stopwatch.js

var ms = 0, s = 0, m = 0,h = 0
var timer

var display = document.querySelector('.timer-display')
var laps = document.querySelector('.laps')

function start(){
    if(!timer){
        timer = setInterval(run,10)
    }
}

function run(){
    display.innerHTML = getTimer()
    ms++
    if(ms === 100){
    ms = 0
    s++;

    if (s === 60){
        s=0
        m++
    }

    if(m === 60){
        m=0
        h++
    }
}
}

function getTimer(){
    return (h<10? "0" + h :h)+":"+(m<10?"0"+m:m) + ":"+ (s<10? "0" + s:s) + ":" +(ms<10? "0" + ms :ms)
}

function pause(){
    stopTimer()
}

function stopTimer(){
    clearInterval(timer)
    timer = false
}

function reset(){
    stopTimer()
    ms = 0
    s = 0
    m = 0
    h = 0
    display.innerHTML = getTimer()
}

function restart(){
    if(timer)
    reset()
    start()
}

function lap(){
    if(timer){
        var li = document.createElement("li")
        li.innerHTML = getTimer()
        laps.appendChild(li)
    }
}

function resetLap(){
    laps.innerHTML = ""
}