I am new to Node and Mongo, I was trying to connect to a mongo database in one file and export the database handler to many other files, so that I need not to connect to the database in all files which need a connection to it. Here is how I attempted to do it

```js
// db.js
var client = require('mongodb').MongoClient
var assert = require('assert')
var url = 'mongodb://localhost:27017/test'

client.connect(url, (err, db) => {
  assert.equal(err, null)
  module.exports = db
})

```

After exporting the db handler, I tried to access methods on it in another file as follows

```js
var db = require('./db')
console.log(db.collection('col'))

```

but it throws a TypeError, saying that `db.collection` is not a function. How can I access the methods on db handler in other files?