How to properly reuse connection to Mongodb across NodeJs application and modules

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You can create a mongoUtil.js module that has functions to both connect to mongo and return a mongo db instance:

const MongoClient = require( 'mongodb' ).MongoClient;
const url = "mongodb://localhost:27017";

var _db;

module.exports = {

  connectToServer: function( callback ) {
    MongoClient.connect( url,  { useNewUrlParser: true }, function( err, client ) {
      _db  = client.db('test_db');
      return callback( err );
    } );
  },

  getDb: function() {
    return _db;
  }
};

To use it, you would do this in your app.js:

var mongoUtil = require( 'mongoUtil' );

mongoUtil.connectToServer( function( err, client ) {
  if (err) console.log(err);
  // start the rest of your app here
} );

And then, when you need access to mongo somewhere else, like in another .js file, you can do this:

var mongoUtil = require( 'mongoUtil' );
var db = mongoUtil.getDb();

db.collection( 'users' ).find();

The reason this works is that in node, when modules are require‘d, they only get loaded/sourced once so you will only ever end up with one instance of _db and mongoUtil.getDb() will always return that same instance.

Note, code not tested.

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