we use Kendo Ui for ANgualr 2 grid with sorting. Out of the box the column with sorting enabled can be in 3 possible states: sorted asc, sorted desc or not sorted. chages in grid sorting update boutd sort property wit current array of SortDescriptors. When column is sorted SortDescriptor is added to array of Sortdescriptors and specifies the field and direction of sort.
If column is not sorted it should not be present in SortDescriptor array. However if column is clicked until it is unsorted again, its SortDesriptor stays in array with direction 'undefined'. This is incorrect, it should disappear from array. Also according to Kendo type definition direction can only have value 'asc' or 'desc' but not 'undefined'