Luhn Algorithm - Credit Card Number Checker

luhn-algorithm.js

const checkByLuhn = (card: string): boolean => {
  if (!card) {
    return false;
  }

  let checksum: number = 0;

  const digits: number[] = card.split("").map(Number);

  for (let i = 0; i < digits.length; i += 1) {
    const digit = digits[i];

    if (i % 2 === 0) {
      const doubled = digit * 2;
      checksum += doubled > 9 ? doubled - 9 : doubled;
    } else {
      checksum += digit;
    }
  }

  return checksum % 10 === 0;
};

console.log(checkByLuhn("5559490000000007"));