Adds a select all option to a SharePoint list of checkboxes.

SharePoint-AddSelectAll

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.4.1/jquery.js" integrity="sha256-WpOohJOqMqqyKL9FccASB9O0KwACQJpFTUBLTYOVvVU=" crossorigin="anonymous"></script>
<script type="text/javascript">
$(function() {
  var statesTable = $('table[id^="State"');
  var selectAll = $('<a href="#" id="select-all-states">Select All</a>').insertBefore(statesTable);
  selectAll.on('click', function(e) {
    e.preventDefault();
    statesTable.find('.ms-RadioText :checkbox').prop('checked', true);
  });
  $('<span>&nbsp;</span>').insertBefore(statesTable);
  var unselectAll = $('<a href="#" id="unselect-all-states">Unselect All</a>').insertBefore(statesTable);
  unselectAll.on('click', function(e) {
    e.preventDefault();
    statesTable.find('.ms-RadioText :checkbox').prop('checked', false);
  });
});
</script>