javascript 26 lines · 7 steps

Validating card numbers with Luhn

How the Luhn checksum verifies a credit card number by doubling alternate digits and testing divisibility by ten.

Explained by highlit
Intermediate checksum validation luhn-algorithm string-parsing
1function isValidCardNumber(input) {
2 const digits = String(input).replace(/[\s-]/g, '');
3 
4 if (!/^\d{13,19}$/.test(digits)) {
5 return false;
6 }
7 
8 let sum = 0;
9 let double = false;
10 
11 for (let i = digits.length - 1; i >= 0; i--) {
12 let digit = digits.charCodeAt(i) - 48;
13 
14 if (double) {
15 digit *= 2;
16 if (digit > 9) {
17 digit -= 9;
18 }
19 }
20 
21 sum += digit;
22 double = !double;
23 }
24 
25 return sum % 10 === 0;
26}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1The Luhn algorithm catches most single-digit typos and adjacent transpositions cheaply, which is why cards use it.
  2. 2Doubling a digit and subtracting 9 when it exceeds 9 is a shortcut for summing the digits of the doubled value.
  3. 3Structural validation like length and format should run before the expensive checksum math.

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