Credit card validation

creditCardValidator.js

const validCreditCard = (cardNumber) => {
  const newInput = cardNumber.toString();

  let first = 0;
  let second = 0;
  if (newInput.length !== 16) {
    return false;
  }
  for (let i = 15; i >= 0; i--) {
    if (i % 2 === 1) {
      first += Number(newInput[i]);
    } else {
      const temp = Number(newInput[i]) * 2;

      if (temp < 10) {
        second += temp;
      } else {
        const newTemp = temp.toString();
        const secondNumber = Number(newTemp[0]) + Number(newTemp[1]);
        second += secondNumber;
      }
    }
  }

  if ((first + second) % 10 === 0) {
    return true;
  }
};

console.log(validCreditCard(4659420912881451));