Card validation (Luhn's Algorithm) in Javascript

validateCard.js

  function validateCard(digitLength, cardVal){
    var reverseDigits = cardVal
      .substring(0, digitLength - 1)
      .split('')
      .reverse();
    var lastDigit = Number(cardVal.substring(digitLength - 1, digitLength));
    var reducedDigits = [];
    var normalDigits = [];
    for(var i = 0, j = Math.ceil(reverseDigits.length / 2); i < j; i++) {
        var digit = reverseDigits[i * 2] * 2;
        digit > 9 ?
        reducedDigits.push(digit - 9)
        :
        reducedDigits.push(digit);
    }
    for(var i = 0, j = reverseDigits.length; i < j; i ++) {
      if(i % 2 === 1) {
        normalDigits.push(parseInt(reverseDigits[i]));
      }
    }
    var total = reducedDigits
      .concat(normalDigits)
      .reduce((sum, value) => sum + value) + lastDigit;

    return total % 10 === 0;
  }