如何在表排序表头添加箭头类？

<table>
  <thead>
    <tr>
      <th onclick="sortTable(0)">Name</th>
      <th onclick="sortTable(1)">Age</th>
      <th onclick="sortTable(2)">Country</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>John</td>
      <td>25</td>
      <td>USA</td>
    </tr>
    <tr>
      <td>Jane</td>
      <td>30</td>
      <td>Canada</td>
    </tr>
    <tr>
      <td>Mike</td>
      <td>20</td>
      <td>Australia</td>
    </tr>
  </tbody>
</table>

th.sortable {
  cursor: pointer;
}

th.sortable::after {
  content: "";
  display: inline-block;
  width: 0;
  height: 0;
  vertical-align: middle;
  margin-left: 5px;
  border-style: solid;
  border-width: 4px 4px 0 4px;
  border-color: #000 transparent transparent transparent;
}

th.sortable.asc::after {
  border-color: transparent transparent #000 transparent;
}

th.sortable.desc::after {
  border-color: #000 transparent transparent transparent;
}

function sortTable(columnIndex) {
  var table = document.querySelector("table");
  var rows = Array.from(table.rows).slice(1); // Exclude the header row
  var isAscending = true;

  // Toggle sorting order
  if (table.dataset.sortedColumn === columnIndex.toString()) {
    isAscending = !table.dataset.sortedAscending;
  }

  // Remove arrow classes from all columns
  table.querySelectorAll("th").forEach(function (th) {
    th.classList.remove("asc", "desc");
  });

  // Sort rows based on the selected column
  rows.sort(function (a, b) {
    var aValue = a.cells[columnIndex].textContent;
    var bValue = b.cells[columnIndex].textContent;

    if (isAscending) {
      return aValue.localeCompare(bValue);
    } else {
      return bValue.localeCompare(aValue);
    }
  });

  // Re-append sorted rows to the table
  rows.forEach(function (row) {
    table.tBodies[0].appendChild(row);
  });

  // Add arrow class to the sorted column
  var sortedColumn = table.querySelector("th:nth-child(" + (columnIndex + 1) + ")");
  sortedColumn.classList.add(isAscending ? "asc" : "desc");

  // Update dataset attributes for sorting order
  table.dataset.sortedColumn = columnIndex;
  table.dataset.sortedAscending = isAscending;
}