Monday 4 March 2019

Node JS Interview Questions and Answers

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



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