Looks like I was too smart by half. I already had the value. Thanks to the people who responded, it made me think harder :0)

var invoiceItemCntResult = query.runSuiteQL({
    query: 'select trandisplayname
    from transaction
    left join transactionline
    on transaction.id=transactionline.transaction
    WHERE custcol_cp_year > 0
    and custcol_cp_carrierid !== null
    and transaction.externalID = ?', params: [externalID]});              
var invoiceItemCnt = invoiceItemCntResult.asMappedResults();       

//Log ItemCnt
log.debug({
title: '1a. ItemCnt',
details: JSON.stringify(invoiceItemCnt.length)});