app.js

let [seconds, minutes, hours] = [0, 0, 0];

let displayTime = document.getElementById("displayTime");
let timer = null;

function stoptWatch() {
  seconds++;
  if (seconds == 60) {
    seconds = 0;
    minutes++;
    if (minutes == 60) {
      minutes = 0;
      hours++;
    }
    if (hours == 12) {
      hours = 0;
    }
  }
  let h = hours < 10 ? "0" + hours : hours;
  let m = minutes < 10 ? "0" + minutes : minutes;
  let s = seconds < 10 ? "0" + seconds : seconds;
  displayTime.innerHTML = h + ":" + m + ":" + s;
}

function watchStart() {
  if (timer !== null) {
    clearInterval(timer);
  }
  timer = setInterval(stoptWatch, 1000);
}

function watchStop() {
  clearInterval(timer);
  console.log(seconds, minutes, hours);
}

function watchReset() {
  if (timer !== null) {
    clearInterval(timer);
  }
  seconds = 0;
  minutes = 0;
  hours = 0;
  displayTime.innerHTML = "00:00:00";
  console.log(seconds, minutes, hours);
}