pomodoro.js

let timer = 1500;

let startTimer = function(){
  
  if (timer !== 0){
    timer -= 1;
    timerToTime();
  } else{
    timer = 1500;
    window.clearTimeout(timeoutID);
  }  
}

let timeoutID = window.setTimeout(startTimer, 1000);

pomodoroStartButton.onclick = function(){
  startTimer();
}

pomodoroStopButton.onclick = function(){
  window.clearTimeout(timeoutID);
}