let data = [];
function startGame() { // 4 × 4 표 그리기
  const $fragment = document.createDocumentFragment();
  [1, 2, 3, 4].forEach(function () {
    const rowData = [];
    data.push(rowData);
    const $tr = document.createElement('tr');
    [1, 2, 3, 4].forEach(() => {
      rowData.push(0);
      const $td = document.createElement('td');
      $tr.appendChild($td);
    });
    $fragment.appendChild($tr);
  });
  $table.appendChild($fragment);
  put2ToRandomCell();
  draw();
}
function put2ToRandomCell() { // 빈칸을 찾아 무작위로 숫자 2 넣기
  const emptyCells = [];
  data.forEach(function (rowData, i) {
    rowData.forEach(function (cellData, j) {
      if (!cellData) {
        emptyCells.push([i, j]);