luhn.js

/**
 * Implementation of Luhn algorithm.
 * Validates ISO/IEC 7812-1 Cards (including credit cards)
 *
 * @param cardNumber
 * @return true, if card is valid.
 */
function luhnCheck(cardNumber) {
  const digits = cardNumber.split("").filter(item => !/\s/.test(item));
  // The last digit is check digit.
  const checkDigit = parseInt(digits.pop());
  const sum = digits
    // Map them to ints
    .map((item, i) => {
      const num = parseInt(item);
      // Step 1: Double the value of every second digit.
      const result = i % 2 ? num : num * 2;
      // Split them into single digits
      return String(result).split("");
    })
    // flatten
    .reduce((collector, item) => collector.concat(item), [])
    // step 2: add them all together
    .reduce((collector, item) => parseInt(item) + collector, checkDigit);
  // Step 3
  return sum % 10  === 0;
}