Credit cards, bank accounts (IBAN), books (ISBN) and products (EAN/UPC barcodes) all carry a check digit — a number computed from the rest so typos can be caught instantly. The math (Luhn, mod-97, weighted sums) is exact and well-defined… and exactly the kind of thing that's tedious to implement per-format and easy for an AI to get subtly wrong.

checkdigit validates and generates all of them with the correct algorithms, zero dependencies, and 100% locally — your card number never touches a server.

📸 Screenshot / demo GIF: ./web/screenshot.png — record the live app validating a card and generating a test EAN-13.

• AI can't reliably do this. Check-digit algorithms (Luhn doubling, IBAN mod-97-10, ISBN/EAN weighted sums) are precise and unforgiving — an LLM will cheerfully produce an "almost right" number that fails validation. You want a tested, deterministic implementation.
• Privacy. Validating a card on a random website means typing your number into their server. checkdigit runs on your device.
• One tool, every format. Stop pulling a separate micro-library (or copy-pasted snippet) for each of Luhn / IBAN / ISBN / EAN.

Developers (form validation without N tiny deps), QA & testers (generate valid test cards/IBANs/barcodes), e-commerce & ops (verify SKUs, orders, barcodes), finance, publishers/librarians (ISBN), and anyone who needs to check or convert an identifier.

No install — just open the web app.

Command line:

npx @didrod2539/checkdigit validate 4111111111111111   # ✓ valid  credit-card (Visa)
echo "DE89 3704 0044 0532 0130 00" | npx @didrod2539/checkdigit validate
npx @didrod2539/checkdigit generate card --brand Amex  # a format-valid TEST card

Library:

npm install @didrod2539/checkdigit

Zero runtime dependencies. ESM + CJS + TypeScript types. Runs in the browser, Node, Deno and Bun.

checkdigit validate <id>     # auto-detect (card/IBAN/ISBN/barcode) + validate
checkdigit generate <kind>   # make format-valid TEST data

validate exits non-zero on an invalid check digit — handy in scripts. Generate kinds: card (--brand), isbn, barcode (--type), luhn (--length). Generated values are test data (correct checksum) — not real accounts.

import { validate, detect, luhn, creditCard, iban, isbn, barcode } from "@didrod2539/checkdigit";

// Auto-detect & validate anything
validate("4111 1111 1111 1111"); // { type: "credit-card", valid: true, detail: "Visa" }
validate("DE89 3704 0044 0532 0130 00"); // { type: "iban", valid: true, detail: "DE" }
validate("978-0-306-40615-7");   // { type: "isbn", valid: true, detail: "ISBN-13" }

// Or use a specific module
creditCard.brand("378282246310005");      // "American Express"
creditCard.generateTest("Visa");           // a Luhn-valid test number
iban.isValid("GB82 WEST 1234 5698 7654 32"); // true
isbn.to13("0306406152");                    // "9780306406157"
barcode.isValid("036000291452");            // true (UPC-A)
luhn.complete("7992739871");                // "79927398713"

Is my card number sent anywhere? No. Everything runs on your device — no server, no telemetry, works offline.

Are the generated card numbers real? No. They're format-valid test numbers (correct brand prefix, length and Luhn check) for testing forms and payment flows — they are not real, usable accounts.

Does validating a card mean it will work for a payment? No — it only confirms the number is structurally valid (right format + check digit). Whether an account exists/has funds is a separate, server-side matter.

Which IBAN countries are supported? Any IBAN — validation uses the universal mod-97-10 check plus the general structure. (Per-country length tables can be added; PRs welcome.)

Can I add VIN / routing numbers / ISIN? Yes please — see CONTRIBUTING; each is a small module with its own check.

Contributions welcome! See CONTRIBUTING.md and the Code of Conduct.

git clone https://github.com/didrod205/checkdigit.git
cd checkdigit
npm install
npm test          # run the suite
npm run dev       # run the web app locally

checkdigit is free, MIT-licensed, and built in spare time. If it saved you a dependency (or a debugging session), please consider supporting it:

• ⭐ Star this repo — free, and it genuinely helps others find it.
• 🍋 Sponsor via Lemon Squeezy — one-time or recurring support.

Where your support goes: more identifiers (VIN, routing/ABA, ISIN, NPI, national IDs), per-country IBAN structure validation, more card brands, a CLI, keeping the free web app online, and fast issue responses.

MIT © checkdigit contributors