Credit Card Validation and the Luhn Algorithm Explained

Payment gateways and checkout forms validate card numbers client-side before sending requests to payment processors like Stripe or PayPal. This prevents obvious typos without transmitting sensitive data.

Test and validate card numbers:

• Check numbers and IIN prefixes with the Credit Card Validator
• Generate valid synthetic test card numbers with the Credit Card Generator

How the Luhn Algorithm (MOD 10) Works

1. Starting from the rightmost digit (excluding the check digit), double the value of every second digit.
2. If doubling results in a number greater than 9, subtract 9 (or sum its digits: $16 \to 1+6 = 7$).
3. Sum all the digits.
4. If the total modulo 10 equals 0, the number is valid.

function isValidLuhn(cardNumber) {
  const digits = cardNumber.replace(/\D/g, "");
  let sum = 0;
  let isEven = false;

  for (let i = digits.length - 1; i >= 0; i--) {
    let digit = parseInt(digits.charAt(i), 10);
    if (isEven) {
      digit *= 2;
      if (digit > 9) digit -= 9;
    }
    sum += digit;
    isEven = !isEven;
  }
  return sum % 10 === 0;
}

Generate dummy card numbers for test suites with the Credit Card Generator.