Fetch API: how to determine if an error is a network error

So I have some code like this:

async function getData() {
  const response = await fetch(/* ... */);
  const json = await response.json();
  return transform(json);
}

Where transform can throw some of its own errors.

I'm try to catch for network errors from the fetch API.

try {
  const data = await getData();

  // ...
  return // ...
} catch (e) {
  if (isNetworkError(e)) {
    return localStorage.getItem('...');
  }

  throw e;
}

My question is how do I implement isNetworkError that works across browsers? Note: that this should only return true if the network is offline.

It seems like both chrome and firefox throws a TypeError but the messages they have are different on each.

Firefox: TypeError: "NetworkError when attempting to fetch resource."
Chrome: TypeError: Failed to fetch

Solution

If the first promise rejects, it's a network error. That's the only time it does.

The Promise returned from fetch() won’t reject on HTTP error status even if the response is an HTTP 404 or 500. Instead, it will resolve normally (with ok status set to false), and it will only reject on network failure or if anything prevented the request from completing.

From Mozilla developer page: https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API

Edit: As pointed out in the comments and other answers, you should also pay attention to the line "or if anything prevented the request from completing". Which means that the initial Promise of fetch will reject on network errors in addition to other problems. Like for example, an invalid URL or a CORS error.

If fetch is able to successfully reach the server with your request, it will resolve the first Promise successfully, otherwise the first promise will reject. In the case of CORS, the error occurs before your request is actually sent out (in the OPTIONS request), which is why the error occurs in the first Promise.