HTMLTableRowElement: sectionRowIndex 属性

<table>
  <thead>
    <tr>
      <th>Item</th>
      <th>Price</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Bananas</td>
      <td>$2</td>
    </tr>
    <tr>
      <td>Oranges</td>
      <td>$8</td>
    </tr>
    <tr>
      <td>Top Sirloin</td>
      <td>$20</td>
    </tr>
  </tbody>
  <tfoot>
    <tr>
      <td>Total</td>
      <td>$30</td>
    </tr>
  </tfoot>
</table>

const rows = document.querySelectorAll("tbody tr");

rows.forEach((row) => {
  const z = document.createElement("td");
  z.textContent = `(row #${row.sectionRowIndex})`;
  row.appendChild(z);
});