How to set default value for a field of type 'Sp.FieldMultiChoice' using JavaScriptObject Model

Hi,

How to set default value for a field of type 'Sp.FieldMultiChoice'(dropdown) using JavaScriptObject Model.

Able to set values for that field(dropdown field) as below. 

var clientContext = SP.ClientContext.get_current();

if (clientContext != undefined && clientContext != null) {

var webSite = clientContext.get_web();

taskList = webSite.get_lists().getByTitle("MyList");                

var fieldCollection = taskList.get_fields();

var item = clientContext.castTo(fieldCollection.getByInternalNameOrTitle("Priority"), SP.FieldMultiChoice);

item.set_choices(choices);

item.update();