I found the client-side sorting method in one of the answers to not work as expected. So I changed it and came up with following method, that I found worked perfectly. The second parameter of sortOrder can be either 'asc' or 'desc'.
functionsortByItemText(comboBox, sortOrder) { comboBox.trackChanges(); varitems = comboBox.get_items(); vartempArray = items._array; if(sortOrder === "asc") { tempArray.sort(function(a, b) { if(a && b && a.get_text() > b.get_text()) return1; if(a && b && a.get_text() === b.get_text()) return0; if(a && b && a.get_text() < b.get_text()) return-1; }); } else{ tempArray.sort(function(a, b) { if(a && b && a.get_text() > b.get_text()) return-1; if(a && b && a.get_text() === b.get_text()) return0; if(a && b && a.get_text() < b.get_text()) return1; }); } items.clear(); for(vari = 0; i < tempArray.length; i++) { varcomboItem = newTelerik.Web.UI.RadComboBoxItem(); comboItem.set_text(tempArray[i].get_text()); comboItem.set_value(tempArray[i].get_value()); items.insert(i, comboItem); } comboBox.commitChanges(); return;}