Looping thru Cash Sale to Fill-In missing Information.

I have this code (Post Sourcing Function) that fills in the missing department and class for an item on a Cash Sale:

function xPopulateCashSale(fldname, typ)
{
var ItemID = nlapiGetCurrentLineItemValue('item','item');
var LCurrentLineDSB = nlapiLookupField('item',ItemID,'custitemdepartment1');
var LCurrentLineASB = nlapiLookupField('item',ItemID,'custitemactivity1');

if(type == 'item' && fldname == 'item' && nlapiGetCurrentLineItemText('item','item').length >= 1)
{
nlapiSetCurrentLineItemValue('item','department',LCurrentLineDSB);
nlapiSetCurrentLineItemValue('item','class',LCurrentLineASB);
}
}

I want to add a button that loops thru a Cash Sales’ line items to fill in the missing fields? I can't get the loop to work, can someone give me a hand with that?

I have to use client side scripting.