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

Space play ←→ step click any line
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.

Related explainers

javascript 
import { useDeferredValue, useMemo, useState } from "react";
 
function ProductSearch({ products }) {
  const [query, setQuery] = useState("");

Keeping search input snappy with useDeferredValue in React

concurrent-rendering deferred-value memoization
Intermediate 7 steps
typescript 
import { InjectionToken, inject, Provider, isDevMode } from '@angular/core';
import { WINDOW } from './window.token';
 
export interface AnalyticsConfig {

Layered config with an Angular InjectionToken

dependency-injection configuration factory-provider
Intermediate 8 steps
python 
import re
from dataclasses import dataclass, field
 
 

Building a table of contents from Markdown

regular-expressions parsing slugification
Intermediate 9 steps
javascript 
class ToastQueue {
  constructor(container, { duration = 4000, max = 3 } = {}) {
    this.container = container;
    this.duration = duration;

Building a rate-limited toast queue

queue dom manipulation throttling
Intermediate 8 steps
go 
func UploadDocument(c *gin.Context) {
	fileHeader, err := c.FormFile("file")
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "file is required"})

Handling multipart uploads in Gin

multipart-upload validation error-handling
Intermediate 9 steps
javascript 
import { useCallback, useState } from "react";
 
const MIN = 0;
const MAX = 1000;

Building a dual-thumb price slider in React

controlled-components state-clamping usecallback
Intermediate 8 steps

Share this explainer

Here's the card — post it anywhere.

How IBAN validation works in JavaScript — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code