mongodb bulk insert with bluebird disposer

bulk.js

module.exports = function(col, limit=5000) {
    this.bulk = col.initializeUnorderedBulkOp()
    let cnt = 0

    this.add = record => {
        cnt++
        this.bulk.insert(record)
        if(cnt >= limit) {
            this.flush()
        }
        return this.bulk
    }
    this.flush = () => {
        if(cnt > 0) {
            this.bulk.execute()
            this.bulk = col.initializeUnorderedBulkOp() 
            cnt=0
        }
        return this.bulk
    }
}

connect.js

const MongoClient=require('mongodb').MongoClient
const Promise = require('bluebird')

module.exports = (connOpts) => {
    return MongoClient
        .connect(connOpts, {promiseLibrary: Promise})
        .then(db => db)
        .disposer(db => {
            db.close()
        })
}

main.js

const N=2000000
const Promise = require('bluebird')
const conn = require('./connect')
const Bulker = require('./bulk')

Promise.using(conn('mongodb://localhost:27017/test'), db => {
    const col = db.collection('colTest')
    const bulker = new Bulker(col)
    console.time('prepare')
    let task = []
    for(let k=0;k<N;k++) {
        task.push(k)
    }
    console.timeEnd('prepare')
    console.time('task')
    return Promise.each(task, t=> {
        return bulker.add({v: t})
    }).then(() => {
        console.timeEnd('task')
        return bulker.flush()
    })
})
// npm install mongodb bluebird
// node main.js