java script

I have a java script function and getting "pitems" list values in select, and i want to send the values in the form of List to my CommonService class. Below code is not working...

function f_getPop(){
CommonService.getAmount(document.getElementById("pitems").value , populateAmount);
}
function populateAmount(data){
document.getElementById("amount").value=data;
}

<td><s:select label="Items" headerKey="1" headerValue="--Please Select--" list="items" name="purchaseitems" id="pitems" listKey="desc" listValue="desc" multiple="true" onchange="f_getPop()"></s:select></td>

<td><s:textfield label="Amount" name="amount" id="amount"/></td>


Can u plz help me out???