MQTTJS example bridge from https://github.com/mcqn/mqtt-socketio-bridge

index.js

// mqtt-socketio-bridge
//
// (c) Copyright 2018 MCQN Ltd
//

var mqtt = require('mqtt')
var express = require('express')
var app = express()
var http = require('http').Server(app)
var io = require('socket.io')(http)

// Port that the web server should listen on
var port = process.env.PORT || 3000;

// Enter details of the MQTT broker that you want to interface to
// By default we'll use the public HiveMQ one, so any Arduino clients using
// the PubSubClient library can easily talk to it
var MQTT_BROKER = process.env.MQTT_BROKER || "mqtt://161.35.167.71"

var options = {
        protocolId: 'MQTT', // Or 'MQIsdp' in MQTT 3.1 and 5.0
        protocolVersion: 4, // Or 3 in MQTT 3.1, or 5 in MQTT 5.0
        clean: true, // Can also be false
        clientId: 'my-device34242',
        username: 'yourUserName',
        password: Buffer.from('Qdfs867&Jf8RFayL'), // Passwords are buffers
}

// We need to cope with wildcards in topics, so can't just do a simple comparison
// This function returns true if they match and false otherwise
// topic1 can include wildcards, topic2 can't
var topicMatch = function(topic1, topic2) {
        // Switch our wildcards from MQTT style to Regexp style
        var matchStr = topic1.replace(/#/g, ".*")
        return (topic2.match("^"+matchStr+"$") != null)
}

var client = mqtt.connect(MQTT_BROKER, options);
client.on('connect', function() {
        console.log("Connected to "+MQTT_BROKER);
})

client.on('message', function(topic, payload) {
        console.log("topic: "+topic)
        console.log("payload: "+payload)
        // Send it to any interested sockets
        Object.keys(io.sockets.adapter.rooms).map(function(room_name) {
                // See if this room matches the topic
                if (topicMatch(room_name, topic)) {
                        // It does.  Send the message
                        for (var clientId in io.sockets.adapter.rooms[room_name].sockets) {
                                io.sockets.connected[clientId].emit('mqtt', { 'topic': topic, 'payload': payload.toString() })
                        }
                }
        })
})

io.sockets.on('connection', function(sock) {
        // New connection, listen for...
        console.log("New connection from "+sock.id)

        // ...subscribe messages
        sock.on('subscribe', function(msg) {
                console.log("Asked to subscribe to "+msg.topic)
                if (msg.topic !== undefined) {
                        sock.join(msg.topic)
                        if (io.sockets.adapter.rooms[msg.topic].length == 1) {
                                // We're the first one in the room, subscribe to the MQTT topic
                                client.subscribe(msg.topic)
                        }
                        // else someone is already here, so we'll have an MQTT subscription already
                }
                // FIXME else It'd be nice to report the error back to the user
        })

        // ...publish messages
        sock.on('publish', function(msg) {
                console.log("socket published ["+msg.topic+"] >>"+msg.payload+"<<")
                client.publish(msg.topic, msg.payload)
        })

        // ...and disconnections
        sock.on('disconnect', function(reason) {
                console.log("disconnect from "+sock.id)
                // The socket will have left all its rooms now, so see if there
                // are any empty ones
                for (var sub in client._resubscribeTopics) {
                        if (io.sockets.adapter.rooms[sub] == undefined) {
                                // There's no "room" for this subscription, so no clients
                                // are watching it, so we should unsubscribe
                                console.log("Unsubscribing from "+sub)
                                client.unsubscribe(sub)
                        }
                        // else someone is watching, so leave this MQTT subscription in place
                }
        })

})

// Serve static files from the 'static_files' folder
app.use(express.static('static_files'))

// Set up web server to serve 
app.get('/', function(req, res) {
        res.sendFile(__dirname+"/static_files/mqtt-socket.html")
})

http.listen(port, function() {
        console.log("listening on "+port)
})