<button id="buttonStartControlAlgorithm" onClick="startControlAlgorithm()">Start Ctrl Alg</button>

<button id="buttonStopControlAlgorithm" onClick="stopControlAlgorithm()">Stop Ctrl Alg</button>

function startControlAlgorithm () {

   socket.emit("startControlAlgorithm");

}

function stopControlAlgorithm () {

    socket.emit("stopControlAlgorithm");

}

var controlAlgorihtmStartedFlag = 0; // flag in global scope to see weather ctrlAlg has been started

var intervalCtrl; // var for setInterval in global space

intervalCtrl = setInterval(function() {controlAlgorithm(); }, 30); // na 30ms klic

if (controlAlgorihtmStartedFlag == 0)

controlAlgorihtmStartedFlag = 1; // set flag that the algorithm has started

    if (controlAlgorihtmStartedFlag == 0) {

        controlAlgorihtmStartedFlag = 1; // set flag that the algorithm has started

        intervalCtrl = setInterval(function() {controlAlgorithm(); }, 30); // na 30ms klic

        console.log("Control algorithm started");

    }

};

function stopControlAlgorithm () {

    clearInterval(intervalCtrl); // clear the interval of control algorihtm

    board.analogWrite(3,0); // write 0 on pwm pin to stop the motor

    controlAlgorihtmStartedFlag = 0; // set flag that the algorithm has stopped

};

ws.on("message", function (msgString) { // message comes as string -> msgString

    var msg = JSON.parse(msgString); // string from ws which comes as a string is put to JSON

    switch(msg.type) {

        case "startControlAlgorithm":

            startControlAlgorithm();

        break;

        case "stopControlAlgorithm":

            stopControlAlgorithm();                

        break;

    }

}); // end of wss.on code

var pwm = 0;

wss.broadcast(JSON.stringify({"type": "clientReadValues", "desiredValue": desiredValue, "actualValue": actualValue, "error": (desiredValue - actualValue), "pwm": (pwm).toFixed(0)}));

log(potValue1 + "|" + potValue2 + "|" + msg.error + "|" + msg.pwm);

var http = require("http").createServer(handler); // on req - hand

var fs = require("fs"); // variable for file system for providing html

var firmata = require("firmata");

const WebSocket = require('ws'); // for permanent connection between server and client

const wss = new WebSocket.Server({port: 8888}); // websocket port is 8888

wss.broadcast = function broadcast(data) {

  wss.clients.forEach(function each(client) {

      if (client.readyState === WebSocket.OPEN) {

    client.send(data);

      }

  });

};

wss.sendToLastWs = function sendToLastWs(data) {

    if (wss.clients[wss.clients.length-1] !== undefined) { // if websocket exist

        if (wss.clients[wss.clients.length-1].readyState === WebSocket.OPEN) { // if it is connected

            wss.clients[wss.clients.length-1].send(data); // send to the last connected client - to only one client's websocket

        }

    }

};

var messageJSON;

var controlAlgorithmStartedFlag = 0; // variable for indicating weather the Alg has benn sta.

var intervalCtrl; // var for setInterval in global scope

console.log("Starting the code");

var board = new firmata.Board("/dev/ttyACM0", function(){

    console.log("Connecting to Arduino");

    console.log("Enabling analog Pin 0");

    board.pinMode(0, board.MODES.ANALOG); // analog pin 0

    board.pinMode(1, board.MODES.ANALOG); // analog pin 1

    board.pinMode(2, board.MODES.OUTPUT); // direction of DC motor

    board.pinMode(3, board.MODES.PWM); // PWM of motor i.e. speed of rotation

    board.pinMode(4, board.MODES.OUTPUT); // direction DC motor

});

function handler(req, res) {

    fs.readFile(__dirname + "/example14.html",

    function (err, data) {

        if (err) {

            res.writeHead(500, {"Content-Type": "text/plain"});

            return res.end("Error loading html page.");

        }

    res.writeHead(200);

    res.end(data);

    })

}

var desiredValue = 0; // desired value var

var actualValue = 0; // variable for actual value (output value)

var pwm;

var factor = 0.1; // proportional factor that determines the speed of aproaching toward desired value

http.listen(8080); // server will listen on port 8080

board.on("ready", function() {

    board.analogRead(0, function(value){

        desiredValue = value; // continuous read of analog pin 0

    });

    board.analogRead(1, function(value) {

        actualValue = value; // continuous read of pin A1

    });

    startControlAlgorithm(); // to start control alg.

    wss.on('connection', function (ws, req) { // start of wss code

        messageJSON = {"type": "message", "content": "Srv connected, board OK"};

        ws.send(JSON.stringify(messageJSON));

        setInterval(sendValues, 40); // on 40ms we send message to client

        ws.on("message", function (msgString) { // message comes as string -> msgString

            var msg = JSON.parse(msgString); // string from ws which comes as a string is put to JSON

            switch(msg.type) {

                case "startControlAlgorithm":

                    startControlAlgorithm();

                break;

                case "stopControlAlgorithm":

                    stopControlAlgorithm();                

                break;

            }

        }); // end of wss.on code

    }); // end of sockets.on connection

}); // end of board.on(ready)

function controlAlgorithm () {

    pwm = factor*(desiredValue-actualValue);

    if(pwm > 255) {pwm = 255}; // to limit the value for pwm / positive

    if(pwm < -255) {pwm = -255}; // to limit the value for pwm / negative

    if (pwm > 0) {board.digitalWrite(2,1); board.digitalWrite(4,0);}; // določimo smer če je > 0

    if (pwm < 0) {board.digitalWrite(2,0); board.digitalWrite(4,1);}; // določimo smer če je < 0

    board.analogWrite(3, Math.abs(pwm));

};

function startControlAlgorithm () {

    if (controlAlgorithmStartedFlag == 0) {

        controlAlgorithmStartedFlag = 1;

        intervalCtrl = setInterval(function(){controlAlgorithm();}, 30); // call the alg. on 30ms

        console.log("Control algorithm has been started.");        

    }

};

function stopControlAlgorithm () {

    clearInterval(intervalCtrl); // clear the interval of control algorihtm

    board.analogWrite(3, 0);

    controlAlgorithmStartedFlag = 0;

    console.log("Control algorithm has been stopped.");

};

function sendValues () {

    wss.broadcast(JSON.stringify({"type": "clientReadValues", "desiredValue": desiredValue, "actualValue": actualValue, "error": (desiredValue - actualValue), "pwm": (pwm).toFixed(0)}));

};

<!DOCTYPE html>

<meta charset = utf8>

<html>

<head>

<title>Example with potentiometers</title>

</head>

<body onload="load()";>

<div>

<canvas id="canvas1" width ="200" height = "100" style="border: 1px dashed #00c3c3;"></canvas>    

</div>

<button id="buttonStartControlAlgorithm" onClick="startControlAlgorithm()">Start Ctrl Alg</button>

<button id="buttonStopControlAlgorithm" onClick="stopControlAlgorithm()">Stop Ctrl Alg</button>

<div id="print1"></div>    

<br>

  <script type="text/javascript">

  "use strict"; // in order to use clasess

var divElement = document.getElementById("print1"); // var for div el.

var numberOfLinesBeforeScroll = 10;

var linesPrintCounter = 0;

var potValue1 = 0; // value of the first potentiometer

var x1 = new Array(); // array for x1

var y1 = new Array(); // array for y1

var potValue2 = 0; // value of second potentiometer

var x2 = new Array(); // array for x2 variable

var y2 = new Array(); // array for y2 variable

var canvas1;

var ctx1;     

function log(msg) {

  var node=document.createElement("tr"); // we create the variable node as the a table row (tr)

  var textnode=document.createTextNode(linesPrintCounter + " | " + msg); // we create element with the text adding the counter

  node.appendChild(textnode); // adding text to "node", i.e. table row

  divElement.insertBefore(node, divElement.childNodes[0]); // inserting into variable node

  if (linesPrintCounter > numberOfLinesBeforeScroll-1) { // if the lines are more than limit -> start with scroll

      divElement.removeChild(divElement.childNodes[numberOfLinesBeforeScroll]); // we remove the oldest printout

  }

  linesPrintCounter++; // increasing the number of printouts

}

let ws = new WebSocket("ws://192.168.254.149:8888"); // create socket - connect to it

var messageJSON;

function load() { // function that is started, when we open the page

    canvas1 = document.getElementById("canvas1");

    ctx1 = canvas1.getContext("2d");

    // initialization of graph with points

    ctx1.lineWidth = "1";

    ctx1.strokeStyle = "#ff0000";

    // initializaction of first time series

    for (var i=0; i<200; i++) {

        x1[i] = i; // for x values are 0, 1, 2, ...

        y1[i] = 0; // for y values are 0

    };

    // initializaion of second graph

    for (var i=0; i<200; i++) {

        x2[i] = i; // x2 values are 0, 1, 2, ...

        y2[i] = 0; // for y2 values are 0

    };

};

ws.onmessage = function(event) {

    var msg = JSON.parse(event.data); // string from ws is put to JSON

    switch(msg.type) {

        case "message":

            log(msg.content); // add msg to div

        break;

        case "clientReadValues":

            potValue1 = msg.desiredValue;

            potValue2 = msg.actualValue;

            // Draw first graph *****************************************

            ctx1.clearRect(0, 0, canvas1.width, canvas1.height); // clear the canvas

            ctx1.lineWidth = "1";

            ctx1.strokeStyle = "#ff0000";

            ctx1.beginPath(); // to start drawing new line

            y1.splice(0, 1); // on the position 0 in the field y1 we erase one value

            y1[199] = potValue1; // new value is added at the end

            for (var i=0; i<200; i++) {

                ctx1.lineTo(x1[i], (100 - (y1[i] / 1023) * 100)); // 0,0 x,y coordinate is in upper left corner

            };

            ctx1.stroke(); // to draw the line

            // End of draw graph***********************************

            // Draw second graph **********************************************

            ctx1.lineWidth = "1";

            ctx1.strokeStyle = "#00ff00";

            ctx1.beginPath(); // to start drawing new line

            y2.splice(0, 1); // on the position 0 in the field y2 we erase one value

            y2[199] = potValue2; // new value is added at the end

            for (i=0; i<200; i++) {

                ctx1.lineTo(x2[i], (100 - (y2[i] / 1023) * 100)); // 0,0 x,y coordinate is in upper left corner

            };

            ctx1.stroke(); // to draw the line

            // Draw second graph graph END ************************************    

            log(potValue1 + "|" + potValue2 + "|" + msg.error + "|" + msg.pwm);

        break;        

    }

};

function startControlAlgorithm () {

    messageJSON = {"type": "startControlAlgorithm"};

    ws.send(JSON.stringify(messageJSON)); // we have to stringify JSON to send it over websocket    

}

function stopControlAlgorithm () {

    messageJSON = {"type": "stopControlAlgorithm"};

    ws.send(JSON.stringify(messageJSON)); // we have to stringify JSON to send it over websocket    

}

</script>

</body>

</html>