Luhn Algorithm: Validate Card Numbers
Validate credit card and identification numbers with the Luhn check — with formula and JavaScript code.
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Luhn Algorithm: Validate Card Numbers The Luhn algorithm (mod 10) is standardized in ISO/IEC 7812-1, which defines the identification numbering system for payment cards. The PCI DSS standards govern the security requirements for card number handling in payment systems. The Luhn algorithm (also called "mod-10") is a checksum formula that determines whether a sequence of digits could be a valid identification number. It catches single-digit errors and most transposition errors — the most common data-entry mistakes. --- What the Luhn Check Validates? The algorithm does not verify that a number is an active, authorized, or issued card. It only confirms the number follows a valid mathematical pattern. This distinction matters for form validation: a Luhn-valid number might not correspond to any…
Frequently Asked Questions
What is the Luhn algorithm?
The Luhn algorithm is a simple mod-10 checksum formula used to validate identification numbers like credit card numbers and IMEI codes. It doubles alternating digits from right to left, reduces values over 9 by subtracting 9, sums all digits, and checks if the total is divisible by 10.
How do I validate a credit card number?
Strip all non-digit characters, then apply the Luhn formula: starting from the second-to-last digit and moving left, double every other digit; if the result exceeds 9, subtract 9; sum all digits including unchanged ones; if the total is divisible by 10, the number passes the Luhn check.
How does the Luhn check work?
The Luhn check works by computing a weighted sum of the digits in the number. Alternating digits (starting from the penultimate) are doubled, and overshoots over 9 are corrected by subtracting 9. If the entire sum is a multiple of 10, the number is Luhn-valid.
What other numbers use the Luhn algorithm?
Beyond credit and debit card numbers, Luhn validation is used for IMEI numbers (mobile devices), Canadian Social Insurance Numbers, Israeli ID numbers, South African ID numbers, and some airline frequent flyer numbers.
How do I implement Luhn in JavaScript?
Strip non-digit characters, reverse the digit array, and use reduce: double every digit at an odd index (0-based), subtract 9 from any result over 9, then sum all digits. Return true if the total modulo 10 equals zero.
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