もぐらたたき

#stage {
  position: relative;
  width: 300px;
  height: 300px;
  background: #8B4513;
  border-radius: 8px;
  margin-top: 10px;
}

.mole {
  position: absolute;
  font-size: 40px;
  cursor: pointer;
  user-select: none;
}

const startBtn = document.querySelector('#startBtn');
const score    = document.querySelector('#score');
const timer    = document.querySelector('#timer');
const stage    = document.querySelector('#stage');

let count    = 0;
let timeLeft = 10;
let gameInterval;
let timerInterval;

startBtn.addEventListener('click', function() {
  count    = 0;
  timeLeft = 10;
  score.textContent = '0点';
  timer.textContent = '10秒';
  stage.innerHTML   = '';
  startBtn.disabled = true;

  // もぐらを出すインターバル
  gameInterval = setInterval(function() {
    const mole = document.createElement('div');
    mole.textContent = '🐭';
    mole.classList.add('mole');
    mole.style.left = Math.random() * 250 + 'px';
    mole.style.top  = Math.random() * 250 + 'px';
    stage.appendChild(mole);

    mole.addEventListener('click', function() {
      mole.remove();
      count = count + 1;
      score.textContent = count + '点';
    });

    // 1秒後に自動で消える
    setTimeout(function() {
      mole.remove();
    }, 1000);
  }, 10);

  // タイマーのインターバル
  timerInterval = setInterval(function() {
    timeLeft = timeLeft - 1;
    timer.textContent = timeLeft + '秒';

    if (timeLeft === 0) {
      clearInterval(gameInterval);
      clearInterval(timerInterval);
      stage.innerHTML   = '';
      startBtn.disabled = false;
      timer.textContent = '終了！' + count + '点でした！';
    }
  }, 1000);
});