CORS
Cross-Origin Resource Sharing — the browser security policy that controls which web pages can read responses from which APIs.
In long form.
By default, JavaScript on `https://example.com` can't read responses from `https://api.other.com` — that's the same-origin policy, a foundational web-security boundary. CORS is the negotiated exception: the API server includes specific Access-Control-Allow-Origin headers, the browser checks them, and either allows the JavaScript to read the response or blocks it. CORS only applies to browsers reading responses; it does not protect the server itself. Pre-flight OPTIONS requests handle non-simple requests (custom headers, methods other than GET/POST/HEAD).
When a frontend can't talk to an API, the first check is the network tab for a CORS error. The fix is on the API server (allow the right origin), not the frontend. Spoiler: 'fixing CORS' on the frontend is impossible by design.
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