jquery checkbox

To check on/off a checkbox:

$("#cbVoidInvoice").attr('checked'true); 
or 
$("#cbVoidInvoice").attr('checked'false); 
 
To check if checkbox check on:
$(this).is(':checked') 
 

jquery dropdownlist

Get value of the selected item:

$("#ComboBox").val()

Get text of the selected item:

$("#ComboBox option:selected").text()

On select value change:

$("#ComboBox").change(function() { /* do something here */ });

Add item to the list:

$(“<option value=’new item’>new item</option>”).appendTo(“#ComboBox”);