While connecting to a websocket server i set a subprotocol value.

this.websocket = new ReconnectingWebSocket(connUrl, [subprotocol]);

It’s up to server to check whether the subprotocol is allowed and valid. But if it isn’t valid, how can the server provide this info to client? I control the server and this is the check on server-side:

server.on("upgrade", (request, socket, head) => {
        if (request.headers["sec-websocket-protocol"] !== validProtocol) {
                socket.end(
                        "HTTP/1.1 400 Bad Request" +
                                "Connection: closern" +
                                "Content-Type: text/plainrn" +
                                "Content-Length: 25rn" +
                                "rn" +
                                "Subprotocol not supported"
                );
                socket.destroy();
                return;
        }

        this.wss.handleUpgrade(request, socket, head, ws => {
                this.wss.emit("connection", ws, request);
        });
});

Connecting work correctly, if protocol is correct, connection is made, otherwise, client has an error and no connection.
The issue is that on the client, I don’t have access to the headers or response from the socket.end() call. So how otherwise can the server let the client know, that protocols are wrong?