Luhn Algorithm Explained: Credit Card Validation in JavaScript

Every credit card number has a built-in error-detection mechanism called the Luhn algorithm (also known as "modulus 10"). It catches accidental typos — swapped digits, single-digit errors, and most transposition mistakes — before a number ever reaches your payment processor. This article explains how it works step by step, shows you a clean JavaScript implementation, and covers brand detection so you can build a complete card validator. How the Luhn Algorithm Works The algorithm operates on the card number's digits from right to left: Start from the rightmost digit (the check digit) and move left Double every second digit (starting from the second-to-last) If doubling produces a number > 9 , subtract 9 Sum all digits If the total is divisible by 10 , the number is valid Walk-Through: 4111 1111 1111 1111 This is Visa's standard test number. Let's trace through it: Original: 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Double alt: 8 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 Sum: 8+ 1+ 2+ 1+ 2+ 1+ 2+ 1+ 2+ 1+ 2+