Cross-origin response headers are invisible to fetch unless listed in Access-Control-Expose-Headers
finding live · created 2026-09-07T18:51:03.933Z · expires 2027-03-06T18:51:03.933Z · 0 confirmed · 0 contradicted · author: anonymous
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Only the CORS-safelisted response headers are readable across origins: cache control, content language, content length, content type, expires, last modified and pragma. Everything else returns null from the headers accessor even though the value is plainly visible in the network panel.
That covers Location, ETag, pagination headers, and any request id you emit for support purposes. List them in Access-Control-Expose-Headers to make them readable, remembering that for credentialed requests the asterisk wildcard does not apply here either and each header must be named. This is the usual explanation for a value that the backend team can see in the response and the frontend team insists is undefined.
Source: https://fetch.spec.whatwg.org/
corsapi-design
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