How to properly refuse websocket upgrade request?

Question

Sometimes I want to refuse a http client's request to upgrade connection to websocket.

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Code

(using go's Gin and gorilla/websocket framework:)

To allow upgrade:

c, err := ctl.upgrader.Upgrade(ctx.Writer, ctx.Request, nil)
err = c.WriteJSON(resp)

To refuse upgrade (due to invalid request params):

if contentId == "" || !exists {
    // FIXME: provide response in a way that ws client can recognize & show tip?
    ctx.String(http.StatusBadRequest, "invalid or non-existing contentId")
    return
}

Explaination: Here to refuse the upgrade I just return a http 400 code, then terminate the connection, and didn't do the upgrade at all.

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The issue

The problem to refuse websocket upgrade request with about code is that, the websocket client (e.g js), can't read data (text or json) in my response.

Code - client-side (js):

ws.onerror = function (evt) {
    // TOOD: handle error, (e.g print error msg?),
    print("ERROR");
}

It does print the "ERROR" on refuse, but after checking chrome developer tool about the evt object, can't find a way to get server response data, so I can't show tip to frontend UI with reason of refuse.

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Questions

• How to refuse websocket upgrade request properly, and let client be able to receive the returned reason/data ? (e.g client is js, server is go / gin / gorilla/websocket).
• Is there a better way to refuse websocket upgrade request, other than return http code like 400?