
      sum += +num.charAt(i);
      dbl = true;
    }
  }

  let total = sum + +checkDigit;
  return total % 10 == 0;
}