Serial Output from P5.js - Arduino and p5js code

OutputFromP5js.ino

// Arduino code for Redboard
const int ledPin = 5; // the pin that the LED is attached to
int incomingByte;      // a variable to read incoming serial data into

void setup() {
  // initialize serial communication:
  Serial.begin(9600);  // note baudrate set at 9600. Other speeds available/must be matched in P5.js options
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
}

void loop() {
  // see if there's incoming serial data:
  if (Serial.available() > 0) {
    // read the oldest byte in the serial buffer:
    incomingByte = Serial.read();
    // if it's a capital H (ASCII 72), turn on the LED:
    if (incomingByte == 'H') {
      analogWrite(ledPin, 255);
    }
    // if it's an L (ASCII 76) turn off the LED:
    if (incomingByte == 'L') {
      analogWrite(ledPin, 0);
    }
    else {
      analogWrite(ledPin,incomingByte);
    }
   
    Serial.write(incomingByte);
  }
}

sketch.js

// Be sure to choose File > Import Library > p5.serialport.js from the P5 IDE main menu to load serial library

// Terminal command to install p5.serialserver:  npm install p5.serialserver
// Terminal command to start server:  node ~/node_modules/p5.serialserver/startserver.js

var serial;                            // variable to hold an instance of the serialport library
var options = { baudrate: 9600};      // set baudrate to 9600; must match Arduino baudrate
var portName = '/dev/cu.usbserial-DA00WVK6'; // fill in your serial port name here
var inData;                            // for incoming serial data
var outByte = 0;                       // for outgoing data
var slider;
function setup() {
  createCanvas(400, 300);          // make the canvas
  serial = new p5.SerialPort();    // make a new instance of the serialport library
  serial.on('data', serialEvent);  // callback for when new data arrives
  serial.on('error', serialError); // callback for errors
  serial.open(portName, options);           // open a serial port @ 9600 
  
  slider = createSlider(0, 255, 100);
  slider.position(30, 50);
  slider.changed(transmitSlider);
}

function draw() {
  // black background, white text:
  background(0);
  fill(255);
  // display the incoming serial data as a string:
  text("incoming value: " + inData, 30, 30);
  outByte = slider.value();
}

function transmitSlider(){
  serial.write(outByte);
}


function keyPressed() {
  if (key ==='H' || key ==='L') {   // if the user presses H or L
    serial.write(key);      // send it out the serial port
    text("incoming value: " + key, 30, 30);
  }
}

function serialEvent() {
  // read a byte from the serial port:
  var inByte = serial.read();
  // store it in a global variable:
  inData = inByte;
}

function serialError(err) {
  println('Something went wrong with the serial port. ' + err);
}