ボタン押すゲーム

ボタンをできるだけ早く押してください。
<!DOCTYPE html>
<html>
  <head>
    <title>ボタン押すゲーム</title>
  </head>
  <body>
    <h1>ボタン押すゲーム</h1>
    <p>ボタンをできるだけ早く押してください。</p>
    <button id="startBtn">スタート</button>
    <button id="resetBtn" style="display:none;">リセット</button>
    <p id="timer"></p>
    <p id="score"></p>

    <script>
      let startTime;
      let endTime;
      let score;
      let timerInterval;
      const TIMER_LENGTH = 5000;

      const startGame = () => {
        startTime = Date.now();
        score = 0;
        document.getElementById("score").innerText = `Score: ${score}`;
        document.getElementById("timer").innerText = `Time left: ${TIMER_LENGTH/1000} seconds`;
        document.getElementById("startBtn").style.display = "none";
        document.getElementById("resetBtn").style.display = "inline";
        timerInterval = setInterval(countdown, 1000);
      };

      const countdown = () => {
        let timerText = `Time left: ${(TIMER_LENGTH - (Date.now() - startTime))/1000} seconds`;
        document.getElementById("timer").innerText = timerText;
        if (Date.now() - startTime >= TIMER_LENGTH) {
          endGame();
        }
      };

      const endGame = () => {
        clearInterval(timerInterval);
        endTime = Date.now();
        let gameTime = (endTime - startTime) / 1000;
        let scoreText = `Your score is ${score} in ${gameTime} seconds.`;
        document.getElementById("score").innerText = scoreText;
        document.getElementById("resetBtn").style.display = "none";
        document.getElementById("startBtn").style.display = "inline";
      };

      document.getElementById("startBtn").addEventListener("click", startGame);

      document.addEventListener("keydown", (event) => {
        if (event.code === "Space" && timerInterval) {
          score++;
          document.getElementById("score").innerText = `Score: ${score}`;
        }
      });

      document.getElementById("resetBtn").addEventListener("click", endGame);
    </script>
  </body>
</html> 

ジョーク：「ボタンを押すのはいいけど、何か重大なことでも起こるんじゃないかとドキドキしてしまう…」