javascript 47 lines · 8 steps

How IBAN validation works in JavaScript

Validate an international bank account number by normalizing it, checking its shape and length, then running the ISO 7064 mod-97 checksum.

Explained by highlit
Intermediate validation checksum modular-arithmetic regex parsing
1const IBAN_LENGTHS = {
2 DE: 22, FR: 27, GB: 22, ES: 24, IT: 27,
3 NL: 18, BE: 16, CH: 21, AT: 20, PT: 25,
4};
5 
6function normalize(iban) {
7 return iban.replace(/\s+/g, '').toUpperCase();
8}
9 
10function mod97(digits) {
11 let remainder = '';
12 for (const digit of digits) {
13 remainder += digit;
14 if (remainder.length >= 9) {
15 remainder = String(Number(remainder) % 97);
16 }
17 }
18 return Number(remainder) % 97;
19}
20 
21function validateIban(input) {
22 const iban = normalize(input);
23 
24 if (!/^[A-Z]{2}[0-9]{2}[A-Z0-9]+$/.test(iban)) {
25 return { valid: false, reason: 'malformed' };
26 }
27 
28 const country = iban.slice(0, 2);
29 const expectedLength = IBAN_LENGTHS[country];
30 if (!expectedLength) {
31 return { valid: false, reason: 'unsupported_country' };
32 }
33 if (iban.length !== expectedLength) {
34 return { valid: false, reason: 'wrong_length' };
35 }
36 
37 const rearranged = iban.slice(4) + iban.slice(0, 4);
38 const numeric = rearranged.replace(/[A-Z]/g, (c) => c.charCodeAt(0) - 55);
39 
40 if (mod97(numeric) !== 1) {
41 return { valid: false, reason: 'checksum' };
42 }
43 
44 return { valid: true, formatted: iban.match(/.{1,4}/g).join(' ') };
45}
46 
47export { validateIban, normalize };
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1The mod-97 check catches transposed or mistyped digits that a simple length or format check would miss.
  2. 2Processing the number in chunks keeps the arithmetic within safe integer range instead of parsing one giant number.
  3. 3Layering cheap checks before expensive ones lets you fail fast and return a precise reason.

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