Using the pulse function

drag-enter.js

function drop_target_enter(e) {
    var $dt = $(context.target_id)
    $dt.addClass('dt-entered');
    pulse($dt,
          500,
          'linear',
          {opacity: 0.25},
          {opacity: 1},
          function() {
              return $dt.hasClass('dt-entered') == false;
          });
}