script.js

var timer
var hour = 0
var minute = 0
var seconds = 0

var click = false

function start(){

    if (!click){
    
        timer = window.setInterval( function watch() {

            seconds++

            if (seconds == 60){
                seconds = 0
                minute++
            }
            if (minute == 60){
                minute = 0
                hour++
            }

            var stopWatch = number(hour) + ":" + number(minute) + ":" + number(seconds)
            document.getElementById("h1").innerText = stopWatch
        
        }, 1000)
        click = true
    }
}
function number(element) {
    
    return element < 10 ? '0' + element : element;

}

function stop(){
    clearInterval(timer)
    click = false
}

function reset(){
    clearInterval(timer)
    click = false
    hour = 0
    minute = 0
    seconds = 0
    let h = document.getElementById("h1")
    h.innerText =  "00:00:00"
}