async function getReply(replyId){
    let db = await MongoClient.connect(MONGO_URI);
    let testDb = db.db("test");

    let reply = await testDb.collection('replies').findOne({'_id': replyId})

    if (typeof reply.replies !== 'undefined' && reply.replies.length > 0){
        let superArr = reply.replies.map(
            async (replyId) => {
                return await getReply(replyId)
            }
        )

        return superArr
    } else {
        return null
    }
}

getReply(ObjectID("5ee29a83b6f54879ebec9f45")).then(
    reply => console.log(reply)
)