NetworkError for a simple HTTP 404?

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1

Hi, folks. I’m rediscovering Elm. I’m adapting the “Book” example to display the details of the Http.Error on error responses. I was very surprised to discover that when I try to request text from this URL, I get NetworkError instead of BadStatus 404.

https://elm-lang.org/assets/public-opinion-this-does-not-exist.txt

To be clear, I changed only the URL in the init function:

init : () -> ( Model, Cmd Msg )
init _ =
    ( Loading
    , Http.get
        { url = "https://elm-lang.org/assets/public-opinion-this-does-not-exist.txt"
        , expect = Http.expectString FetchPublicOpinionResponse
        }
    )

As far as I can tell, that’s a perfectly cromulent URL. Both Firefox and cURL answer with HTTP Status 404.

Is there a bug or is the problem coming from inside the brain?

Thanks.

2

Hi!

From my limited testing (see https://ellie-app.com/pH4j93hrLqMa1, the request is done when the application starts), it seems that this URL responds with a restrictive access-control-allow-origin https://worker.elm-lang.org, which means any request done from JavaScript is going to get rejected, unless it’s initiated by code living at https://worker.elm-lang.org.

This is likely why you’re seeing a NetworkError. You’re seeing a regular 404 when you make that request from more traditional means (direct browser request or curl) because CORS enforcement does not work the same in those cases.

Hope that helps!

3

Thank you! Based on your description, I’d expect even a successful request to be rejected due to CORS, but the text loads when the URL is correct. I admit, I don’t understand CORS yet. :person_shrugging:

Huh. It seems to be something about the server that’s serving these URLs. When I request the document at the correct URL, I see

access-control-allow-origin: *

and when I request a URL that almost certainly doesn’t have a document on the server, I see

access-control-allow-origin: https://worker.elm-lang.org

I guess the server is doing something sensible: explicitly allowing CORS for the document it knows it has and denying it otherwise.

Did I get it? :person_shrugging:

4

I don’t know for sure but your understanding of the situation makes sense to me =)