script.js

let alarm = new Audio('alarm.mp3');
let start = false;
let button = document.getElementById('button');
let timer = document.getElementById('timer');
let interval;

function starte() {
  if (start == false) {
    start = true;
    button.src = 'images/stop.png';
    startTimer();
  } else {
    start = false;
    button.src = 'images/start.png';
    stopTimer();
  }
}

function startTimer() {
  let startTime = new Date().getTime();
  let fiveMinutes = 1000 * 60 * 5;
  let endTime = startTime + fiveMinutes;

  let updateTimer = function() {
    let currentTime = new Date().getTime();
    let timeLeft = endTime - currentTime;

    if (timeLeft > 0) {
      let minutes = Math.floor(timeLeft / (1000 * 60));
      let seconds = Math.floor((timeLeft / 1000) % 60);
      let text = ('0' + minutes).slice(-2) + ' : ' + ('0' + seconds).slice(-2);
      timer.innerHTML = text;
    } else {
      if (start === true) {
        alarm.play();
        timer.innerHTML = '05 : 00';
        start = false;
        button.src = 'images/start.png';
        clearInterval(interval);
      }
    }
  };

  updateTimer();
  interval = setInterval(updateTimer, 500);
}

function stopTimer(){
  start = false;
  clearInterval(interval);
  timer.innerHTML = '05 : 00';
}