①

let startTime; 
let endTime;

$screen.addEventListener('click', function () {
  if ($screen.classList.contains('waiting')) { // 대기 화면
    $screen.classList.replace('waiting', 'ready');
    $screen.textContent = '초록색이 되면 클릭하세요';
    setTimeout(function () {

      startTime = new Date(); ---------------------------------- ②

      $screen.classList.replace('ready', 'now');
      $screen.textContent = '클릭하세요!';
    }, Math.floor(Math.random() * 1000) + 2000); // 2000~3000
  } else if ($screen.classList.contains('ready')) { // 준비 화면
  } else if ($screen.classList.contains('now')) { // 측정 화면

    endTime = new Date(); -------------------------------------- ③
    $result.textContent = `${endTime - startTime}ms`; ---------- ④

⑤

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

  }
});