Server Sent Events not receiving messages from stream with React and NodeJs

Question

I have a very simple set up.
NodeJs

export default class StreamController {
    public static newMessage = async (req: Request, res: Response) => {
        const { userid } = req.params;
        res.writeHead(200, {
            "Connection": "keep-alive",
            "Content-Type": "text/event-stream",
            "Cache-Control": "no-cache",
        });
        setInterval(async () => {
            console.log(await MessagesController.hasNewMessage(userid));
            res.write(`${JSON.stringify({ hasUnread: await MessagesController.hasNewMessage(userid) })}`);
            res.write("\n\n");
        }, 5000);
    }
}

React

 constructor() {
        super();
        const uid = getUserId();
        const eventSource = new EventSource(`http://localhost:8080/stream/messages/${uid}`)

        eventSource.onmessage = (e) => {
            console.log(e)
        }
    }

I can see that stream was opened but none event was passed from the server, while data was issued on the server side. What I'm doing wrong?

Answer 1

You are passing the data on server side, but are not passing the event name as event. This is how I solved the issue (simplified reproduction):

Server:

app.get('/stream', (req, res) => {
  console.log('request received');
  res.writeHead(200, {
    "Connection": "keep-alive",
    "Content-Type": "text/event-stream",
    "Cache-Control": "no-cache",
  });
  setInterval(async () => {
    res.write('event: ping\n');  // added these
    res.write(`data: ${JSON.stringify({ hasUnread: true })}`);
    res.write("\n\n");
  }, 5000);
});

Client:

constructor(props) {
  super(props);
  const eventSource = new EventSource(`http://localhost:3030/stream`);
  eventSource.onopen = e => {
    console.log(e);
  }
  eventSource.onmessage = e => {
    console.log('onmessage');
    console.log(e);
  }
  eventSource.addEventListener('ping', e => {
    console.log(e);
  });
}

The ping event will be firing every 5 seconds.

Now, if you want the eventSource.onmessage to be called, you should name the event as message on server side:

...
setInterval(async () => {
  res.write('event: message\n');  // message event
  res.write(`data: ${JSON.stringify({ hasUnread: true })}`);
  res.write("\n\n");
}, 5000);
...

Now you will see in the console:

onmessage MessageEvent {..., data: "{'hasUnread': true}", ...}

Note that in this case the ping event won't be fired any more. More info here.