NetworkError when attempting to fetch resource

What the error means

Your code calls fetch(), Firefox throws TypeError: NetworkError when attempting to fetch resource., and the catch block has nothing more to go on. That is on purpose. When a request fails for a network or security reason, browsers give scripts one generic error, so a page cannot probe which servers exist on your network. Chrome calls the same thing “Failed to fetch”, and Safari says “Load failed”. Our guide to what “Failed to fetch” means compares the three.

The details still exist. Firefox writes them to the console as a separate line, and the Network tab shows what happened to the request.

Twelve causes, one error

We set up small test servers and made fetch() fail in every way we could think of, in Firefox, then recorded the console line that came with each failure.

Six of the twelve rows have nothing to do with CORS: the server was down, the domain did not exist, the connection broke, the browser was offline, the certificate was self-signed, or an extension stopped the request. Firefox still labelled all six “CORS request did not succeed”. MDN's page for that message says the error “is not directly related to CORS”, and it lists the same kind of network failures. If you see it, check that the server is reachable before you touch any CORS settings.

The other six are about permission. A missing Access-Control-Allow-Origin header, one that names another site, or a * combined with credentials: 'include' are all server settings. Firefox shows the status code in the first case, and seeing Status code: 200 next to a failure is a strong hint that the server did its job and only the header is missing.

The form that reloads under its own request

One cause comes from the form's own script rather than the network. If a submit handler calls fetch() but does not call event.preventDefault(), the browser also does its normal form submit and reloads the page. Firefox cancels the request that was still running, and the handler logs the NetworkError just before the page goes away. In our test the page reloaded to /?a=1 and the last thing in the console was form fetch rejected: TypeError: NetworkError when attempting to fetch resource.

So the server may have received the message, or may not, and the user sees a page that simply reloaded with their text gone.

// The bug: fetch() runs, but the form also submits and the page reloads under it.
form.addEventListener("submit", () => {
  fetch(url, { method: "POST", body: new FormData(form) });
});

// The fix: stop the browser's own submit first.
form.addEventListener("submit", async (event) => {
  event.preventDefault();
  const response = await fetch(url, { method: "POST", body: new FormData(form) });
  // show a message based on response.ok
});

How to find your cause

Open the console (Ctrl+Shift+K, or Cmd+Option+K on a Mac) and read the line just before the NetworkError. Then open the Network tab, send the form again and click the failed request. A request with no status and a transfer error means it never got an answer: check the address, the server and your extensions, and try a private window. A request with a status code means the server answered and the browser blocked the answer, so the fix is a header on the server.

If the request is an OPTIONS one, that is the preflight Firefox sends before a JSON POST. It must answer with the CORS headers too, not with a 404 as in our test. Sending the form as normal FormData instead of JSON avoids the preflight altogether.

A form that does not depend on fetch()

A plain HTML form that posts to a form backend has none of these failure modes, because the browser's normal submit is not subject to CORS. Formgong takes that post, emails the message, keeps it in an inbox, and shows the visitor a thank-you page. If you prefer to send with fetch(), the endpoint sends the CORS headers, so the permission rows in the table do not apply.

<form action="https://formgong.com/submit" method="POST">
  <input type="hidden" name="access_key" value="fk_your_access_key">
  <input type="text" name="botcheck" style="display:none" tabindex="-1" autocomplete="off">
  <label>Email <input type="email" name="email" required></label>
  <label>Message <textarea name="message" required></textarea></label>
  <button type="submit">Send</button>
</form>
<!-- Works without JavaScript. Formgong answers CORS for fetch() too. -->

Seeing a console line we did not list?

Paste it to us at support@formgong.com with your Firefox version. We will reproduce it and add a row.

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