script.js

    const startButton = document.querySelector('[data-action="start"]');
    const stopButton  = document.querySelector('[data-action="stop"]');
    const resetButton = document.querySelector('[data-action="reset"]');
    const minutes     = document.querySelector('.minutes');
    const seconds     = document.querySelector('.seconds');
    let timerTime     = 00;
    let isRunning     = false;
    let interval;    

    function startTimer() {
      if (isRunning) return;

      isRunning = true;
      interval  = setInterval(incrementTimer, 1000);
    }

    function stopTimer() {
      if (!isRunning) return;

      isRunning = false;
      clearInterval(interval);
    }

    function resetTimer() {
      stopTimer();

      timerTime = 0;
      minutes.innerText = '00';
      seconds.innerText = '00';
    }

    function incrementTimer() {
      timerTime++;

      const numOfMinutes = Math.floor(timerTime / 60);
      const numOfSeconds = timerTime % 60;

      minutes.innerText = pad(numOfMinutes);
      seconds.innerText = pad(numOfSeconds);
    }

    function pad(number) {
      return (number < 10) ? '0' + number : number;
      // if (number < 10) {
      //   return '0' + number;
      // } else {
      //   return number;
      // }
    }

    startButton.addEventListener('click', startTimer);
    stopButton.addEventListener('click', stopTimer);
    resetButton.addEventListener('click', resetTimer);