let spans = document.getElementsByTagName("span");
let startButton = document.getElementById("start");
let stopButton = document.getElementById("stop");
let resetButton = document.getElementById("reset");
let t;
let ms = 0,
  s = 0,
  m = 0,
  h = 0;

function startChrono() {
  t = setInterval(updateChrono, 100);
  startButton.disabled = true;
}

function updateChrono() {
  ms += 1;
  if (ms == 10) {
    ms = 0;
    s += 1;
  } else if (s == 60) {
    s = 0;
    m += 1;
  } else if (m == 60) {
    m = 0;
    h += 1;
  }
  spans[0].innerHTML = h + " h";
  spans[1].innerHTML = m + " m";
  spans[2].innerHTML = s + " s";
  spans[3].innerHTML = ms + " ms";
}

function stopChrono() {
  clearInterval(t);
  startButton.disabled = false;
}

function resetChrono() {
  clearInterval(t);
  startButton.disabled = false;
  (ms = 0), (s = 0), (m = 0), (h = 0);
  spans[0].innerHTML = h + " h";
  spans[1].innerHTML = m + " m";
  spans[2].innerHTML = s + " s";
  spans[3].innerHTML = ms + " ms";
}