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Top NodeJS Questions

Q (1): What is Node.js?

Node.js is an open-source server side runtime environment built on Chrome's V8 JavaScript engine. It provides an event driven, non-blocking (asynchronous) I/O and cross-platform runtime environment for building highly scalable server-side applications using JavaScript.

Q (2): How do Node.js works?

Node is completely event-driven. Basically the server consists of one thread processing one event after another.

A new request coming in is one kind of event. The server starts processing it and when there is a blocking IO operation, it does not wait until it completes and instead registers a callback function. The server then immediately starts to process another event (maybe another request). When the IO operation is finished, that is another kind of event, and the server will process it (i.e. continue working on the request) by executing the callback as soon as it has time.

So the server never needs to create additional threads or switch between threads, which means it has very little overhead. If you want to make full use of multiple hardware cores, you just start multiple instances of node.js

Node JS Platform does not follow Request/Response Multi-Threaded Stateless Model. It follows Single Threaded with Event Loop Model. Node JS Processing model mainly based on Javascript Event based model with Javascript callback mechanism.

Single Threaded Event Loop Model Processing Steps:

  • Clients Send request to Web Server.
  • Node JS Web Server internally maintains a Limited Thread pool to provide services to the Client Requests.
  • Node JS Web Server receives those requests and places them into a Queue. It is known as “Event Queue”.
  • Node JS Web Server internally has a Component, known as “Event Loop”. Why it got this name is that it uses indefinite loop to receive requests and process them.
  • Event Loop uses Single Thread only. It is main heart of Node JS Platform Processing Model.
  • Even Loop checks any Client Request is placed in Event Queue. If no, then wait for incoming requests for indefinitely.
  • If yes, then pick up one Client Request from Event Queue
    • Starts process that Client Request
    • If that Client Request Does Not requires any Blocking IO Operations, then process everything, prepare response and send it back to client.
    • If that Client Request requires some Blocking IO Operations like interacting with Database, File System, External Services then it will follow different approach
      • Checks Threads availability from Internal Thread Pool
      • Picks up one Thread and assign this Client Request to that thread.
      • That Thread is responsible for taking that request, process it, perform Blocking IO operations, prepare response and send it back to the Event Loop
      • Event Loop in turn, sends that Response to the respective Client.

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Q (3): Where Node.js can be used?

NodeJS can be used to develop:

  • Real-Time Web Applications
  • Network Applications
  • Distributed Systems
  • General Purpose Applications Real world NodeJS examples
  • LinkedIn
  • Paypal
  • Netflix
  • Twitter
  • Walmart
  • Uber
  • Medium
  • EBay

And there are so many companies using NodeJS in there organization.

Q (4): What is npm in Node.js?

NPM stands for Node Package Manager. It provides following two main functionalities.

  • It works as an Online repository for node.js packages/modules which are present at <>.
  • It works as Command line utility to install packages, do version management and dependency management of Node.js packages. NPM comes bundled along with Node.js installable. We can verify its version using the following command-
$ npm --version

NPM helps to install any Node.js module using the following command.

$ npm install <Module Name>

For example, following is the command to install a famous Node.js web framework module called express-

$ npm install express

Q (5): What is package.json?

The package.json file in Node.js is the heart of the entire application. It is basically the manifest file that contains the metadata of the project where we define the properties of a package.

  "name": "name-of-your-application",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  "keywords": [
  "author": "Ajay",
  "license": "ISC",
  "dependencies": {
    "body-parser": "^1.19.0",
    "express": "^4.17.1",
    "mysql2": "^2.1.0",
    "sequelize": "^5.21.9"
  "devDependencies": {
    "unique-names-generator": "^4.2.0"

Reference : Check out official documentation

Q (6): How to create a simple server in Node.js that returns Hello World?

Step 01: Create a project directory

cmd> mkdir myapp
cmd> cd myapp

Step 02: Initialize project and link it to npm

npm init

This creates a package.json file in your myapp folder. The file contains references for all npm packages you have downloaded to your project. The command will prompt you to enter a number of things. You can enter your way through all of them EXCEPT this one:

entry point: (index.js) 

Rename this to:


Step 03: Install Express in the myapp directory

npm install express --save

Step 04: app.js

var express = require('express');
var app = express();
app.get('/', function (req, res) {
  res.send('Hello World!');

app.listen(3000, function () {
  console.log('Example app listening on port 3000!');

Step 05: Run the app

node app.js

Q (7): What is Event loop in Node.js? And How does it work?

The event loop is what allows Node.js to perform non-blocking I/O operations — despite the fact that JavaScript is single-threaded — by offloading operations to the system kernel whenever possible.

Node.js is a single-threaded application, but it can support concurrency via the concept of event and callbacks. Every API of Node.js is asynchronous and being single-threaded, they use async function calls to maintain concurrency. Node uses observer pattern. Node thread keeps an event loop and whenever a task gets completed, it fires the corresponding event which signals the event-listener function to execute.

Event-Driven Programming
In an event-driven application, there is generally a main loop that listens for events, and then triggers a callback function when one of those events is detected.

Although events look quite similar to callbacks, the difference lies in the fact that callback functions are called when an asynchronous function returns its result, whereas event handling works on the observer pattern. The functions that listen to events act as Observers. Whenever an event gets fired, its listener function starts executing. Node.js has multiple in-built events available through events module and EventEmitter class which are used to bind events and event-listeners as follows

// Import events module
var events = require('events');

// Create an eventEmitter object
var eventEmitter = new events.EventEmitter();


// Import events module
var events = require('events');

// Create an eventEmitter object
var eventEmitter = new events.EventEmitter();

// Create an event handler as follows
var connectHandler = function connected() {
   console.log('connection succesful.');
   // Fire the data_received event 

// Bind the connection event with the handler
eventEmitter.on('connection', connectHandler);
// Bind the data_received event with the anonymous function
eventEmitter.on('data_received', function() {
   console.log('data received succesfully.');

// Fire the connection event 

console.log("Program Ended.");

Q (8): What is REPL? What purpose it is used for?

REPL (READ, EVAL, PRINT, LOOP) is a computer environment similar to Shell (Unix/Linux) and command prompt. Node comes with the REPL environment when it is installed. System interacts with the user through outputs of commands/expressions used. It is useful in writing and debugging the codes. The work of REPL can be understood from its full form:

  • Read: It reads the inputs from users and parses it into JavaScript data structure. It is then stored to memory.
  • Eval: The parsed JavaScript data structure is evaluated for the results.
  • Print: The result is printed after the evaluation.
  • Loop: Loops the input command. To come out of NODE REPL, press ctrl+c twice

Simple Expression

$ node
> 10 + 20
> 10 + ( 20 * 30 ) - 40

Q (9): What is an error-first callback?

The pattern used across all the asynchronous methods in Node.js is called Error-first Callback. Here is an example:

fs.readFile( "file.json", function ( err, data ) {
  if ( err ) {
    console.error( err );
  console.log( data );

Any asynchronous method expects one of the arguments to be a callback. The full callback argument list depends on the caller method, but the first argument is always an error object or null. When we go for the asynchronous method, an exception thrown during function execution cannot be detected in a try/catch statement. The event happens after the JavaScript engine leaves the try block.

In the preceding example, if any exception is thrown during the reading of the file, it lands on the callback function as the first and mandatory parameter.

Q (10): What is callback hell in Node.js?

Callback hell is a phenomenon that afflicts a JavaScript developer when he tries to execute multiple asynchronous operations one after the other.

An asynchronous function is one where some external activity must complete before a result can be processed; it is “asynchronous” in the sense that there is an unpredictable amount of time before a result becomes available. Such functions require a callback function to handle errors and process the result.

    getMoreData(a, function(b){
        getMoreData(b, function(c){ 
            getMoreData(c, function(d){ 
	            getMoreData(d, function(e){ 

Techniques for avoiding callback hell

  1. Using Async.js
  2. Using Promises
  3. Using Async-Await
  • Managing callbacks using Async.js
    Async is a really powerful npm module for managing asynchronous nature of JavaScript. Along with Node.js, it also works for JavaScript written for browsers.

Async provides lots of powerful utilities to work with asynchronous processes under different scenarios.

npm install --save async
var async = require('async');
    function(callback) {
        callback(null, paramx); //paramx will be availaible as the first parameter to the next function
            The 1st parameter passed in callback.
            @null or @undefined or @false control moves to the next function
            in the array
            if @true or @string the control is immedeatly moved
            to the final callback fucntion
            rest of the functions in the array
            would not be executed
    function(arg1, callback) {
        //doSomething else
      // arg1 now equals paramx
        callback(null, result);
    function(arg1, callback) {
        //do More
        // arg1 now equals 'result'
        callback(null, 'done');
    function(arg1, callback) {
        //even more
        // arg1 now equals 'done'
        callback(null, 'done');
], function (err, result) {
    //final callback function
    //finally do something when all function are done.
    // result now equals 'done'
var async = require('async');
        // do some stuff ...
        callback(null, 'one');
            The 1st parameter passed in callback.
            @null or @undefined or @false control moves to the next function
            in the array
            if @true or @string the control is immedeatly moved
            to the final callback fucntion with the value of err same as
            passed over here and
            rest of the functions in the array
            would not be executed
        // do some more stuff ...
        callback(null, 'two');
// optional callback
function(err, results){
    // results is now equal to ['one', 'two']
  • Managing callbacks hell using promises
    Promises are alternative to callbacks while dealing with asynchronous code. Promises return the value of the result or an error exception. The core of the promises is the .then() function, which waits for the promise object to be returned. The .then() function takes two optional functions as arguments and depending on the state of the promise only one will ever be called. The first function is called when the promise if fulfilled (A successful result). The second function is called when the promise is rejected.
var outputPromise = getInputPromise().then(function (input) {
    //handle success
}, function (error) {
    //handle error
  • Using Async Await
    Async await makes asynchronous code look like it’s synchronous. This has only been possible because of the reintroduction of promises into node.js. Async-Await only works with functions that return a promise.
const getrandomnumber = function(){
    return new Promise((resolve, reject)=>{
        setTimeout(() => {
            resolve(Math.floor(Math.random() * 20));
        }, 1000);

const addRandomNumber = async function(){
    const sum = await getrandomnumber() + await getrandomnumber();