exports.getJoinedTransactions = function(firmId, page, pageSize, billerId, matterId, words, billingStatus, cb) {

    let firmIdToObjId = new mongoose.Types.ObjectId(global.user.firmId);
    let query = { firmId: firmIdToObjId };

    if ( typeof billerId === "object" ) {
        billerId = new mongoose.Types.ObjectId(billerId);
        query.contactId = billerId;
    }

    if ( typeof matterId === "object" ) {
        matterId = new mongoose.Types.ObjectId(matterId);
        query.matterId = matterId;
    }

    ....

    myAggregate = [
        { $sort: { date: -1 } },
        {
            $match: query
        },

    ....

    Transactions.aggregate(myAggregate).exec( function(err, transactions) {

        if (err) {
            console.log(err);
        }

        someVar = transactions.length;

    ....

if (typeof billingStatus === "string" && billingStatus != "") {

    if (billingStatus === "billed") {
        query.billId = { $not: null } ;          <-- This does not work.
    } else if (billingStatus === "unbilled") {
        query.billId = null;                     <-- This works!
    }
}