ISQA project- personal library

Can someone tell me what’s wrong with my second post.

Here’s my code:

var MongoClient = require('mongodb').MongoClient;
var ObjectId = require('mongodb').ObjectId;
const MONGODB_CONNECTION_STRING = process.env.DB;
//Example connection: MongoClient.connect(MONGODB_CONNECTION_STRING, function(err, db) {});

module.exports = function (app) {

  app.route('/api/books')
    .get(function (req, res){
      //response will be array of book objects
      //json res format: [{"_id": bookid, "title": book_title, "commentcount": num_of_comments },...]
      MongoClient.connect(MONGODB_CONNECTION_STRING, (err, db) => {
        if(err) {return err;}
        let collection = db.collection("library");
        collection.find({title:String,_id:String,commentcount:Number})
                  .toArray(function(err,docs){
                            res.json(docs)
                          })
      });
    })
    
    .post(function (req, res){
      var title = req.body.title;
      //response will contain new book object including atleast _id and title
      var bookid = new ObjectId();
      res.json({ title: title, _id: bookid });
    })
    
    .delete(function(req, res){
      //if successful response will be 'complete delete successful'
    });



  app.route('/api/books/:id')
    .get(function (req, res){
      var bookid = req.params.id;
      //json res format: {"_id": bookid, "title": book_title, "comments": [comment,comment,...]}
      MongoClient.connect(MONGODB_CONNECTION_STRING, (err, db) => {
        let collection = db.collection('library');
        collection.findOne({_id:bookid},(err,doc)=>{
          if(err){res.send(`Error ${err}`);
          } else {
            if(doc!==null){
              res.json(doc);
            }else{
              res.send('no book exists');}
          }
      });
    });                            
    })
    
    .post(function(req, res){
      var bookid = req.params.id;
      var comment = req.body.comment;
      //json res format same as .get
      MongoClient.connect(MONGODB_CONNECTION_STRING,(err, db)=>{
      const collection = db.collection('library');
      collection.findAndModify({_id:bookid},{$push:{comments:comment},$inc:{commentcount:1}},{new: true},function(err,doc){
        if(err){res.send(`Error ${err}`);
        } else{
          res.json(doc);
        }
      });
    });
    })
    
    .delete(function(req, res){
      var bookid = req.params.id;
      //if successful response will be 'delete successful'
    });
  
};

I get:
{“lastErrorObject”:{“n”:0,“updatedExisting”:false},“value”:null,“ok”:1,“operationTime”:“6630766216044609555”,"$clusterTime":{“clusterTime”:“6630766216044609555”,“signature”:{“hash”:“VcUyx7/KhCrMknLsXgSo7BpHEv0=”,“keyId”:“6619253573387550721”}}}

Link to my Glitch project: https://billowy-november.glitch.me