A Javascript implementation of the Luhn Algorithm used for validating credit card numbers.

creditCardValidator.js

const isCardNumberValid = cardNumber => {
  const normalizedStr = cardNumber.replace(/\s/g, '');
  const tokens = Array.from(normalizedStr).map(Number);
  const checkDigit = tokens.pop();

  const nonCheckDigitsSum = tokens
    .reverse()
    .reduce((sum, token, i) => {
      if (!(i % 2)) {
        const strigifiedDouble = (token * 2).toString();
        const s = Array
          .from(strigifiedDouble)
          .map(Number)
          .reduce(
            (acc, v) => acc + v, 0
          );

        return sum + s;
      }

      return sum + token;
    }, 0);

  return (nonCheckDigitsSum + checkDigit) % 10 === 0;
};