const doubleEveryOtherDigit = (creditCardArray) => {
  let doubledCreditCardArray = creditCardArray.slice(0);

  for(let i = doubledCreditCardArray.length-2; i>=0; i=i-2) {
    let digit = doubledCreditCardArray[i];
    let digitTimesTwo = digit * 2;
    doubledCreditCardArray[i] = digitTimesTwo > 9
        ? digitTimesTwo - 9
        : digitTimesTwo;
  }
  return doubledCreditCardArray;
}

const sumReducer = (accumulator, currentSum) => {
  return accumulator + currentSum;
}

const validateCred = (creditCardArray) => {
  const sum = doubleEveryOtherDigit(creditCardArray).reduce(sumReducer);
  const sumModTen = sum % 10;

  return sumModTen === 0;
}