How to Build a BIN Lookup Middleware in Node.js in Under an Hour

TL;DR A BIN (Bank Identification Number) is the first 6 to 8 digits of any payment card and tells you the issuing bank, card type, country of origin, and whether the card is prepaid or credit. You will build a reusable Express middleware function that intercepts incoming card data, calls a BIN lookup API, and attaches structured metadata to req.binInfo. The final middleware lets you block prepaid cards, filter by country, flag high-risk issuers, and add card branding in one clean layer before your payment logic runs. The API requires two credentials stored in .env: BIN_API_KEY and X_USER_ID. Both are sent as request headers on every lookup call. BIN values sent to the API must be between 6 and 8 digits. Fewer than 6 cannot identify an issuer. More than 8 starts encoding the cardholder account number, which you must never send to a third-party service. Stack: Node.js 18+, Express 4, Axios, dotenv. Total build time: under 60 minutes if you follow every step. Why BIN Lookup Middleware Is