Luhn Validator

Invented by IBM scientist Hans Peter Luhn in 1954, the Luhn Algorithm (Modulus 10) is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and IMEI numbers.

It is critical to understand that the Luhn algorithm is not designed to be a cryptographically secure hash. Its sole purpose is to protect against accidental human errors, such as a user swapping two adjacent digits (typing 45 instead of 54) or mistyping a single digit during checkout.

1. Starting from the rightmost digit (the check digit) and moving left, double the value of every second digit.
2. If the result of a doubling is greater than 9, add the digits of the product together (e.g., 8 * 2 = 16, then 1 + 6 = 7).
3. Take the sum of all the resulting digits.
4. If the total modulo 10 is equal to 0 (the sum ends in 0), the number is mathematically valid.

Why do e-commerce sites use this? Because it prevents unnecessary network requests.

By running the Luhn check locally in JavaScript in the user's browser, the application can instantly alert the user of a typo. If they didn't use this, they would have to send the mistyped card number to the payment gateway (like Stripe or Visa), wait for a network response, and pay a potential API fee just to be told the number was invalid.