Backend Interview Questions and Answers related to Node JS , Golang/Go , Express JS , MongoDB

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• ### What is package json?

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• ### Explain the purpose of module exports?

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• ### List down the major security implementations within Nodejs?

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• ### Explain the concept of URL module?

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• ### Explain the concept of middleware in Nodejs?

Execute any type of code
Update or modify the request and the response objects
Finish the request-response cycle
Invoke the next middleware in the stack

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• ### Explain libuv?

Full-featured event loop backed
File system events
Asynchronous file & file system operations
Asynchronous TCP & UDP sockets
Child processes

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• ### List down the two arguments that async.queue takes as input?

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• ### Differentiate between spawn and fork methods in Nodejs?

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• ### Explain the purpose of ExpressJS package?

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• ### Explain the usage of a buffer class in Nodejs?

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• ### How does Nodejs handle the child threads?

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• ### Explain stream in Nodejs along with its various types?

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• ### Describe the exit codes of Nodejs?

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• ### Is cryptography supported in Nodejs?

const crypto = require'crypto');
	const secret = 'akerude';
	const hash = crypto.createHmac('swaEdu', secret).update('Welcome to Edureka').digest('hex');
	console.log(hash);

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• ### Explain the reason as to why Express app and server folder must be kept separate?

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• ### What is the role of asset module in nodejs?

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• ### What is the role of async_hooks module in nodejs?

const asynchooks = require('asynchooks');

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• ### What are buffer objects in nodejs?

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• ### What are the different ways of implementing Addons in NodeJS?

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• ### How can we spawn the child process asynchronously without blocking the Nodejs event loop?

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• ### How can we take advantage of multi-core system in Nodejs as nodejs works on single thread?

const cluster = require('cluster');
	const http = require('http');
	const numCPUs = require('os').cpus().length;
if (cluster.isMaster) { 	console.log(Master ${process.pid} is running);
// Fork workers. 		for (let i = 0; i < numCPUs; i++) {     cluster.fork(); 	}
cluster.on('exit', (worker, code, signal) => {     console.log(worker ${worker.process.pid} died); 		}); 	}  	else { 		// Workers can share any TCP connection 		// In this case it is an HTTP server 	http.createServer((req, res) => {     res.writeHead(200);     res.end('hello world\n'); 	}).listen(8000);
console.log(Worker ${process.pid} started); 	} 	 	//Running Node.js will now share port 8000 between the workers: 	 	$ node server.js 	Master 3596 is running 	Worker 4324 started 	Worker 4520 started 	Worker 6056 started 	Worker 5644 started

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• ### What is the datatype of console?

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• ### Which are the different console methods available?

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• ### Can node js perform cryptographic functions?

Use require('crypto') to access this module.

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• ### How can we read or write files in node js?

const fs = require('fs');
There are a few methods like
fs.readFile(file, data[, options], callback)
fs.writeFile(file, data[, options], callback)

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• ### Which are the global objects in Node JS?

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• ### How can we perform asynchronous network API in Node JS?

It can be accessed using:

const net = require('net');

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• ### What are the utilities of OS module in NodeJS?

const os = require('os');.

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• ### Which are the areas where it is suitable to use NodeJS?

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• ### Which are the areas where it is not suitable to use NodeJS?

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• ### What Are The Key Features Of NodeJs?

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• ### Explain REPL In NodeJs?

READ - It Reads the input from the user, parses it into JavaScript data structure and then stores it in the memory.
EVAL - It Executes the data structure.
PRINT - It Prints the result obtained after evaluating the command.
LOOP - It Loops the above command until the user presses Ctrl+C two times.

.

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• ### Can you write CRUD operations in Node js without using frameworks?

var http = require('http');//create a server object:
	http.createServer(function (req, res) {
	res.writeHead(200, {'Content-Type': 'text/html'}); // http headervar 	url = req.url;
	if(url ==='/about'){
		res.write('<h1>about us page<h1>'); //write a response
		res.end(); //end the response
	}else if(url ==='/contact'){
		res.write('<h1>contact us page<h1>'); //write a response
		res.end(); //end the response
	}else{
		res.write('<h1>Hello World!<h1>'); //write a response
		res.end(); //end the response
	}}).listen(3000, function(){
	console.log("server start at port 3000"); //the server object listens on port 3000
	});

.

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• ### What Is The Difference Between Nodejs AJAX And JQuery?

Node.Js :

It is a server-side platform for developing client-server applications. For example, if we’ve to build an online employee management system, then we won’t do it using client-side JS. But the Node.js can certainly do it as it runs on a server similar to Apache, Django not in a browser.

AJAX (Aka Asynchronous Javascript And XML) :

It is a client-side scripting technique, primarily designed for rendering the contents of a page without refreshing it. There are a no. of large companies utilizing AJAX such as Facebook and Stack Overflow to display dynamic content.

JQuery :

It is a famous JavaScript module which complements AJAX, DOM traversal, looping and so on. This library provides many useful functions to help in JavaScript development. However, it’s not mandatory to use it but as it also manages cross-browser compatibility, so can help you produce highly maintainable web applications.

.

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• ### What Is EventEmitter In NodeJs?

// Import events module
	var events = require('events');
// Create an eventEmitter object 	var eventEmitter = new events.EventEmitter();

EventEmitter provides multiple properties like “on” and “emit”. The “on” property is used to bind a function to the event and “emit” is used to fire an event. .

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• ### What Is A Child_process Module In NodeJs?

The Child processes always have three streams , child.stdout, and child.stderr. The stream of the parent process shares the streams of the child process.

Node.js provides a module which supports following three methods to create a child process.

exec – method runs a command in a shell/console and buffers the output. spawn – launches a new process with a given command. fork – is a special case of the spawn() method to create child processes. .

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Table of Contents - Express JS

| No. | Questions | | --- | --------- | | | Express JS | | 1 | What is ExpressJS?| | 2 | What are some of the salient features of express?| | 3 | Explain with an example a working of a simple express app?| | 4 | Mention few properties of request parameter in express?| | 5 | How to get the name parameters in express?| | 6 | How to retrieve the get query string parameters using express? ?| | 7 | How to send a response back using express?| | 8 | How to set http response status using express?| | 9 | What are the different http status codes?| | 10 | Mention few properties of request parameter in express?| | 11 | How can you change http header value of a response?| | 12 | How to redirect to other pages server-side?| | 13 | How does routing work in express?| | 14 | What are the tasks that a middleware can do?| | 15 | What are the different types of middleware?| | 16 | How to serve static assests from express? | | 17 | How to provide file download using express?| | 18 | How to use the Response.cookie() method to manipulate your cookies?-method-to-manipulate-your-cookies)| | 19 | How to manage sessions using express?| | 20 | How to provide file download using express?| | 21 | How To Allow Cors In Expressjs Explain With An Example?|

Express Js

• ### What is ExpressJS?

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• ### What are some of the salient features of express?

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• ### Explain with an example a working of a simple express app?

const express = require('express')
	const port = 3000
	const app = express()
	app.get('/', function(req, res) {
      res.send('Hello World!')
	})
	app.listen(port, function(){ 
      console.log('listening on port',port)
	})

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• ### Mention few properties of request parameter in express?

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• ### How to get the name parameters in express?

// GET /user/tj
	req.params.name
	// => "tj"

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• ### How to retrieve the get query string parameters using express?

?height=6&weight=60
	//req.query.height - 6
	//req.query.weight - 60

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• ### How to send a response back using express?

function(req, res) {
		res.send('Hello World!')
	}
	function(req, res) {
		res.end('Hello World!')
	}
	function(req, res) {
		res.json({title:'Hello World!'})
	}

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• ### How to set http response status using express?

res.status(404).send('File not found')
	
	//if sendStatus we no need to write send method , i will pre send a few inbuilt messages upon using that
	
	res.sendStatus(200)
	// === res.status(200).send('OK')
res.sendStatus(403) 	// === res.status(403).send('Forbidden')
res.sendStatus(404) 	// === res.status(404).send('Not Found')
res.sendStatus(500) 	// === res.status(500).send('Internal Server Error')

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• ### What are the different http status codes?

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• ### Mention few properties of request parameter in express?

app.get('/', (req, res) => {
	console.log(req.headers)
	})
	
	app.get('/', (req, res) => {
	req.header('User-Agent')
	})

• ### How can you change http header value of a response?

res.set('Content-Type', 'text/html')
	res.type('json')
	// => 'application/json'
res.type('application/json') 	// => 'application/json'
res.type('png') 	// => image/png:

• ### How to redirect to other pages server-side?

res.redirect('/go-there')
	//it can be either a url or a path of file
	res.redirect(301, '/go-there')

• ### How does routing work in express?

In the Hello World example we used this code

app.get('/', function(req, res) { 
     / / 
	})
	//This creates a route that maps accessing the root domain URL / using the HTTP GET method to the response we want to provide.

• ### What are the tasks that a middleware can do?

Execute any code.
Make changes to the request and the response objects.
End the request-response cycle.
Call the next middleware function in the stack.

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• ### What are the different types of middleware?

Application-level middleware
Router-level middleware
Error-handling middleware
Built-in middleware
Third-party middleware

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• ### How to serve static assests from express?

const express = require('express')
	const app = express()
app.use(express.static('public'))
app.listen(3000, () => console.log('Server ready'))

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• ### How to provide file download using express?

Once a user hits a route that sends a file using this method, browsers will prompt the user for download.

The Response.download() method allows you to send a file attached to the request, and the browser instead of showing it in the page, it will save it to disk.

app.get('/', (req, res) => res.download('./file.pdf'))

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• ### How to use the Response.cookie() method to manipulate your cookies?

res.cookie('username', 'Adam')
	
	This method accepts a third parameter which contains various options:
	res.cookie('username', 'Adam', { domain: '.bangalore.com', path: '/administrator', secure: true })
res.cookie('username', 'Adam', { expires: new Date(Date.now() + 900000), httpOnly: true }) 	 	//clear cookie 	res.clearCookie('username')

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• ### How to manage sessions using express?

const express = require('express')
	const session = require('express-session')
const app = express() 	app.use(session( 	'secret': '343ji43j4n3jn4jk3n' 	))

We’ll reference that server-side to associate the session id with the data stored locally.

Memory is the default, it requires no special setup on your part, it’s the simplest thing but it’s meant only for development purposes.

The best choice is a memory cache like Redis, for which you need to setup its own infrastructure.

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• ### How to process forms using Express?

To extract it, you will use the express.urlencoded() middleware, provided by Express:

const express = require('express') 	const app = express()

app.use(express.urlencoded())

app.post('/submit-form', (req, res) => { 	const username = req.body.username 	//... 	res.end() 	})

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• ### How To Allow Cors In Expressjs Explain With An Example?

app.all('*', function(req, res, next) {
	res.set('Access-Control-Allow-Origin', '*');
	res.set('Access-Control-Allow-Methods', 'GET, POST, DELETE, PUT');
	res.set('Access-Control-Allow-Headers', 'X-Requested-With, Content-Type');
	if ('OPTIONS' == req.method) return res.send(200);
	next();
	});

var express = require('express')
	var cors = require('cors')
	var app = express()
 
	app.use(cors())

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Table of Contents - MongoDB and Mongoose

| No. | Questions | | --- | --------- | | | MongoDB and Mongoose | | 1 | What is MongoDB?| | 2 | What are the difference between NoSQL and SQL| | 3 | How to establish MongoDB database connection in a node application?| | 4 | What are virtual property in mongoose| | 5 | How can we add or create our own instance methods in mongoose| | 6 | How can we add or create our own static methods in mongoose| | 7 | What are the mongoose middlewares?| | 8 | How to query data using mongoose?| | 9 | What is Population in mongoose| | 10| What is Datamasking?| | 11| What is hashing and explain how it works?| | 12| What are salts and why are they so important?| | 13| What are pepper and why are they so important?| | 14| What are JWT?| | 15| What are different authentication methods?| | 16| What are disadvantages of using session based authentication?| | 17| What are disadvantages of using jwt based authentication?|

• ### What is MongoDB?

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• ### What are the difference between NoSQL and SQL?

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• ### How to establish MongoDB database connection in a node application?

Database Connection

Next, we will add code that connects to the database.

in database.js file

const mongoose = require('mongoose'); 	const server = '127.0.0.1:27017'; // REPLACE WITH YOUR DB SERVER 	const database = 'fcc-Mail';      // REPLACE WITH YOUR DB NAME

mongoose.connect(mongodb://${server}/${database})        .then(() => {          console.log('Database connection successful')        })        .catch(err => {          console.error('Database connection error')        })   	module.exports = { mongoose }

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• ### What are virtual property in mongoose?

userSchema.virtual('fullName').get(function() {
	return this.firstName + ' ' + this.lastName
	})
	
	userSchema.virtual('fullName').set(function(name) {
	let str = name.split(' ')
  
	this.firstName = str[0]
	this.lastName = str[1]
	})
	
	const user = new User()
	user.fullName = 'Thomas Anderson'
	console.log(user.toJSON())  // Output model fields as JSON
	console.log()
	console.log(user.fullName)  // Output the full name
	//The code above will output the following:
{ _id: 5a7a4248550ebb9fafd898cf, 		firstName: 'Thomas', 		lastName: 'Anderson' } 	//Thomas Anderson

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• ### How can we add or create our own instance methods in mongoose?

userSchema.methods.details = function() {
	return this.username + ' - ' +  this.email
	}
	//This method will be accessible via a model instance:
	const user = new User({
	username: 'user2',
	email: 'user2@gmail.com'
	})

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• ### How can we add or create our own static methods in mongoose?

userSchema.statics.getUsers = function() {
  return new Promise((resolve, reject) => {
    this.find((err, docs) => {
      if(err) {
        console.error(err)
        return reject(err)
      }
      resolve(docs)
			})
		})
	}

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• ### What are the mongoose middlewares?

Aggregate
Document
Model
Query

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• ### How to query data using mongoose?

In this example, we are going to:

Find all users
Skip the first 100 records
Limit the results to 10 records
Sort the results by the firstName field
Select the firstName
Execute that query

User.find()                   // find all users          .skip(100)                // skip the first 100 items          .limit(10)                // limit to 10 items          .sort({firstName: 1}      // sort ascending by firstName          .select({firstName: true} // select firstName only          .exec()                   // execute the query          .then(docs => {             console.log(docs)           })          .catch(err => {             console.error(err)           })

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• ### What is Population in mongoose?

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• ### What is Datamasking?

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• ### What is hashing and explain how it works?

• ### What are salts and why are they so important?

• ### What are pepper and why are they so important?

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• ### What are JWT?

Information Exchange: JSON Web Tokens are a good way of securely transmitting information between parties. Because JWTs can be signed—for example, using public/private key pairs—you can be sure the senders are who they say they are. Additionally, as the signature is calculated using the header and the payload, you can also verify that the content hasn't been tampered with.

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• ### What are different authentication methods?

Use API keys if you expect developers to build internal applications that don’t need to access more than a single user’s data.
Use OAuth access tokens if you want users to easily provide authorization to applications without needing to share private data or dig through developer documentation.
Use session cookies, here server is responsible for creating a session for the particular user when the user log's in, after that the id of the session is stored in a cookie on the user browser. For every request sent by the user, the cookie will be sent too, where the server can compare the session id from the cookie with the session information stored on the server so the user identity is verified.

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• ### What are disadvantages of using session based authentication?

Compromised Secret Key : The best and the worst thing about JWT is that it relies on just one Key. Consider that the Key is