Should NetworkError extend TypeError?

Is your feature request related to a problem? Please describe. A fetch() promise will reject with a TypeError when a network error is encountered or CORS is misconfigured on the server-side (mdn, but MSW throws a custom type NetworkError, which extends Error. This makes it impossible to test error handling depending on instanceof TypeError checks to handle for network errors explicitly.

catch (error) {
  if (error instanceof TypeError) return doSomething()
  throw error;
}

Describe the solution you’d like I think NetworkError should extend the more specific TypeError instead of Error.

export class NetworkError extends TypeError {
  constructor(message: string) {
    super(message)
    this.name = 'NetworkError'
  }
}

Describe alternatives you’ve considered Add an option to throw specific kinds of errors.

ctx.throw(new TypeError('something went wrong'))

Additional context https://github.com/mswjs/msw/blob/2e7ecd87e5568c6e59a408e812535f088498e437/src/utils/NetworkError.ts#L1-L6

https://github.com/mswjs/msw/blob/2e7ecd87e5568c6e59a408e812535f088498e437/src/mockServiceWorker.js#L210-L217

[kettanaito]

Hey, @smeijer. Zero time has been wasted! It was a great opportunity for me to refresh my knowledge of client errors. Yeah, in the end, each request client is in charge of what errors it produces. This also makes sense that node-fetch would have a difference with window.fetch as it’s, effectively, a different client based on Fetch API.

[smeijer]

Let’s check what class does the network error have when it happens without MSW around. We should adhere to that class in MSW as well.

So that would be TypeError. If you’d run a fetch while being offline or blocking the URL, the browser throw an Error of instance TypeError. That’s the use case I need to be able to reproduce, so I can inform the user that something is wrong with the network, and not so much with the entered form data or our server.

fetch('/graphql', { method: 'POST' }).catch((e) => { 
  console.log('is TypeError', e instanceof TypeError);
})

https://user-images.githubusercontent.com/1196524/150381407-e033fb8c-5d82-4e6e-ad29-78392edbf1b1.mp4

Can you tell me more about when do you need this?

I don’t need the option to throw any other errors than TypeError to be honest. The issue template asked about alternatives, so that would be one. Feel free to ignore that.