Using bluebird to scroll through (in pages/chunks) a large dataset in mongodb and nodejs

sample.js

/**
 * Motivation:
 * Wanted to put together some code that used:
 *  - BlueBird (promises)
 *  - MongoDB NodeJS Driver
 *  - and paging that did not rely on skip()
 *
 * References:
 * Based on articles such as:
 * https://scalegrid.io/blog/fast-paging-with-mongodb/
 * and GitHub puclic code searches such as:
 * https://github.com/search?utf8=%E2%9C%93&q=bluebird+MongoClient+_id+find+limit+gt+language%3Ajavascript+&type=Code&ref=searchresults
 * which yielded smaple code hits such as:
 * https://github.com/HabitRPG/habitrpg/blob/28f2e9c356d7053884107d90d04e28dde75fa81b/migrations/api_v3/coupons.js#L71
 */

  var Promise = require('bluebird'); // jshint ignore:line
  var _ = require('lodash');
  var MongoClient = require('mongodb').MongoClient;
  var dbHandleForShutDowns;

  // option a: great for debugging
  var logger = require('tracer').console();
  // option b: general purpose use
  //var logger = console;

  //...

    var getPage = function getPage(db, collectionName, query, projection, pageSize, processPage) {
      //console.log('DEBUG', 'filter:', JSON.stringify(query,null,2));
      projection = (projection) ? projection['_id']=true : {'_id':true};
      return db
        .collection(collectionName)
        .find(query)
        .project(projection)
        .sort({'_id':1}).limit(pageSize)
        .toArray() // cursor methods return promises: http://mongodb.github.io/node-mongodb-native/2.1/api/Cursor.html#toArray
        .then(function processPagedResults(documents) {
          if (!documents || documents.length < 1) {
            // stop - no data left to traverse
            return Promise.resolve();
          }
          else {
            if (documents.length < pageSize) {
              // stop - last page
              return processPage(documents);
            }
            else {
              return processPage(documents) // process the results of the current page
                .then(function getNextPage(){ // then go get the next page
                  var last_id = documents[documents.length-1]['_id'];
                  query['_id'] = {'$gt' : last_id};
                  return getPage(db, collectionName, query, projection, pageSize, processPage);
                });
            }
          }
        });
    };

    //...

    return MongoClient
      .connect(params.dbUrl, {
        promiseLibrary: Promise
      })
      .then(function(db) {
        dbHandleForShutDowns = db;
        return getPage(db, collectionName, {}, {}, 5, function processPage(pagedDocs){console.log('do something with', pagedDocs);})
          .finally(db.close.bind(db));
      })
      .catch(function(err) {
        console.error("ERROR", err);
        dbHandleForShutDowns.close();
      });