Javascript表筛选器不显示表头

<table id="myTable">
  <thead>
    <tr>
      <th>姓名</th>
      <th>年龄</th>
    </tr>
  </thead>
  <tbody>
    <!-- 表格数据行 -->
  </tbody>
</table>

function filterTable() {
  var input, filter, table, tr, td, i, txtValue;
  input = document.getElementById("myInput");
  filter = input.value.toUpperCase();
  table = document.getElementById("myTable");
  tr = table.getElementsByTagName("tr");

  for (i = 0; i < tr.length; i++) {
    td = tr[i].getElementsByTagName("td")[0];
    if (td) {
      txtValue = td.textContent || td.innerText;
      if (txtValue.toUpperCase().indexOf(filter) > -1) {
        tr[i].style.display = "";
      } else {
        tr[i].style.display = "none";
      }
    }
  }
}