Depandant dropdown in pdf

var AddSizeList = this.getField("AddSize");

if(event.willCommit)
{
switch(event.value){

case "Back cover":
  AddSizeList.setItems(["-Select Back cover-", "Back Cover(10x10")", "Back Cover(8x8")", "Back Cover(10.5x10.5")"]);
break;

default:
 AddSizeList.clearItems();}}