Fix JavaScript Fetch “Failed to Fetch” Errors
TypeError: Failed to fetch usually means JavaScript did not receive an accessible HTTP response. Use the browser network and security details to find the blocked or failed stage.
Distinguish network failure from HTTP failure
Fetch normally resolves for HTTP statuses such as 400, 404, and 500. Application code must inspect response.ok or response.status. Promise rejection instead points toward DNS, connection, TLS, CORS, mixed content, browser policy, cancellation, or an invalid request URL. Preserve the error and network entry without logging tokens or private bodies.
Inspect the browser network panel
Look for the actual request, an OPTIONS preflight, redirects, blocked-reason details, timing, and the final hostname. If no request appears, inspect URL construction, synchronous exceptions, extension blocking, Content Security Policy, and whether an AbortSignal was already canceled. If the request remains pending, examine connectivity, server response, and client timeout or cancellation logic.
Check CORS and preflight
A server can return a response that JavaScript is not allowed to read. Confirm that the requested origin, method, and non-safelisted headers are permitted. A failed OPTIONS route, missing Access-Control-Allow-Origin, credentialed wildcard, or redirect to a different CORS policy can surface as a generic Fetch error. Fix the server policy; adding mode: "no-cors" produces an opaque response and does not grant access.
Rule out mixed content and TLS
An HTTPS page should call an HTTPS API. Browsers block many active HTTP requests from secure pages. Open the API URL directly to inspect certificate validity, hostname, trust chain, and expiry, but remember that navigation and Fetch can follow different security rules. Do not disable certificate verification as a production fix.
Compare cURL and Fetch precisely
When cURL succeeds, compare final URL, method, body bytes, content type, authorization, cookies, redirects, proxy, and DNS environment. cURL is not restricted by browser CORS or service workers. Convert a known safe request using the cURL to Fetch tool, then remove unnecessary headers rather than copying browser-controlled headers manually.
Inspect service workers and browser state
A stale service worker can intercept requests, serve old routes, or fail its own network strategy. Test in a clean profile, unregister the worker for diagnosis, and check its console separately. Also consider privacy extensions, corporate proxies, captive portals, VPNs, and offline state. Re-enable each layer to identify the real cause.
Handle cancellation explicitly
Differentiate intentional AbortController cancellation from connectivity failure in application messaging. Bound requests with a clear timeout, clean up controllers when UI state changes, and do not retry an intentionally canceled operation. Retry only safe operations with a small backoff budget.
Frequently Asked Questions
Does Fetch reject on HTTP 500?
Normally no. Check response.ok yourself.
Can CORS cause this message?
Yes. The browser can hide a disallowed response.
Why does cURL still work?
It does not use browser CORS, mixed-content, or service-worker rules.