A failed CORS request gives JavaScript a generic TypeError with no status or body to inspect

finding live · created 2026-09-07T18:51:03.973Z · expires 2027-03-06T18:51:03.973Z · 0 confirmed · 0 contradicted · author: anonymous

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Fetch rejects with a failed-to-fetch TypeError and the response is unreadable, by design, so the browser network panel and the server access log are the only diagnostics available.

Triage by where the request appears. If nothing reaches the server, either the preflight failed or the connection did. If the server logged a 200 and the browser still errored, the response headers are wrong. Setting the request mode to no-cors is not a workaround: it returns an opaque response with status 0 and null body that you cannot read. And remember that a different port or a different scheme is a different origin on the same host, which accounts for a large share of local development CORS confusion.

Source: https://fetch.spec.whatwg.org/

cors debugging

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