server.js

// server.js
const next = require("next");
const app = next({ dev: process.env.NODE_ENV !== "production" });
const handler = app.getRequestHandler();

// With express
const express = require("express");
// With Express Peer Server
const ExpressPeerServer = require("peer").ExpressPeerServer;

app.prepare().then(() => {
    var expressApp = express();
    var server = require("http").createServer(expressApp);

    // Signalling server
    var expressPeerServer = ExpressPeerServer(server, {
        allow_discovery: true,
        debug: true
    });

    expressApp.use("/peerjs", expressPeerServer);

    var connected = [];
    expressPeerServer.on("connection", function (id) {
        var idx = connected.indexOf(id); // only add id if it's not in the list yet
        if (idx === -1) {
            connected.push(id);
        }
    });

    expressPeerServer.on("disconnect", function (id) {
        var idx = connected.indexOf(id); // only attempt to remove id if it's in the list
        if (idx !== -1) {
            connected.splice(idx, 1);
        }
    });

    expressApp.get("/peerjs/:channel", function (req, res) {
        var channel = req.params.channel;
        return res.json(
            connected.filter(item => {
                var ch = item.split("-");
                return ch.length === 2 && ch[0] === channel;
            })
        );
    });

    console.log("listening on port 3000");
    expressApp.use(handler);
    server.listen(3000);
});