A string or a function controlling the preview of the element being dragged.

When not set or set to "original", the original element will be moved.
Setting it to "clone" will make a jQuery.clone() of the original element and show it for movement.
Any other string will be treated as a jQuery selector of the helper to move.

If you specify a function for this option, it must return a jQuery object.

Defaults to undefined, which means that the original element will be moved.

<div id="div1">Hello DIV!</div>

<script type="text/javascript">
    jQuery(function ($) {
            helper: function(e) {
                return $(this.element).clone().css("opacity", "0.5");