Cloudflare Developers•6d ago

WebSockets: Uncaught (in response) Error: The script will never generate a response.

I have a very simple function that listens for WebSocket connections and sends the current time whenever a message is received:

export const onRequest: PagesFunction = context => {
  const upgradeHeader = context.request.headers.get("Upgrade")
  if (upgradeHeader !== "websocket") {
    return new Response("Expected websocket", { status: 400 })
  }

  const [client, server] = Object.values(new WebSocketPair());
  
  server.accept();
  server.addEventListener("message", () => {
    server.send((Date.now()).toString());
  });

  return new Response(null, {
    status: 101,
    webSocket: client
  });
}

export const onRequest: PagesFunction = context => {
  const upgradeHeader = context.request.headers.get("Upgrade")
  if (upgradeHeader !== "websocket") {
    return new Response("Expected websocket", { status: 400 })
  }

  const [client, server] = Object.values(new WebSocketPair());
  
  server.accept();
  server.addEventListener("message", () => {
    server.send((Date.now()).toString());
  });

  return new Response(null, {
    status: 101,
    webSocket: client
  });
}

The script is working fine but every time the WebSocket is closed by the client, the following error is logged:

✘ [ERROR] Uncaught (in response) Error: The script will never generate a response.

✘ [ERROR] Uncaught (in response) Error: The script will never generate a response.

The only information I've seen about this error has to do with either Promises or external libraries, neither of which I am using so I am confused as to what is causing this error message.

1 Reply

LeftyLlamaOP•6d ago

I figured it out, the issue is that there that the WebSocket is never properly closed, meaning that the function reaches a state where it can never be resolved. Adding a listener like this fixes it:

server.addEventListener("close", () => {
  server.close();
});

server.addEventListener("close", () => {
  server.close();
});

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WebSockets: Uncaught (in response) Error: The script will never generate a response.