HTMLTableCellElement: cellIndex プロパティ

<table>
  <thead>
    <tr>
      <th>商品</th>
      <th>価格</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>バナナ</td>
      <td>$2</td>
    </tr>
    <tr>
      <td>ç±³</td>
      <td>$2.5</td>
    </tr>
  </tbody>
</table>

table {
  border-collapse: collapse;
}

th,
td,
table {
  border: 1px solid black;
}

button {
  margin: 1em 1em 1em 0;
}

const rows = document.querySelectorAll("tbody tr");
const cells = rows[0].cells;

for (const cell of cells) {
  cell.textContent = `${cell.textContent} (セル #${cell.cellIndex})`;
}