Real-Time VAT Validation in a React Checkout Form

The article explains how to validate VAT IDs as customers type into a React checkout field without exposing API credentials. Calling the vatnode API directly from the browser would leak the key in the client bundle and fail due to CORS restrictions, so the author recommends a backend proxy route that stores the key server-side. The architecture routes the React component through the developer's own endpoint, which forwards requests to vatnode and returns normalized results. Before any network call, the implementation performs a local format check, with special handling for Greek (EL) and Northern Ireland (XI) prefixes. The guide also provides a minimal Next.js route handler that reads the key from a server-only environment variable and standardizes the response shape.