Express and Node JS Interview Questions

Node Js is one of the most popular and powerful server technologies today.
It allows you built the entire website only in one programming Language i.e Javascript. Node js is free and open source server technology that uses Javascript to create complete web software.It runs on various platforms like Windows, Linux, Unix, Mac OS X, etc.

• Bash on Linux.
• Terminal of Mac.
• Command Prompt or Powershell on Windows
• Shell/ Command line/terminal on Unix and Ubuntu

Node js is written in C, C++,JavaScript.It uses Google’s open source V8 Javascript Engine to convert Javascript code to C++.

Modules are reusable block of code whose existence does not impact other code in any way. It is not supported by Javascript. Modules are introduced in ES6. Modules are important for Maintainability, Reusability, and Namespacing of Code.

CommonJS Modules is the Standard how to code modules are structured. It specifies an ecosystem for JavaScript outside on the server or for native desktop applications.

require() is used to include modules from external files in Node Js. It is the easiest way to include a module in Node. Basically require is a function that takes a string parameter which contains the location of the file that you want to include. It reads the entire javascript file, executes the file, and then proceeds to return the exports object.
Syntax:

require('path');

Syntax and usage:

// greet.js
var greet=function(){
console.log("hello World");
}
module.exports=greet;

//In app.js

var greet=require('./greet.js');

greet();

Yes, Node Js is single threaded to perform asynchronous processing. Doing async processing on a single thread could provide more performance and scalability under typical web loads than the typical thread-based implementation.

By writing following line of code, you can create a server in Node Js.

var http =require('http');
http.createServer(function(req,res){

res.writeHead(200,{'Content-Type':'text/plain'});
res.end('Hello World\n');

}).listen(1320,'127.0.0.3');

Some of the main features of Express JS are listed below: –

• It is used for setting up middlewares so as to provide a response to the HTTP or RESTful requests.
• With the help of express JS, the routing table can be defined for performing various HTTP operations.
• It is also used for dynamically rendering HTML pages which are based on passing arguments to the templates.
• It provides each and every feature which is provided by core Node JS.
• The performance of Express JS is adequate due to the presence of a thin layer prepared by the Express JS.
• It is used for organizing the web applications into the MVC architecture.
• Everything from routes to rendering view and performing HTTP requests can be managed by Express JS.

Following are the steps used to set up an express JS application: –

1. A folder with the same name as the project name is created.
2. A file named package.json is created inside the folder created.
3. “npm install” command is run on the command prompt. It installs all the libraries present in package.json.
4. A file named server.js is created.
5. “Router” file is created inside the package which consists of a folder named index.js.
6. “App” is created inside the package which has the index.html file.

This way, an express JS application is set up.

Express JS is an application framework which is light-weighted node JS. A number of flexible, useful and important features are provided by this JavaScript framework for the development of mobile as well as web applications with the help of node JS.

Single-page, multi-page, and hybrid web applications can be built using Express JS.

Express.js is a lightweight web application which helps in organizing the web application into MVC architecture on the server side.

The arguments which are available to an Express JS route handler-function are-

• Req – the request object
• Res – the response object
• Next (optional) – a function which is used to pass control to one of the subsequent route handlers.

The third argument is optional and may be omitted, but in some cases, it is useful where there is a chain of handlers and control can be passed to one of the subsequent route handlers skipping the current one.

In Express JS, there are two ways for configuring the properties:

• With process.ENV:
• A file with the name “.env” is to be created inside the project folder.
• All the properties are to be added in the “.env” file.
• Any of the properties can be used in server.js.
• With require JS:
• A file with the name “config.json” is to be created in the config folder inside the project folder.
• The config properties are to be added in the config.json file.
• Now, require should be used to access the config.json file.

There is no notion of any database in Express JS. So, the concept of models is left up to third-party node modules, allowing the users to interface with nearly any type of database.

Since authentication is an opinionated area which is not ventured by express JS, therefore any authentication scheme can be used in express JS for the authentication of users.

Express JS supports any template engine that conforms to the (path, locals, callback) signature.

There’s no need to render HTML with the res.render () function. If there’s a specific file, then you should use the res.sendFile () function. If any assets are being served from a dictionary, then express.static () middleware function needs to be used.

Below are the few reasons why to use Express with Node.js

• Express js is built on top of Node.js. It is the perfect framework for ultra-fast Input / Output.
• Cross Platform
• Support MVC Design pattern
• Support of NoSQL databases out of the box.
• Multiple templating engine support i.e. Jade or EJS which reduces the amount of HTML code you have to write for a page.
• Support Middleware, basic web-server creation, and easy routing tools.

• readFile load the whole file which you had marked to read whereas createReadStream reads the complete file in the parts of the size you have declared.
• The client will receive the data faster in the case of createReadStream in contrast with readFile.
• In readFile, a file will first completely read by memory and then transfers to a client but in later option, a file will be read by memory in a part which is sent to clients and the process continue until all the parts finish.

Http defines a set of request methods to perform the desired actions. These request methods are:

1. GET: The GET method asked for the representation of the specifies resource. This request used to retrieve the data.
2. POST: The POST technique is utilized to present an element to the predetermined resource, generally causing a change in state or reactions on the server.
3. HEAD: The HEAD method is similar to the GET method but asks for the response without the response body.
4. PUT: This method is used to substitute all current representations with the payload.
5. DELETE: It is used to delete the predetermined resource.
6. CONNECT: This request is used to settle the TCP/IP tunnel to the server by the target resource
7. OPTION: This method is used to give back the HTTP strategies to communicate with the target resource.
8. TRACE: This method echoes the message which tells the customer how much progressions have been made by an intermediate server.
9. PATCH: The PATCH method gives partial modifications to a resource.

The main difference between the put and the patch is

Run below command on the terminal to set default node version along multiple installed versions of node. You can list all install versions of the node by running nvm ls

nvm alias default v7.3.0

Use node-uuid package to generate unique UUIDs/ guid in Node Js. Below code demonstrates how to generate it.

var uuid = require('node-uuid');

// Generate a v1 (time-based) id 
uuid.v1(); 

// Generate a v4 (random) id 
uuid.v4(); 

CORS stands for Cross-Origin Resource Sharing. It a is a mechanism that uses additional HTTP headers to tell a browser to let a web application running at one origin (domain) have permission to access selected resources from a server at a different origin.

Use below code to enable CORS on NodeJS

app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
  next();
});

Using Node Js you can build applications like:

• Internet of Things
• Real-Time Chats Applications
• Complex Single-Page Applications
• Real-Time Collaboration Tools
• Streaming apps
• Microservices / API’s

libuv is Cross-platform I/O abstraction library that supports asynchronous I/O based on event loops.It is written in C and released under MIT Licence.

libuv support Windows IOCP, epoll(4), kqueue(2), and Solaris event ports. Initially, it was designed for Node.js but later it is also used by other software projects.

Reference : https://en.wikipedia.org/wiki/Libuv

Zlib is Cross-platform data compression library. It was written by Jean-loup Gailly and Mark Adler. In Node js, you can Zlib for Threadpool, HTTP requests, and responses compression and Memory Usage Tuning. In order to use zlib in node js, you need to install node-zlib package. After installation below is sample code to use Zlib.

var Buffer = require('buffer').Buffer;
var zlib = require('zlib');

var input = new Buffer('lorem ipsum dolor sit amet');
var compressed = zlib.deflate(input);
var output = zlib.inflate(compressed);

Further Reading https://nodejs.org/api/zlib.html

https://en.wikipedia.org/wiki/Zlib

Following are the list of few new Features introduced in ES6

• const and let keywords
• Array helper functions like map, forEach, filter, find, every, some, reduce
• Arrow functions
• Classes and enhanced object literals
• Template strings
• Default function arguments
• Rest and spread operators
• Promises
• Modules
• Multi-line Strings
• Destructuring Assignment

JIT stands for Just-in-time. A JIT compiler is a program which is used to send bytecode (it consists of instruction that can be interpreted) to the processor by converting it into instruction. After you have done with writing a program, the compiler compiles the source language statements into bytecode instead of compiling it into the code that carries the information which is similar to the specific hardware platform's processor.

Relation of JIT with Node: Virtual machine of Nodejs has JIT compilation which improves the execution speed of the code. The virtual machine takes the source code and converts to machine code in runtime. By this, the hot functions which are called very often are compiled to machine code and, hence increasing speed.

Aggregations are a set of functions that are used to manipulate the data that has been returned from a MongoDB query. In Mongoose, aggregations work as a pipeline. The aggregate function accepts the array of data transformations which are applied by data using different methods in terms of arguments.

Syntax: db.customers.aggregate([ ... aggregation steps go here ...]);

In above, aggregation is applied to data of customers.

Normally NodeJs reads the content of a file in non-blocking, asynchronous way. Node Js uses its fs core API to deal with files. The easiest way to read the entire content of a file in nodeJs is with fs.readFile method. Below is sample code to read a file in NodeJs asynchronously and synchronously.

Reading a file in node asynchronously/ non-blocking

var fs = require('fs'); 
fs.readFile('DATA', 'utf8', function(err, contents) {
    console.log(contents);
});
console.log('after calling readFile');

Reading a file in node asynchronously/blocking

var fs = require('fs'); 
var contents = fs.readFileSync('DATA', 'utf8');
console.log(contents);

Streams are special types of objects in Node that allow us to read data from a source or write data to a destination continuously. There are 4 types of streams available in Node Js, they are

• Readable − For reading operation.
• Writable − For writing operation.
• Duplex − Used for both reading and write operation.
• Transform − A type of duplex stream where the output is computed based on the input.

In Node Js all core modules, as well as most of the community-published modules, follows a pattern where the first argument to any callback handler is an error object. this object is optional, if there is no error then in that case null or undefined is passed to the callback.

Example of the callback function

function callback(err, results) {
	// usually we'll check for the error before handling results
	if(err) {
		// handle error somehow and return
	}
		// no error, perform standard callback handling
}

Revealing module pattern is similar to Module Pattern.IT ExposES only the properties and methods you want via an returned Object.

var greeting = 'Hello world'
function greet() {
 console.log(greeting)
}
module.exports = {
  greet: greet
}

A Closure is a function defined within another scope that has access to all the variables within the outer scope.

Global variables can be made local (private) with closures.