app.js

const startBtn = document.querySelector('.btn-start'); 
const session = document.querySelector('.minutes'); 
let myInterval; 
let state = true;

const appTimer = () => {
    const sessionAmount = Number.parseInt(session.textContent)
  
    if(state) {
      state = false;
      let totalSeconds = sessionAmount * 60;
  
      const updateSeconds = () => {
        const minuteDiv = document.querySelector('.minutes');
        const secondDiv = document.querySelector('.seconds');
      
        totalSeconds--;
      
        let minutesLeft = Math.floor(totalSeconds/60);
        let secondsLeft = totalSeconds % 60;
      
        if(secondsLeft < 10) {
          secondDiv.textContent = '0' + secondsLeft;
        } else {
          secondDiv.textContent = secondsLeft;
        }
        minuteDiv.textContent = `${minutesLeft}`
      
        if(minutesLeft === 0 && secondsLeft === 0) {
          clearInterval(myInterval);
        }
      }
      myInterval = setInterval(updateSeconds, 1000);
    } else {
      alert('Session has already started.')
    }
  }

  startBtn.addEventListener('click', appTimer);