node-mongodb-native - cursor returns null as last value during an each call

Question

So. I have a very basic script, that connects to a database and does a find on a collection that has a lot of documents and limit it to 3 items. Everything runs smoothly except that at the end of my results, a null and the script doesn't terminate instead of closing the connection quietly after having succeedeed.

Here I declare my params and create my database object:

var SERVER = 'localhost',
 PORT = 27017,
 DATABASE = 'test',
 COLLECTION = 'coll',
 mongo = require('mongodb'),
 db = new mongo.Db(DATABASE,
  new mongo.Server(SERVER, PORT, {auto_reconnect: true}),
  {});

And here I connect to the database and proceed to query it with a find cursor and the each function:

db.open(function(err, db) {
 if(err) throw err;
 var collection = new mongo.Collection(db, COLLECTION),
  cursor = collection.find({}, {}).limit(3);
 cursor.each(function(err, doc) {
  if(err) throw err;
  console.log(doc);
 });
 db.close();
});

Results are nice:

{ _id: '1',
  a: 'first object' }
{ _id: '2',
  a: 'second object' }
{ _id: '3',
  a: 'third object' }

up until the point where a

null

appears.

As described above, the script then proceeds not to terminate.

I don't get why and would appreciate pointers on how to make it terminate nicely.

Tetherball

Answer 1

Well, that was stupid. Too much time without programming in an async setup and you make mistakes: the db.close() call came before the each looping was over. Returned null and hanged because of that. Here is the correct code:

db.open(function(err, db) {
 if(err) throw err;
 var collection = new mongo.Collection(db, COLLECTION),
  cursor = collection.find({}, {}).limit(3);
 cursor.each(function(err, doc) {
  if(err) throw err;
  if(doc !== null) console.log(doc);
  else db.close();
 });
});

I hope it'll save some minutes to someone, somehow.

Tetherball

Answer 2

This question's a few years old, and already has a possible answer, but I just wanted to point out another possibility for anyone who's still confused after reading this. Turns out, each repeatedly calls nextObject which actually is intended to return null when the cursor is exhausted.

The answer here was the one I needed: Node Mongo Native - how to tell when a cursor is exhausted?

From the Node MongoDB driver docs: http://mongodb.github.io/node-mongodb-native/api-generated/cursor.html#nextobject

Rainbow

This should be the actual answer. – Lithotomy

Even, I was getting a null as the last value during call to cursor.each. nodejs version : 0.12.9 , mongodb ddriver version : ^2.2.33. Good that I saw this answer. – Querida