// AFTER SUBMIT && Only on STATUS PENDING FULFILLMENT
var recType = nlapiGetRecordType();
var recId = nlapiGetRecordId();
var salesRec = nlapiLoadRecord(recType, recId);

var count = salesRec.getLineItemCount('item');
var hasBackOrder = false;

for (var i = 1; i <= count; i++) {
  var soLine = {};
  soLine.backOrder = parseFloat(nvl(salesRec.getLineItemValue('item', 'quantitybackordered', i), 0));

  // OR /////

  soLine.quantity = parseFloat(nvl(salesRec.getLineItemValue('item', 'quantity', i), 0));
  soLine.qtyCommitted = parseFloat(nvl(salesRec.getLineItemValue('item', 'quantitycommitted', i), 0));

  soLine.backOrder = soLine.quantity - soLine.qtyCommitted;

  if (soLine.backOrder > 0) {
    hasBackOrder = true;
  }
}