updateMonsterStat() { // 몬스터 정보 표시 메서드
    const { monster } = this; 
    if (monster === null) {
      $monsterName.textContent = '';
      $monsterHp.textContent = '';
      $monsterAtt.textContent = ''; 
      return;
    }
    $monsterName.textContent = monster.name;
    $monsterHp.textContent = `HP: ${monster.hp}/${monster.maxHp}`;
    $monsterAtt.textContent = `ATT: ${monster.att}`;
  }
  showMessage(text) {
    $message.textContent = text;
  }

}