script.js

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


var display = document.querySelector(".timer-display");
var laps = document.querySelector(".laps");
var lapRecord = document.querySelector(".lapRecord");

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 ul = document.createElement('ul');
    ul.innerHTML = getTimer();
    laps.appendChild(ul);
  }
}


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