CODE EXAMPLE FOR HTML

select2 select all option

$('.select2').select2({
  placeholder: 'Press CTRL+A for selecr or unselect all options'
});

$('.select2[multiple]').siblings('.select2-container').append('<span class="select-all"></span>');

$(document).on('click', '.select-all', function (e) {
  selectAllSelect2($(this).siblings('.selection').find('.select2-search__field'));
});

$(document).on("keyup", ".select2-search__field", function (e) {
  var eventObj = window.event ? event : e;
  if (eventObj.keyCode === 65 && eventObj.ctrlKey)
     selectAllSelect2($(this));
});
        
        
function selectAllSelect2(that) {

  var selectAll = true;
  var existUnselected = false;
  var item = $(that.parents("span[class*='select2-container']").siblings('select[multiple]'));

  item.find("option").each(function (k, v) {
      if (!$(v).prop('selected')) {
          existUnselected = true;
          return false;
      }
  });

  selectAll = existUnselected ? selectAll : !selectAll;

  item.find("option").prop('selected', selectAll);
  item.trigger('change');
}