script.js

let [seconds, minutes, hours] = [0,0,0];
let displayTime = document.getElementById("displayTime");
let timer = null;

function stopwatch() {
    seconds++;
    if(seconds == 60){
        seconds=0;
        minutes++;
        if (minutes == 60){
           minutes = 0;
           hours++;
           }
        }
        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(stopwatch,1000);
}
function watchStop(){
    clearInterval(timer);
}

function watchReset(){
    clearInterval(timer);
    [seconds, minutes, hours] = [0,0,0];
    displayTime.innerHTML = "00:00:00";
}