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sudo :
Switch to super user
sudo su
chmod :



ssh : protocol to log into remote system.

grep : Global Regular Expression Print

nohup :
example : run java without holding the current session.

nohup java -jar a.jar &

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ReferenceError: MongoClient is not defined

Hi Guys ,

In this blog I will share information related to error : ReferenceError: MongoClient is not defined .


This error is related to the node.js code which is trying to connect to the database . To fix this check the following lines of code highlighted in yellow  . You need to make sure module mongodb is imported and the MongoClient is referenced using the variable .

	var mongo = require('mongodb');
	var url = "mongodb://localhost:27017/qanda";
	mongo.MongoClient.connect(url, function(err, db) {
	if (err) throw err;
	console.log("Database created!");
	db.close();
	});

Please make changes to the code and retest . Hope this will help you in resolving this issue .

MongoNetworkError: failed to connect to server MongoNetworkError: connect ECONNREFUSED

Hi Guys ,

You might have faced an error like MongoNetworkError: failed to connect to server [localhost:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017]

We generally face this error when the database server is down , if you bring the database server up you will not face this error .

The full error is as follows :


(node:15564) DeprecationWarning: current URL string parser is deprecated, and will be removed in a future version. To use the new parser, pass option { useNewUrlParser: true } to MongoClient.connect.
F:\\study\nodejs\node_modules\mongodb\lib\topologies\server.js:240
            throw err;
            ^

MongoNetworkError: failed to connect to server [localhost:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017]
    at Pool.<anonymous> (F:\\study\nodejs\node_modules\mongodb-core\lib\topologies\server.js:431:11)
    at emitOne (events.js:116:13)
    at Pool.emit (events.js:211:7)
    at connect (F:\\study\nodejs\node_modules\mongodb-core\lib\connection\pool.js:557:14)
    at makeConnection (F:\\study\nodejs\node_modules\mongodb-core\lib\connection\connect.js:39:11)
    at callback (F:\\study\nodejs\node_modules\mongodb-core\lib\connection\connect.js:261:5)
    at Socket.err (F:\\study\nodejs\node_modules\mongodb-core\lib\connection\connect.js:286:7)
    at Object.onceWrapper (events.js:315:30)
    at emitOne (events.js:116:13)
    at Socket.emit (events.js:211:7)

The code snipped to connect to mongo db is as follows :

	var mongo = require('mongodb');
	var url = "mongodb://localhost:27017/qanda";
	mongo.MongoClient.connect(url, function(err, db) {
	if (err) throw err;
	console.log("Database created!");
	db.close();
	});

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