Question

I am playing around with node.js and based on this wonderful tutorial I created two providers (SchemaProvider and EntityProvider).

They look both like:

var Db = require('mongodb').Db;
var Connection = require('mongodb').Connection;
var Server = require('mongodb').Server;
var BSON = require('mongodb').BSON;
var ObjectID = require('mongodb').ObjectID;

EntityProvider = function(host, port) {
this.db = new Db('timerange', new Server(host, port, {auto_reconnect: true}, {}));
this.db.open(function() {
    console.log("Schema Provider has connected and may be used as of now.");
});
};

EntityProvider.prototype.getCollection = function(callback) {
this.db.collection('entity', function(error, collection) {
    if (error) {
        callback(error)
    } else {
        callback(null, collection);
    }

});
};

EntityProvider.prototype.findById = function(id /* The id to be found */, callback) {
this.getCollection(function(error, collection) {
    if (error) {
        callback(error);
    } else {
        collection.findOne({_id: id}, function(error, result) {
            if (error) {
                callback (error);
            } else {
                callback(null, result);
            }
        });
    }
});
};

In app.js I require(‘provider’) where both providers are defined.

Then I do:

schemaProvider = new SchemaProvider('192.168.0.50', 27017); 
entityProvider = new EntityProvider('192.168.0.50', 27017); 

Now, I created a module named dao (I come from a java/spring perspective :-)). As I didn’t use “var”, both variables and hence providers are accessible in my DAO. If I used “var”, the providers wouldn’t be accessible.

My question is :

If I wanted to use only one instance of the provider throughout the whole application, how would I do that?

Thanks in advance!

Answer 1

Setting globals (not using var) is a really bad practice, you should always avoid that.

If you want to have only an instance of the provider in your whole application you can do something like this:

provider.js

var providerInstance;

// define provider here

module.exports = function() {
  providerInstance = providerInstance || new Provider('192.168.0.50', 27017);
  return providerInstance;
}

That way the provider object gets created only once and then reused each time you require it:

app.js

var provider = require('./provider')();

app2.js

// using the same object as in app.js
var provider = require('./provider')();