Why does CORS block localhost:3000 but allow localhost:5000?
Because they are different origins. Different ports count as different origins, and so do http and https. A common mistake is allowing one origin on the server and being blocked from another.
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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.
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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