What should a GetSelectedIndex method return when no rows are selected

Question

I'm creating a UI table component that returns an array of selected indices.

Some typical return values would be

var myRetVal1 = [0];//one value selected 
var myRetVal2 = [0,1,2,3,8,11];//multiple value selected

as you can see I'm always returning an array.

I had an idea to return -1 when nothing is selected, but then I thought that might be confusing when in every other condition an array is returned.

So checking for an empty set of values would be either

//returns -1
var selectedItems = tbl.GetSelectedIndex();
if(selectedItems !== -1){
   //we have data to process
}

OR

//returns []
var selectedItems = tbl.GetSelectedIndex();
if(selectedItems.length > 0){
   //we have data to process
}

OR

//returns null
var selectedItems = tbl.GetSelectedIndex();
if(selectedItems){
   //we have data to process
}

Maybe I'm making too big a deal over this, but is there a standard expectation for this type of control?

As I build other controls should they conform to a standard empty return value or should they always return a "empty" version of their expected return type?