Programming Tutorials

  You have built a clean React frontend and a robust Node.js/Express backend. You fire up both servers, attempt a simple   fetch   request, and your browser console turns red with the most infamous error in web development: "Access to fetch at ' http://localhost:4000/api ' from origin ' http://localhost:5173 ' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource." This error is a rite of passage for full-stack developers. It halts progress immediately, preventing your frontend from consuming your backend API. While it feels like a bug, this is actually a browser security feature working exactly as intended. This guide covers the root cause of Cross-Origin Resource Sharing (CORS) errors, provides the production-ready fix using Express middleware, and explains how to handle authenticated requests correctly. Understanding the Root Cause: The Same-Origin Policy To fix CORS, you must understand why it...