Node.js

Following are some of the Commonly asked Node.js interview questions and answers

Explain what is Node.js ?

Node.js is a server framework.It has following features:
Node.js is an open source server frameworkNode.js allows running JavaScript on the server.
Node.js provides event-driven,non-blocking I/O programming.It is based on the Google Chrome’s V8 Engine.

Which applications are more suitable to be developed using Node.js?

Following applications will see improvement in performance when developed using Node.js :

web applications
web tools such as task runners
I/O based applications
Streaming applications
Network applications

What is Asynchronous programming in Node.js?

In normal synchronous programming code is executed sequentially.This means that the first block of code should be executed before next bloxk ofcode could be executed.This means that the first block of code blocks the next block of code.In asynchronous programming ,which is used in Node.js ,the code to execute does not depend on the previous block of code.In other words there is no dependency on the sequence of code.This results in applications which executes faster.

What is a callback?

A callback is a function which executes when a task is completed.This prevents blocking operations.You call a task and define a callback for this task.The callback is executed after the task executes.This is how asynchronous applications ,developed in Node.js,works.
What is a Promise? Promise is a object which stands for the result of a callback.You register a callback using the promise method then()
asyncFunction(argumen1, argumen2).then(result => {//});

What is an Event Loop?

Node.js applications uses the concepts of loops.This means that you register a callback for an event.When the event is executed the callback is pushed on the event loop.The event loop can consist of multiple callbacks.Event loop executes callbacks in a FIFO manner.

How many threads can Event Loop utilize?

Event Loop runs under a single thread.But the callbacks can run in different background threads.

How Node.js improves the performance of application?

Node.js uses a single non-blocking thread.This means that thread runs time consuming tasks as I/O asynchronously.So instead of creating a new thread for every request ,Node.js uses the same thread for every request.This improves the performance of the application.

What is npm?

NPM is Node Package Manager. NPM is a repository of node.js pcakges.

What is ExpressJS?

ExpressJS is a Node.js web framwork for developing web and mobile applications.

What is MEAN stack?

MEAN is a group of technolgies used for developing web application.Following tehnologies are part of the MEAN stack:

Node.js Javascript based server side framework
Express Framework to develop web applications
MongoDB NoSQL database
AngularJS JavaScript framework to develop front end of web applications.

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Gulp can be used to perform different types of tasks.To automate tasks we define tasks in Gulp.In these tasks we use the npm plugins.There are npm plugins for different types of tasks.For example there are plugins for

Minifying files
Unit testing JS code
LESS css preprocessor
Running module bundlers such as WebPack

You can check the available plugins here plugins

Here we will see how to perform some of the common tasks using Gulp.

Minifying file using uglify plugin

Minifying file means compressing the file so that extra characters such s spaces and unused characters are removed.This is of much significance specially in web applications.This is because bandwidth is a very critical aspect of web applications.So reducing size of files even up to few kbs may impact performance.
To get idea baout how to define tasks in gulp please go through the following .

Before we can use the mify plugin we need to install the plugin using npm.We can install the plugin by executing the following command from within the project directory:

npm install --save-dev gulp-uglify

Once the uglify plugin is installed we will add the following code to the gulpfile.js file.Following will define a task called minify.This task is reading files defined in the js directory within the main directory.The output of gulp.src() method is passed to the uglify plugin by using pipe().

Passing the output of one method to next is very useful when using gulpfile to define tasks.

var gulp = require('gulp'),
uglify = require('gulp-uglify');

gulp.task('minify', function () {
gulp.src('js/Greet.js')
.pipe(uglify())
.pipe(gulp.dest('minified'))
});

The above task will minify all the files defined in the js directory and put the output in the minified directory.If you see the minified directory you can see that it contains the minified files corresponding to the files defined in the js directory.

Concatenate files

To concatenate all the JavaScript files in a single file we will use the concat plugin.To use the concat plugin we need to install the dependency of the plugin in project.json file as:

npm install --save-dev gulp-concat

Add the following code in the gulpfile.js.

gulp.task('minify', function () {
gulp.src('js/*.js')
.pipe(concat('main.js'));
.pipe(uglify())
.pipe(gulp.dest('minified'))
});

The ‘minify’ task will first concatenate all the .js files in the js folder to a file called main.js.It will then minify the main.js file.

Copy files from one location to another

To copy files from one location to another we can use the src() and dest() functions.The following task will copy the files from the js folder to the copy folder:

gulp.task('copy', function () {
gulp.src('./js/*.*')
.pipe(gulp.dest('./copy'));
});

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