Determine if MongoClient instance is already connected

The documentation is incorrect. Looking at the source code (line 395), the only supported parameter is an optional options object.

MongoClient.prototype.isConnected = function(options) {  options = options || {};  if (!this.topology) return false;  return this.topology.isConnected(options);};

So ignore the docs and don't pass a database name.

I have a suggestion about that:

const MongoClient = require('mongodb').MongoClient    , async = require('async')const state = {    db: null,    mode: null,}// In the real world it will be better if the production uri comes// from an environment variable, instead of being hard coded.const PRODUCTION_URI = 'mongodb://127.0.0.1:27018/production'    , TEST_URI = 'mongodb://127.0.0.1:27018/test'exports.MODE_TEST = 'mode_test'exports.MODE_PRODUCTION = 'mode_production'// To connect to either the production or the test database.exports.connect = (mode, done) => {    if (state.db) {        return done()    }    const uri = mode === exports.MODE_TEST ? TEST_URI : PRODUCTION_URI    MongoClient.connect(uri, (err, db) => {        if (err) {            return done(err)        }        state.db = db        state.mode = mode        done()    })}

You can call the database in your models as below:

const DB = require('../db')const COLLECTION = 'comments'let db// Get all commentsexports.all = (cb) => {    db = DB.getDB()    db.collection(COLLECTION).find().toArray(cb)}...

See the full node-testing project:

https://github.com/francisrod01/node-testing