index.js

window.onload = function(){

    var milliSeconds = 00;
    var seconds = 00;
    var minutes = 00;
    var hours = 00;
    var appendMilliSeconds = document.getElementById("milliSeconds");
    var appendSeconds = document.getElementById("seconds");
    var appendMinutes = document.getElementById("minutes");
    var appendHours = document.getElementById("hours");
    var start = document.getElementById("start-btn");
    var stop = document.getElementById("stop-btn");
    var reset = document.getElementById("reset-btn");
    var Interval

    //Start button event
    start.onclick = function(){
        clearInterval(Interval);
        Interval = setInterval(setTimer, 10);
    }

    //stop button event
    stop.onclick = function(){
        clearInterval(Interval);
        start.innerHTML = "Resume";   }

    //reset button event
    reset.onclick = function(){
        clearInterval(Interval);
        milliSeconds = setZero(appendMilliSeconds);
        seconds = setZero(appendSeconds);
        minutes = setZero(appendMinutes);
        hours = setZero(appendHours);
        start.innerHTML = "Start"
    }

   function setTimer(){
        milliSeconds++;
        (milliSeconds <= 9)? appendMilliSeconds.innerHTML = "0"+milliSeconds : appendMilliSeconds.innerHTML = milliSeconds;

        if(milliSeconds>99){
            seconds++;
            milliSeconds = setZero(appendMilliSeconds);
            (seconds <= 9)? appendSeconds.innerHTML = "0"+seconds : appendSeconds.innerHTML = seconds;
        }

        if(seconds >= 59){
            minutes++;
            milliSeconds = setZero(appendMilliSeconds);
            seconds = setZero(appendSeconds);
            (minutes <= 9)? appendMinutes.innerHTML = "0"+minutes : appendMinutes.innerHTML = minutes;
        }

        if(minutes >= 59){
            hours++;
            milliSeconds = setZero(appendMilliSeconds);
            seconds = setZero(appendSeconds);
            minutes = setZero(appendMinutes);
            (hours <= 9)? appendHours.innerHTML = "0"+hours : appendHours.innerHTML = hours;
        } 
    }

    function setZero(appendElement){
        var element = 0;
        appendElement.innerHTML = "0"+element;
        return element;
    }
}