LED:

// variable to hold an instance of the p5.webserial library:
const serial = new p5.WebSerial();

// HTML button object:
let portButton;
let inData; // for incoming serial data
let outByte = 0; // for outgoing data

function setup() {
  createCanvas(400, 300); // make the canvas
  // check to see if serial is available:
  if (!navigator.serial) {
    alert("WebSerial is not supported in this browser. Try Chrome or MS Edge.");
  }
  // if serial is available, add connect/disconnect listeners:
  navigator.serial.addEventListener("connect", portConnect);
  navigator.serial.addEventListener("disconnect", portDisconnect);
  // check for any ports that are available:
  serial.getPorts();
  // if there's no port chosen, choose one:
  serial.on("noport", makePortButton);
  // open whatever port is available:
  serial.on("portavailable", openPort);
  // handle serial errors:
  serial.on("requesterror", portError);
  // handle any incoming serial data:
  serial.on("data", serialEvent);
  serial.on("close", makePortButton);
}

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

function mouseDragged() {
  // map the mouseY to a range from 0 to 255:
  outByte = byte(map(mouseY, 0, height, 0, 255));
  // send it out the serial port:
  serial.write(outByte);
}

function keyPressed() {
  if (key >= 0 && key <= 9) {
    // if the user presses 0 through 9
    outByte = key * 25; // map the key to a range from 0 to 225
    serial.write(outByte); // send it out the serial port
  }
}

// if there's no port selected,
// make a port select button appear:
function makePortButton() {
  // create and position a port chooser button:
  portButton = createButton("choose port");
  portButton.position(10, 10);
  // give the port button a mousepressed handler:
  portButton.mousePressed(choosePort);
}

// make the port selector window appear:
function choosePort() {
  serial.requestPort();
}

// open the selected port, and make the port
// button invisible:
function openPort() {
  // wait for the serial.open promise to return,
  // then call the initiateSerial function
  serial.open().then(initiateSerial);

  // once the port opens, let the user know:
  function initiateSerial() {
    console.log("port open");
  }
  // hide the port button once a port is chosen:
  if (portButton) portButton.hide();
}

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

// pop up an alert if there's a port error:
function portError(err) {
  alert("Serial port error: " + err);
}

// try to connect if a new serial port
// gets added (i.e. plugged in via USB):
function portConnect() {
  console.log("port connected");
  serial.getPorts();
}

// if a port is disconnected:
function portDisconnect() {
  serial.close();
  console.log("port disconnected");
}

function closePort() {
  serial.close();
}

93b70b4266f1664ae6d3984542c2ee88.mp4

Serial Monitor H & L controlling LED ON / OFF.

d3f4de477448d109260e829c12001fb6.mp4

p5js H & L controlling LED ON / OFF.

9167632857cdf6531049387eab1a372b.mp4