How do i send a function from the server to the client

Im new to post requests in NodeJS & ExpressJS, Im trying to pass in a function (not the function's result) to get called in the client, I have this listener and this post request (Im trying to make a discord bot webview tool)

The Server-Side

app.post("/dashboardstartup", jsonParser, (req, res) => {
    // authClient is a discord client (Discord Bot)

    if(authClient)
    {
        authClient.on("messageCreate", (message) => {
             // always send the message once its created
             res.json({
                 message
             })
             // the problem is that when I send the result the fetch request from the client stops
        });
    }
})

The Client-Side

// ...

await fetch("http://localhost:8080/dashboardstartup", {
     method: "POST";
     headers: {
         'Content-type': 'application/json'
     }

}).then(a=>a.json()).then(a=>{
   // once the fetch response is received, the fetch stops and the client can't listen to messages anymore

  console.log(a.messsage) // prints out "Hello world!"
})

> And is there any better way to do this, than fetch method?

I understand your goal of creating a real-time communication channel between your Discord bot and the client using Node.js and ExpressJS. While you attempted to use a POST request to send updates from the server to the client, it's important to note that HTTP POST is not suitable for real-time communication due to its one-time request-response nature.

To achieve real-time updates, I recommend using WebSockets, as they provide full-duplex communication, making it possible to establish a continuous connection between the server and the client.

Here's an outline of how you can implement this:
On the server side (using ws library for WebSocket communication):

const WebSocket = require('ws');

const wss = new WebSocket.Server({ port: 8080 });

// Simulating message creation in your Discord bot
function simulateMessageCreation() {
 return "Hello world!";
}

wss.on('connection', (ws) => {
// Send a message to the client whenever a message is created
 authClient.on("messageCreate", (message) => {
   const messageContent = simulateMessageCreation();
   ws.send(JSON.stringify({ message: messageContent }));
 });
});

On the client side (using WebSocket API in the browser):

const ws = new WebSocket("ws://localhost:8080");

ws.onmessage = (event) => {
 const data = JSON.parse(event.data);
 console.log(data.message); // prints out "Hello world!" when a new 
message is created
};

By using WebSockets, you can establish a persistent connection between the server and the client, enabling real-time updates. This way, your client application will be able to listen for messages continuously without the need for repeated HTTP requests.

Remember to install the required dependencies, such as the ws library, using npm before running the server.

WebSockets are a powerful tool for enabling real-time communication in web applications, and I encourage you to explore their capabilities further.

You can make use of the native response.write() method.

The response.json() method comes from the Express.js package and it internally closes the connection after sending the response.

Here's an example of how you can use .write():

authClient.on("messageCreate", (message) => {
             // always send the message once its created
             res.write(JSON.stringify({ message }), 'utf8');
        });

It doesn't close the connection until you manually do it and the client will keep listening for messages. So remember to end the response when authClient ends as well:

authClient.on('close', () => res.end());