Fixing CORS Errors in Local Development: Causes and Solutions

Developers often hit CORS errors when a frontend on localhost calls a backend API that works fine in tools like Postman. CORS is a browser security mechanism that restricts cross-origin requests, and ports or hostnames that differ count as different origins. Common triggers include mismatched ports, missing Access-Control-Allow-Origin headers, failed preflight OPTIONS requests, and inconsistent use of localhost versus 127.0.0.1. The article recommends configuring the cors middleware in an Express.js backend to specify allowed origins, methods, and headers, and it also covers using a proxy in Vite as an alternative fix.