teletap/server.js

// Import dependencies
const { WebSocket, WebSocketServer } = require("ws");
const express = require("express");
const dotenv = require("dotenv");
dotenv.config();

// Setup Environmental Variables
const PORT = process.env.PORT || 3000;
const PREFIX = process.env.PREFIX || "";
const PUBLIC = process.env.PUBLIC || "";

// Setup Express Router
// Create the routes of the application
// Here are two pages and a wildcard:
// at / there is the index.html page, where to draw
// at /destination there is the destination.html page, where to receive the drawings
// the wildcard /* serves the static files from the public folder

const router = express.Router();
const routes = (app) => {
	app.get("/", (req, res) => {
        res.render("index", {
			address: PREFIX,
		});
	});
	app.get("/destination", (req, res) => {
		res.render("destination", {
			address: PREFIX,
		});
    });
	app.get("/*", (req, res) => {
		res.sendFile(req.url, { root: "public" });
	});
	return app;
};

// Setup the Express server
const server = express()
	.set("view engine", "html")
	.engine("html", require("hbs").__express)
	.use(PREFIX, routes(router, {}))
	.use(express.static("public"))
	.listen(PORT, () => console.log(`Listening on ${PORT}`));

// Setup the Websocket server on top of the Express one
const wss = new WebSocketServer({ server, clientTracking: true });

// Global variables to manage the connected Destination and User clients
let DESTINATIONS = new Set();
let USERS = new Set();
var theme = "";

// The message processor defines which function is associated to every websocket message type.
// It map a type to a function, passing some optional parameters such as the message itself and the websocket client that sent it.
// for example an incoming message like {type: hello} will trigger the registerDest(ws, msg) function.
// In this way to add message types and functionalities gets easier, and avoid long chain of if-else statements.

const messageProcessor = {
	default: (ws, msg) => unknownMsg(msg),
	hello: (ws, msg) => registerDest(ws, msg),
    pulse: (ws, msg) => toDest(msg)
};

// Message processor functions

// Default function, to cactch unkown message types
const unknownMsg = (msg) => {
	console.log("Unknown message type...");
	console.log(msg);
};

// Add the ws client in the destinations set
// Removing it when it disconnets
const registerDest = (ws, msg) => {
	console.log("Destination client connected");
	DESTINATIONS.add(ws);
	ws.on("close", () => {
		DESTINATIONS.delete(ws);
	});
};

// Send a message to all the connected Destinations
const toDest = (msg) => {
	let message = JSON.stringify(msg);
	DESTINATIONS.forEach((DESTINATION) => {
		if (DESTINATION?.readyState === WebSocket.OPEN) {
			DESTINATION.send(message);
		}
	});
};

// Send a message to all the connected Users
const broadcast = (msg) => {
	let message = JSON.stringify(msg);
    console.log('broadcasting: ', message)
	for (const user of USERS.values()) {
		if (user?.readyState === WebSocket.OPEN && !DESTINATIONS.has(user)) {
			user.send(message);
		}
	}
};

// Websocket events listener
wss.on("connection", (ws) => {
	USERS.add(ws);
	ws.send(JSON.stringify({ type: "theme", theme: theme }));

	ws.on("message", (data) => {
		// Parse the incoming data safely
		let message;
		try {
			message = JSON.parse(data);
		} catch (e) {}

		// Call the message processor, eventually falling back to use the default function if the type is not defined.
		if (message) {
			(messageProcessor[message?.type] || messageProcessor.default)(ws, message);
		}
	});

	ws.on("close", () => {
		USERS.delete(ws);
	});
});