Correct way to identify a "fetch error"

[janeklb]

using Node.js 18.17.1

After switching from node-fetch to the native fetch now available in node, im unsure how to best identify "fetch errors".

In node-fetch there was such a thing as FetchError. This is of course not a whatwg fetch spec concept -- just something that node-fetch included as an extension. That said, it was handy because it allowed me to easily determine if something was a "fetch error" by simply evaluating error instanceof FetchError (upon which i could access error.code etc. while keeping TypeScript happy).

Using the native fetch, errors appear as follows:

> fetch('http://xxx.yyy.com').catch((e) => console.log(e));
Promise {
  <pending>,
  [Symbol(async_id_symbol)]: 1867,
  [Symbol(trigger_async_id_symbol)]: 1840
}
> TypeError: fetch failed
    at Object.fetch (node:internal/deps/undici/undici:11576:11)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
  cause: Error: getaddrinfo ENOTFOUND xxx.yyy.com
      at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:108:26)
      at GetAddrInfoReqWrap.callbackTrampoline (node:internal/async_hooks:130:17) {
    errno: -3008,
    code: 'ENOTFOUND',
    syscall: 'getaddrinfo',
    hostname: 'xxx.yyy.com'
  }
}

Could anyone help with the following Qs:
1/ What's the reasoning behind this being a TypeError? Feels misleading..
2/ Is the best way to identify failed fetches to check error instanceof TypeError and error.message === 'failed fetch', and then retrieve error.cause?
3/ Is there a (internal?) type for error.cause? I'm trying to come up with a good (TypeScript-safe) way to get at the error.type and error.code properties in order to tag my application metrics, and am not sure if there's an existing type that fits the cause (I'm aware of the megathread in DT about having TS types for fetch but wondering if there's maybe an undici type that is exposed somewhere?)

[axel7083]

I also face similar issue, when working with https://github.com/octokit/request-error.js they seems to pass fetch underlining error to the cause, so we get something like

> error.cause.cause
Error: getaddrinfo EAI_AGAIN api.github.com
    at GetAddrInfoReqWrap.onlookupall [as oncomplete] (node:dns:120:26)
    at GetAddrInfoReqWrap.callbackTrampoline (node:internal/async_hooks:130:17) {
  errno: -3001,
  code: 'EAI_AGAIN',
  syscall: 'getaddrinfo',
  hostname: 'api.github.com'

I was looking for a way to type the fetch error, so I can check for which syscall failed, and guess that the problem is related to network issue.