let turn = 'O'; ------------------------------------------- ①
const callback = (event) => { ----------------------------- ②
  if (event.target.textContent !== '') { // 빈칸이 아닌가? 
    console.log('빈칸이 아닙니다.');
  } else { // 빈칸이면
    console.log('빈칸입니다'); 
    event.target.textContent = turn;
    turn = turn === 'X' ? 'O' : 'X'; ---------------------- ④
  }
};

for (let i = 1; i <= 3; i++) {
  const $tr = document.createElement('tr'); 
  const cells = [];
  for (let j = 1; j <= 3; j++) {
    const $td = document.createElement('td'); 

    $td.addEventListener('click', callback); -------------- ③

    cells.push($td);
    $tr.appendChild($td);
  }
  rows.push(cells);
  $table.appendChild($tr);
}