How to Fix CORS Policy Blocked Error in React

Nothing disrupts a coding session quite like an unexpected CORS Policy Blocked Error in React. Here's how to diagnose and fix it.

Root Cause

This error occurs when a browser blocks a cross-origin request because the server doesn't include the proper Access-Control-Allow-Origin headers. It's a security mechanism that prevents unauthorized domains from accessing your API.

Step-by-Step Fix

The key is to proxy API requests through your dev server to avoid cross-origin issues:

// setupProxy.js (create-react-app) or vite.config.js
const { createProxyMiddleware } = require("http-proxy-middleware");

module.exports = function(app) {
  app.use("/api", createProxyMiddleware({
    target: "http://localhost:5000",
    changeOrigin: true,
  }));
};

Common Pitfall

Before diving into code changes, double-check your environment variables and React version. Version mismatches between local and deployed environments are a frequent source of this error. While you're at it, check if your logging captures enough context around this error to speed up debugging next time.

Validate the Solution

Verify by triggering the same action that caused the original error. In React, you can also enable verbose logging temporarily to confirm the fix is applied correctly. Once verified, remove or reduce the logging level to keep your logs clean in production.

Stay Ahead of Errors

Want to catch errors like this before they reach production? [Bugsly](https://bugsly.dev) provides real-time error tracking for React applications.