var jpInvoiceLineNum = nlapiSelectValue(invoiceLineNodes[x], 'line-num');
if (jpInvoiceLineNum == 1)
{
    var jpPONumber = nlapiSelectValue(invoiceLineNodes[x], 'po-number');
    if (!record.getFieldValue('custbody_coupa_po_number'))
    {
        record.setFieldValue('custbody_coupa_po_number', jpPONumber);
    }
}