How do I manage MongoDB connections in a Node.js web application?

Question: How do I manage MongoDB connections in a Node.js web application?
You need to install mongoose and bluebird module.
Also you must have mongodb install in server, As you need mongodb URL and port on which mongodb running.

Example

var mongoose = require('mongoose');
mongoose.Promise = require('bluebird');
//mongodb connection with error handing
mongoose.connect(config.MONGO_DB_URL + config.MONGO_DB);
var db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function() {
    console.log("MongoDB connected Successfully.!");
});

Question: mongoose - find all documents with IDs listed in array?

var AjaxChatUser = mongoose.model('AjaxChatUser');
AjaxChatUser.find({
    '_id': { $in: [
        mongoose.Types.ObjectId('4ed3ede8844f0f351100000c'),
        mongoose.Types.ObjectId('4ed3f117a844e0471100000d'), 
        mongoose.Types.ObjectId('4ed3f18132f50c491100000e')
    ]}
}, function(err, docs){
     console.log(docs);
});

Question: How to npm install to a specified directory?
Use --prefix option, to installed in specific directory.
Example

npm install --prefix  -g

Question: How to declare multiple module.exports in Node.js?
You can put multiple function inside module.exports.

Example

module.exports = {
    method: function() {},
    otherMethod: function() {},
};

Question: Can we write a JS code that work for both (node and the browser)?
Yes, We can write.
Suppose we have mymodule.js which have following code.

Example of Code
(function(exports){
   exports.test1 = function(){
        return 'this is test1 function.'
    };
   exports.test2 = function(){
        return 'this is test2 function.'
    };

})(typeof exports === 'undefined'? this['mymodule']={}: exports);

In Node (Server side()

var share = require('./mymodule.js');
share.test1();
share.test2();

In Browser (Client side()

//Include the js with script tag
share.test1();
share.test2();

Question: How we can use global variable in Node?
We can use with global.varname

global.version='1022.55';

Question: How we can access static files with express.js in Node?
Use following to set the folder as static so that we can put public files here.

app.use(express.static('public')); //public folder

How to determine a user's IP address in node

Question: How to determine a user's IP address in node?
We can use request.connection.remoteAddress to detect the IP Address in node but sometimes server is running behind the load balancer, In this case we need to check for x-forwarded-for

Example

exports.check_node = function(req, res) {
    var ip = req.headers['x-forwarded-for'] || 
     req.connection.remoteAddress || 
     req.socket.remoteAddress ||
     (req.connection.socket ? req.connection.socket.remoteAddress : null);
    
    res.json({ip:ip});
};

Question: How can I use wait In Node.js?
You can use setTimeout function similar to javascript

Example

function function1() {    
    console.log('Welcome to My Console,');
}

function function2() {    
    console.log('Console2');
}

setTimeout(function2, 3000); //call second
function1(); //Call first

Question: Command to remove all npm modules in windows?

1. Remove module local
   Go to directory with name of node_modules under current project. DELETE all the folder.
2. C:\Users\DELL\AppData\Roaming\npm\node_modules
   Delete all the files

Question: Node.js check if path is file or directory?
For this, you need to install the fs module.

var fs = require('fs');
stat = await fs.lstat(PATH_HERE)
stats.isFile()
stats.isDirectory()
stats.isBlockDevice()
stats.isCharacterDevice()
stats.isSocket()

Question: What is use of app.use in express?
app.use is a method to configure the middleware used by Express.


Question: Difference between process.stdout.write and console.log?
console.log() calls process.stdout.write with formatted output.
console.log() explains in library

console.log = function (d) {
  process.stdout.write(d + '\n');
};

Question: How to Detect if file called through require or directly by command line?

if (require.main === module) {
    console.log('called directly');
} else {
    console.log('required as a module');
}

Question: What is the use of next() in express?
It passes control to the next matching route.

For Example:

app.get('/user/:id?', function(req, res, next){
    var id = req.params.id;
    if (id) {
        // do something
    } else {
        next(); 
    }
});

Node JS Interview Questions and Answers

Question: How to write in files synchronous in Node.js?

const fs = require('fs');
fs.writeFileSync('/fold/test.txt', 'Write in file');

Question: What is the difference between --save and --save-dev?
--save-dev is used to save the package for development purpose. Example: unit tests, minification
--save is used to save the package required for the application to run.



Question: is node single threaded or multithreaded?
It is a single threaded language which in background uses multiple threads to execute asynchronous code. Also its is non-blocking which means that all functions ( callbacks ) are delegated to the event loop and they are ( or can be ) executed by different threads.


Question: How to print console without a trailing newline?

    process.stdout.write("hello: ");

Question: How to create a directory if it doesn't exist using Node.js?

var fs = require('fs');
var dir = './tmp';
if (!fs.existsSync(dir)){
    fs.mkdirSync(dir);
}

Question: How can we execute binary/exe files in Node ?

const { exec } = require('child_process');
exec('wc -l', (err, stdout, stderr) => {
  if (err) {
    // node couldn't execute the command
    return;
  }
});

Question: Is there a way to get version from package.json?

var pjson = require('./package.json');
console.log(pjson.version);

Question: How can I update npm on Windows?
Simple way
Download and run the latest MSI from nodejs.org. The MSI will update your installed node and npm.


Question: How can node in background?

nohup node server.js &

Question: What is the difference between __dirname and ./ in node.js?
__dirname is always the directory in which the currently executing script exist.
. gives you the directory from which you ran the node command in your terminal window



Question: How to get the full url in Express?

var url = require('url');
function fullUrl(req) {
  return url.format({
    protocol: req.protocol,
    host: req.get('host'),
    pathname: req.originalUrl
  });
}

Node JS Interview Questions and Answers for 3 Years experience

Question: What is the difference between tilde(~) and caret(^) in package.json?
~ update you to all future patch versions, without incrementing the minor version. ~2.2.3 will use releases from 2.2.3 to 2.3.0.
^ update you to all future minor/patch versions, without incrementing the major version. ^2.3.3 will use releases from 2.3.3 to 3.0.0.



Question: How do I pass command line arguments to a Node.js program?
In Node - How to pass arguent

node process-2.js one two=three four

In Node - How to get argument value

process.argv

Question: How do I update each dependency in package.json to the latest version?

npm i -g npm-check-updates
ncu -u
npm install

Question: How can I update NodeJS and NPM to the next versions?

npm install -g npm

Question: How to write in files synchronous in Node.js?

const fs = require('fs');
fs.writeFileSync('/fold/test.txt', 'Write in file');

Question: How to read environment variables?

var mode   = process.env.NODE_ENV;
var apiKey = process.env.apiKey; 

Question: How to parse JSON using Node.js?

//Parse JSON Data
var bodyParser = require('body-parser')
app.use(bodyParser.json());       // to support JSON-encoded bodies
app.use(bodyParser.urlencoded({// to support URL-encoded bodies
    extended: true
}));

Question: How  an HTTP POST request made?

var request = require('request');
request.post(
    'https://www.example.com',
    { json: { key: 'value' } },
    function (error, response, body) {
        if (!error && response.statusCode == 200) {
            console.log(body);
        }
    }
);

Question: How can I get the full object in Node.js's console.log(), rather than Object?

const util = require('util');
console.log(util.inspect(myObject, {showHidden: false, depth: null}));

OR
console.log(JSON.stringify(myObject, null, 4));

Question: How to encode a text using base64?

Buffer.from("Hello World").toString('base64'); //SGVsbG8gV29ybGQ=

Question: How to decoded a text which is encoded with base64?

Buffer.from("SGVsbG8gV29ybGQ=", 'base64').toString('ascii'); //Hello World

Node JS Interview Questions and Answers

Question: What is callback?
A callback function is called at the completion of a given task.


Question: What Are Arrow Functions?
Arrow functions – also called "fat arrow" functions, are a more concise syntax for writing function expressions.

Examples

const multiplyES6 = (x, y) => { return x * y };
const phraseSplitterEs6 = phrase => phrase.split(" ");
var docLogEs6 = () => { console.log(document); };

Question: What is JSON.stringify()?
JSON.stringify() method converts a JavaScript object to a JSON string.


Question: What is Handlebars view engine in nodejs?
Handlebars.js is templating engine in NodeJS.


Question: What is an error-first callback?
Error-first callbacks are used to pass errors and data.
You have to pass the error as the first parameter, and it has to be checked to see if something went wrong.


Question: What are Promises?
The Promise object represents the event completion or failure of an asynchronous operation, and its resulting value.

function readFile(filename, enc){
  return new Promise(function (resolve, reject){
    fs.readFile(filename, enc, function (err, res){
      if (err) reject(err);
      else resolve(res);
    });
  });
}

Question: What is callback hell in Node.js?
Callback hell is the result of heavily nested callbacks that make the code not only unreadable but also difficult to maintain.


Question: What is the role of REPL in Node.js?
Full form of REPL is Read Eval print Loop.


Question: Explain chaining in Node.js?
Chaining is a mechanism whereby the output of one stream is connected to another stream as input.


Question: How to access the GET parameters after ? in Express?

req.query.name;

Question: What is buffer module?
Buffer class provides instances to store binary data similar to an array of integers.


Question: How we exporting a Module?
We use export for exporting the module.

exports.sayHelloInEnglish = function() {
    return "Hello, how are you?";
};

Question: What is meaning of tilde and caret?
the tilde(~) matches the most recent minor version.
the caret(^) matches the most recent major version.


Question: How to access 2nd argument when passed in command line?

process.argv.slice(3,4); //two=222

Question: What is Yargs?
Yargs helps to parsing arguments of command line tools.



Question: What is Async?
Async is a utility module which provides straight-forward, powerful functions for working with asynchronous JavaScript.

async.map(['file1','file2','file3'], fs.stat, function(err, results) {
    // results is now an array of stats for each file
});

Question: How to use expressJs in Node

var express = require('express');
var app = express();

//Add cookie to express
var cookieParser = require('cookie-parser');
app.use(cookieParser());

//Add session to express
var session = require('express-session');
app.use(session({
    secret: 'XEDDKJKXD',
    name: 'testname',
    proxy: true,
    resave: true,
    saveUninitialized: true
}));

//Add static path
app.use(express.static('public'))

//Add "Parse JSOn" to express
var bodyParser = require('body-parser')
app.use(bodyParser.json());       // to support JSON-encoded bodies
app.use(bodyParser.urlencoded({// to support URL-encoded bodies
    extended: true
}));

//Add middleware to express
app.use(function (req, res, next) {
  //console.log('Time2222:', Date.now())
 res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
   
  next()
})


//Add https to expresJs
var https = require('https');
var privateKey  = fs.readFileSync('public/cert/server.key', 'utf8');
var certificate = fs.readFileSync('public/cert/georama.crt', 'utf8');
var caCertificate = fs.readFileSync('public/cert/chain.crt', 'utf8');

var credentials = {key: privateKey, cert: certificate, ca: caCertificate};
var httpsServer = https.createServer(credentials, app);   

Question: How to start node on port?

httpsServer.listen(config['SERVERPORT'],function() {
    console.log('https listening on *:' + config['SERVERPORT']);
});

Question: How to connect socket with https?

var io = require('socket.io')(httpsServer);

Question: What is underscore module in nodeJS?
Underscore is a JavaScript library that provides a whole mess of useful functional programming helpers without extending any built-in objects.

console.log(_.each([1, 2, 3], function(value, key){
     console.log(key+'>>'+value)
 }));

Question: What Are Core Features Of Express Framework?
1) Allows to set up middlewares to respond to HTTP Requests.
2) Defines a routing.
3) Dynamically render HTML Pages.


Question: Give simple example of ExpressJS?

var app = require('express')();
var http = require('http').Server(app);

app.get('/listing', function (req, res) {   
   res.send('This is listing API'); 
});

http.listen('8080', function() {
    console.log('listening on *:8080');
});

Question: What are different Middleware?
1) Application-level middleware
2) Router-level middleware
3) Error-handling middleware
4) Built-in middleware
5) Third-party middleware