A CORS Debugging Checklist for React Developers

A CORS Debugging Checklist for React Developers

When CORS blocks your React app, follow this checklist to diagnose and fix the problem step by step.

Step 1: Confirm It Is CORS

Read the browser console. A CORS error mentions cross-origin or Access-Control headers. Other errors are not CORS even if they happen during a fetch.

Step 2: Identify the Origin

Note your frontend origin, including port and protocol. localhost:3000 and http://localhost:3000 are different from https://yourapp.com. Know which origin is being blocked.

Step 3: Check the Server Response

In the Network tab, look at the failing response headers. Is Access-Control-Allow-Origin present and set to your origin or a wildcard? If not, the server is not allowing you.

Step 4: Check for Preflight

If the request is non-simple, the browser sends an OPTIONS preflight. Does the server respond to OPTIONS with the right headers? If not, the actual request never fires.

Step 5: Check Credentials

If your request sends cookies or auth headers, the server must set Access-Control-Allow-Credentials to true and must not use a wildcard origin. Otherwise the browser blocks it.

Step 6: Apply the Right Fix

If you control the server, set the right headers. If you do not, use a proxy in development or route through your backend in production.

Step 7: Test From the Browser

After fixing, test from your actual React app in the browser, not just Postman. Postman does not enforce CORS, so it cannot confirm your fix works.

The Takeaway

Follow the checklist: confirm CORS, identify the origin, check the server response, check preflight, check credentials, apply the right fix, and test from the browser. This catches every common CORS cause.