timeoutId = setTimeout(function () {

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

    clearTimeout(timeoutId); -------------------------- ①
    $screen.classList.replace('ready', 'waiting'); ---- ②
    $screen.textContent = '너무 성급하군요!'; --------- ③

  } else if ($screen.classList.contains('now')) { // 측정 화면
    endTime = new Date();
    const current = endTime - startTime; 
    records.push(current);
    const average = records.reduce((a, c) => a + c) / records.length;
    $result.textContent = `현재 ${current}ms, 평균: ${average}ms`; 
    startTime = null;
    endTime = null;
    $screen.classList.replace('now', 'waiting');
    $screen.textContent = '클릭해서 테스트를 시작하세요';
  }
});