window.onload = function () {

var seconds = 00;

var tens = 00;

var appendTens = document.getElementById("tens")

var appendSeconds = document.getElementById("seconds")

var buttonStart = document.getElementById('start-btn');

var buttonStop = document.getElementById('stop-btn');

var buttonReset = document.getElementById('reset-btn');

var Interval;

buttonStart.onclick = function () {

clearInterval(Interval);

Interval = setInterval(startTimer, 10);

}

buttonStop.onclick = function () {

clearInterval(Interval);

}

buttonReset.onclick = function () {

clearInterval(Interval);

tens = "00";

seconds = "00";

appendTens.innerHTML = tens;

appendSeconds.innerHTML = seconds;

}

function startTimer() {

tens++;

if (tens <= 9) {

appendTens.innerHTML = "0" + tens;

}

if (tens > 9) {

appendTens.innerHTML = tens;

}

if (tens > 99) {

console.log("seconds");

seconds++;

appendSeconds.innerHTML = "0" + seconds;

tens = 0;

appendTens.innerHTML = "0" + 0;

}

if (seconds > 9) {

appendSeconds.innerHTML = seconds;

}

}

}