How do I know an error is actually CORS?
Read the browser console. A CORS error specifically mentions cross-origin or Access-Control headers. Network errors, wrong URLs, and 404s are different problems and have different fixes.
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More FAQs in Common CORS Mistakes Frontend Developers Make
No. CORS is a browser security feature, not a bug in your code. Blaming your frontend wastes time. The fix is always on the server side, either by setting headers on the API or by routing through your backend.
Because extensions work only for you. Production users do not have them, so your app breaks for everyone else. Never ship code that depends on a CORS extension.
For non-simple requests, the browser sends an OPTIONS preflight first. The server must handle OPTIONS and return the right headers, or the actual request never fires. Forgetting to handle preflight is a common mistake.
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