const records = []; ------------------------------------------------------ ①

$screen.addEventListener('click', function () {
  if ($screen.classList.contains('waiting')) { // 대기 화면
    (중략)
  } else if ($screen.classList.contains('ready')) { // 준비 화면
  } else if ($screen.classList.contains('now')) { // 측정 화면
    endTime = new Date(); 

①

    const current = endTime - startTime; 
    records.push(current); 

    const average = records.reduce((a, c) => a + c) / records.length; --- ②
    $result.textContent = `현재 ${current}ms, 평균: ${average}ms`; ------ ③

④

    startTime = null; 
    endTime = null;

    $screen.classList.replace('now', 'waiting'); 
    $screen.textContent = '클릭해서 테스트를 시작하세요';
  }
});