function showMines() {
  const mines = [CODE.MINE, CODE.QUESTION_MINE, CODE.FLAG_MINE];
  data.forEach((row, rowIndex) => {
    row.forEach((cell, cellIndex) => {
      if (mines.includes(cell)) {
        $tbody.children[rowIndex].children[cellIndex].textContent = 'X';
      }
    });
  });
}

function onLeftClick(event) {
  (중략)
  } else if (cellData === CODE.MINE) { // 지뢰 칸이면

    showMines();

    target.textContent = '펑';
    (중략)
  } // 나머지는 무시
}