typescript 26 lines · 7 steps

Validating card numbers with the Luhn check

How the Luhn algorithm verifies a credit card number by weighting alternate digits and testing the sum modulo 10.

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

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Normalizing input before validation lets you accept human-friendly formatting like spaces and dashes.
  2. 2The Luhn checksum catches most single-digit typos and transpositions cheaply, without any network call.
  3. 3Doubling alternate digits and subtracting 9 keeps each contribution to a single digit's worth of value.

Related explainers

php 
<?php
 
namespace App\Validation;
 

Building a reusable address form validator in PHP

validation error-accumulation regex
Intermediate 9 steps
typescript 
interface UserAgentInfo {
  browser: { name: string; version: string };
  os: { name: string; version: string };
  device: 'mobile' | 'tablet' | 'desktop';

Parsing a user-agent string with ordered rules

regex parsing pattern-matching
Intermediate 9 steps
php 
<?php
 
declare(strict_types=1);
 

Normalizing human names in PHP

unicode text-normalization transliteration
Intermediate 8 steps
typescript 
import { Injectable, inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable, timer, throwError } from 'rxjs';
import { switchMap, takeWhile, filter, take, catchError } from 'rxjs/operators';

Polling a job until it finishes in Angular

rxjs polling observables
Intermediate 7 steps
typescript 
type Flatten = Record<string, unknown>;
 
function isPlainObject(value: unknown): value is Record<string, unknown> {
  return (

Flattening nested objects into dotted keys

recursion reduce type-guards
Intermediate 7 steps
rust 
use once_cell::sync::Lazy;
use regex::Regex;
 
static CREDIT_CARD: Lazy<Regex> = Lazy::new(|| {

Redacting sensitive data from logs in Rust

regex lazy-initialization checksum-validation
Intermediate 9 steps

Share this explainer

Here's the card — post it anywhere.

Validating card numbers with the Luhn check — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code