• This option saves the currently collapsed groups, using the group name.
• Collapsed groups are saved by column and group class name, so that the groups can be dynamically changed and still remember the correct collapsed groups. Try changing the group names (using the sliders) in the demo below & then collapsing some groups several times; go back to previous groups and it will remember which groups were collapsed.
• Because the group name is saved, groups with a number range (e.g. "group-number-10"; see the "Numeric" column below) will show a different range depending on sort direction; in this situation, each sort direction is saved independently.
• This option requires the $.tablesorter.storage utility contained within the jquery.tablesorter.widgets.js file.

• This option should contain a jQuery selector string or jQuery object pointing to an element to be used to reset (clear) the list of collapsed groups.
• You can also clear the saved collapsed groups by calling this function: $.tablesorter.grouping.clearSavedGroups(table); (table would be either the table DOM or table jQuery object).
• This option requires the $.tablesorter.storage utility contained within the jquery.tablesorter.widgets.js file.

// the above example becomes: ["images", "cats", "burger-time", "jpg"]
group_separator : /[/.]/

• txt - current text of the group header.
• col - current table column being grouped (zero-based index).
• table - current table (DOM).
• c - table configuration data from table.config.
• wo - table widget options from table.config.widgetOptions.

group_formatter : function(txt, col, table, c, wo) {
  // If there are empty cells, name the group "Empty"
  return txt === "" ? "Empty" : txt;
}

• $cell - current group header table cell (jQuery object).
• $rows - current group rows (jQuery object).
• column - current table column being grouped (zero-based index).
• table - current table (DOM).

group_callback : function($cell, $rows, column, table){
  $cell.find('.group-name').append(' group'); // adds "group" to the end of the group name
}

$('table').on('groupingComplete', function(){
  // always hide empty groups ("Empty" can be added by the group_formatter function)
  $(this).find('.group-header:contains("Empty")').trigger('toggleGroup');
});

// you can just return date, date.toLocaleString(), date.toLocaleDateString() or d.toLocaleTimeString()
group_dateString : function(date) {
  return date.toLocaleString();
}