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

user_5776910 0
Viewed 4234

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
fetch-api javascript
3 Answers

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

user_131751380

I don't have a perfect answer for you. The easiest to solve your exact problem is to implement isNetworkError as a type guard. Maybe there are common properties on the firefox/chrome error object that allows you to detect specifically these.

The alternative is to catch errors closer to where you do fetch() and rethrow the error as something you can easily detect.

async function getData() {
  let response;

  try {
    response = await fetch(/* ... */);
  } catch (err) {
    throw new MyCustomNetworkError(err.message);
  }
  const json = await response.json();
  return transform(json);
}
user_809110

As a client of fetch, you can't differentiate network errors from other errors caused by building an incorrect request, as both are thrown as TypeErrors. (See https://developer.mozilla.org/en-US/docs/Web/API/fetch#exceptions).

This is quite a flaw, as application defects that cause an incorrectly built request may go unnoticed, masked as if they were circumstantial network errors.

user_5037850
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