Step-by-step troubleshooting solutions for the #1 most common frontend API error across Node.js/Express, Python/FastAPI, Go, Nginx, Next.js, and Spring Boot.
Frequently Asked Questions
Q1. Why does my CORS middleware in Express still throw errors on 500 errors?
If an unhandled exception or 500 error crashes your route before headers are written, Express may send a default error response that bypasses your CORS headers. Ensure your global error handler attaches CORS headers.
Q2. Can I fix CORS by adding headers in the frontend fetch() call?
No! Setting headers like headers: { "Access-Control-Allow-Origin": "" } in client JavaScript does nothing because Access-Control-Allow-Origin is a RESPONSE header sent from server to browser, not a request header.