Node Express MongoDB Native Driver — Where to open the DB connection?

Question

I want to open and initialise the DB via Node-Mongo-Native-Driver once in the app.js file and then leave it open and do reads in the routes. I put following code in the app.js and wrapping the app.gets in order to made them available when the DB is opened:

var mongoClient = new MongoClient(new Server('localhost', 27017));
mongoClient.open(function(err, mongoClient) {
  var db1 = mongoClient.db("dev-db")
    , products = db1.collection('products');
  app.get('/', routes.index);
  app.get('/users', user.list);
});

When I try now to read the DB in the index.js route I get

ReferenceError: products is not defined

I thought that index.js should be able to access products since it was defined in a outer function due to wrapping the app.gets in the initialisation.

Besides a 2nd question: what's the difference between MongoClient.open and MongoClient.connect

Answer 1

JavaScript uses lexical scoping - meaning, if you did this it would work:

var mongoClient = new MongoClient(new Server('localhost', 27017));
mongoClient.open(function(err, mongoClient) {
  var db1 = mongoClient.db("dev-db")
    , products = db1.collection('products');
  app.get('/', function closedOverIndexRoute(req, res, next) {
    console.log(products); // this will work
  });
  app.get('/users', user.list); // however, user.list will not be able to see `products`
});

A function doesn't become a closure (doesn't retain the values of its enclosing function) unless it's lexically (written) inside of the closure.

However, you probably don't want to write your whole app as one big closure. Instead, you can use exports and require to get access to your products collection. For example, in a file called mongoconnect.js:

var mongoClient = new MongoClient(new Server('localhost', 27017));
var products;

mongoClient.open(function(err, mongoClient) {
  var db1 = mongoClient.db("dev-db");
  products = db1.collection('products');
});

module.exports = {
  getProducts: function() { return products; }
};

Then in your index.js file:

var products = require('mongoconnect').getProducts();

Another option (if you want to keep just app and index) is to use a pair of closures:

index.js:

module.exports = function(products) {
  return function index(req, res, next) {
    console.log(products); // will have closed-over scope now
  };
};

app.js, inside of mongoClient.open(), where products is defined:

var index = require('./index')(products);