Problems › JavaScript › payments
Luhn check a card number
Before a card ever reaches the payment gateway, the checkout runs the Luhn checksum so an obvious typo is caught in the browser.
- Working from the right, double every second digit; if doubling gives more than 9, subtract 9.
- The number is valid when the resulting sum divides by 10.
- Spaces and dashes are formatting and are ignored.
- Any other non-digit makes it invalid, as does anything shorter than two digits.
luhnValid(digits: string) → bool
Where you start
function luhnValid(digits) {
}Worked examples
| Call | Result |
|---|---|
luhnValid("79927398713") | true |
luhnValid("79927398710") | false |
luhnValid("4111 1111 1111 1111") | true |
luhnValid("4111-1111-1111-1112") | false |
Hint
Strip the formatting into a clean string first. Then walk it backwards with an index so "every second" is easy to say.
Reference solution in JavaScript
function luhnValid(digits) {
let clean = '';
for (const c of digits) {
if (c === ' ' || c === '-') continue;
if (c < '0' || c > '9') return false;
clean += c;
}
if (clean.length < 2) return false;
let sum = 0;
for (let i = 0; i < clean.length; i++) {
let d = clean.charCodeAt(clean.length - 1 - i) - 48;
if (i % 2 === 1) { d *= 2; if (d > 9) d -= 9; }
sum += d;
}
return sum % 10 === 0;
}