Any particular reason about this isn't matching the element with that class?
I have checked a million times and can't see what is that I'm doing wrong.


$('.lnk-folder').click(function(e) {
  e.preventDefault();
  var header = $(this).parent('thead').find('.folder-header');
  console.log($(header));
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<thead>
  <tr>
    <th colspan="10" style="padding:0px; font-size:120%;background-color:#ff0000;">
      <div style="float:left;min-width:20%;">
        <a style="color: #000" class="lnk-folder folder-close" data-hash="" href="#">
          <i class='fa fa-folder-open'></i> Folder 1
        </a>
      </div>
      <div style="float:left;height:26px;padding-left:10px;">
        <a href="#"><i class='fas fa-file-upload tooltip' style="color:#fff;"><span class="tooltiptext_m">New</span></i></a>
      </div>
    </th>
  </tr>
  <tr class="folder-header">
    <th colspan="2" style='background-color:#0c343d;vertical-align:middle;'> Name </th>
    <th style='width:7%;background-color:#0c343d;vertical-align:middle;'> Code </th>
    <th style='width:30%;background-color:#0c343d;vertical-align:middle;'> Act</th>
    <th style='width:7%;background-color:#0c343d;vertical-align:middle;'> Version</th>
  </tr>
</thead>




Find element by class under the same parent
parent() is your problem. It looks up the DOM exactly one level, to the parent element, but you need to go higher than that. To do so, use closest()


$('.lnk-folder').click(function(e) {
  e.preventDefault();
  var header = $(this).closest('thead').find('.folder-header');
  console.log($(header));
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<table>
  <thead>
    <tr>
      <th colspan="10" style="padding:0px; font-size:120%;background-color:#ff0000;">
        <div style="float:left;min-width:20%;">
          <a style="color: #000" class="lnk-folder folder-close" data-hash="" href="#">
            <i class='fa fa-folder-open'></i> Folder 1
          </a>
        </div>
        <div style="float:left;height:26px;padding-left:10px;">
          <a href="#"><i class='fas fa-file-upload tooltip' style="color:#fff;"><span class="tooltiptext_m">New</span></i></a>
        </div>
      </th>
    </tr>
    <tr class="folder-header">
      <th colspan="2" style='background-color:#0c343d;vertical-align:middle;'> Name </th>
      <th style='width:7%;background-color:#0c343d;vertical-align:middle;'> Code </th>
      <th style='width:30%;background-color:#0c343d;vertical-align:middle;'> Act</th>
      <th style='width:7%;background-color:#0c343d;vertical-align:middle;'> Version</th>
    </tr>
  </thead>
</table>



