Luhn's algorithm: the 70-year-old checksum still validating every card you own

Every credit card number has a built-in checksum digit, and the algorithm that checks it (Luhn's algorithm) predates modern computing — it was patented in 1954, before credit cards as we know them existed.

function luhnValid(number) {
  const digits = number.replace(/\D/g, "").split("").reverse().map(Number);
  const sum = digits.reduce((acc, digit, i) => {
    if (i % 2 === 1) {
      digit *= 2;
      if (digit > 9) digit -= 9;
    }
    return acc + digit;
  }, 0);
  return sum % 10 === 0;
}

This only checks that the number is structurally valid — it says nothing about whether the card exists, is active, or has funds. That's a completely separate (and much more sensitive) concern that needs a real payment processor, not a format checker.

Brand detection (Visa vs Mastercard vs Amex) is a second, separate step based on the IIN/BIN prefix ranges. I bundled both into one endpoint on Validate since they're almost always needed together. Same account also runs QR API and Currency API.