app.js

var min = 0;
var sec = 0;
var msec = 0;
var hour = 0;
var hourHeading = document.getElementById("hour")
var minHeading = document.getElementById("min");
var secHeading = document.getElementById("sec");
var msecHeading = document.getElementById("msec");
var interval;

function timer(){
    msec++;
    msecHeading.innerHTML = msec;
    if(msec >= 100){
        sec++;
        secHeading.innerHTML = sec;
        msec = 0;
    }else if(sec >= 60){
        min++;
        minHeading.innerHTML = min;
        sec = 0;
    }else if(min >= 60){
        hour++;
        hourHeading.innerHTML = hour;
        min = 0;
    }
}

function start(){
    interval = setInterval(timer,10)
    document.getElementById('btn').disabled = true;
}


function stop(){
    clearInterval(interval)
    document.getElementById('btn').disabled = false;
}

function reset(){
    hour = "0"+ 0;
    min = "0"+ 0;
    sec = "0"+ 0;
    msec = "0"+ 0;
    hourHeading.innerHTML = hour;
    minHeading.innerHTML = min;
    secHeading.innerHTML = sec;
    msecHeading.innerHTML = msec;
    stop()
    document.getElementById('btn').disabled = false;
}

var Lap = document.getElementById('lap');
var Laps = document.getElementById('laps');

Lap.onclick = function() {
    
  Laps.innerHTML += "<li>" + hourHeading.innerHTML + ": " + minHeading.innerHTML + ": " + secHeading.innerHTML + "</li>";
}