Web Development with ExpressJS

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Express.js and Node.js Interview Questions

1) What is Node js? 

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.

2) Explain CLI in Node.js? 

CLI stands for Command Line Interface. It is a utility or program on your computer where users type commands to perform some action or run some script rather than clicking on the screen.
There are different types of command line interfaces depending on which operating system you are using. We have listed some of them below.

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

 
3) In which Language Node Js is written? 

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

 
4) Who is the author of Node Js? 

Node Js is written by Ryan Dahl.

5) Explain What is a Javascript Engine? 

A Javascript Engine is a program that converts code written in Javascript to something that computer processor understands.

 
6) Explain V8 Engine? 

V8 is Google’s open source high-performance JavaScript engine, written in C++ and used in Google Chrome, the open source browser from Google, and in Node.js, among others. It implements ECMAScript as specified in ECMA-262, and runs on Windows 7 or later, macOS 10.5+, and Linux systems that use IA-32, ARM, or MIPS processors. V8 can run standalone or can be embedded into any C++ application.
 

7) Explain ECMAScript? 

ECMAScript is the standard on which Javascript is based on. It was created to standardize Javascript. It is commonly used for client-side scripting on the World Wide Web and used by Node Js for writing server applications and services.
 

8) How can you check the installed version of Node Js? 

Use node -v command to check the installed version of Node Js.

 
9) Explain What is NPM? 

NPM stands for node package manager. It is default Package Manager for JavaScript programming language. NPM is used for installing/updating packages and modules of Javascript.

 
10) Explain Modules in Node Js? 

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.

 
9) Explain What is NPM?

NPM stands for node package manager. It is default Package Manager for JavaScript programming language. NPM is used for installing/updating packages and modules of Javascript.

10) Explain Modules in Node Js? 

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.

11) What are CommonJs Modules?

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.

12) For what require() is used in Node Js?

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');

13) What Is Express Js?

Express JS is a framework which helps to develop web and mobile applications. Its works on nodejs plateform. Its sub part of node.js.

 
14) What Type Of Web Application Can Built Using Express Js?

You can build single-page, multi-page, and hybrid web applications.

 
15) What Are Core Features Of Express Framework?

·      Allows to set up middlewares to respond to HTTP Requests
·      Defines a routing table which can works as per HTTP Method and URL
·      Dynamically render HTML Pages
 

16 Why I Should Use Express Js? 

Express 3.x is a light-weight web application framework to help organize your web application into an MVC architecture on the server side.
 

17) How To Install Expressjs? 

Assuming you’ve already installed Node.js, create a directory to hold your application, and make that your working directory.

$ mkdir myapp
$ cd myapp

Use the npm init command to create a package.json file for your application. For more information on how package.json works, see Specifics of npm’s package.json handling.

$ npm init

This command prompts you for a number of things, such as the name and version of your application. For now, you can simply hit RETURN to accept the defaults for most of them, with the following exception:

entry point: (index.js)

Enter app.js, or whatever you want the name of the main file to be. If you want it to be index.js, hit RETURN to accept the suggested default file name.

Now install Express in the myapp directory and save it in the dependencies list. For example:

$ npm install express --save

To install Express temporarily and not add it to the dependencies list, omit the --save option:

$ npm install express
 

18) How To Get Variables In Express.js In Get Method? 

var express = require('express');
var app = express();
app.get('/', function(req, res){
    /* req have all the values **/  
  res.send('id: ' + req.query.id);  
});
app.listen(3000);

19) How To Get Post A Query In Express.js? 

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

20) How To Output Pretty Html In Express.js? 

app.set('view options', { pretty: true });
 

21) How To Get The Full Url In Express? 

var port = req.app.settings.port || cfg.port;

res.locals.requested_url = req.protocol + '://' + req.host  + ( port == 80 || port == 443 ? '' : ':'+port ) + req.path;
 

22) How To Remove Debugging From An Express App? 

var io = require('socket.io').listen(app, { log: false });
io.set('log level', 1);
 

23) How To Do 404 Errors? 

app.get('*', function(req, res){
  res.send('what???', 404);
});

24) How To Download A File? 

app.get('/download', function(req, res){
  var file = __dirname + '/download-folder/file.txt';
  res.download(file); 
});

 
25) What Is The Parameter “next” Used For In Express? 

app.get('/userdetails/:id?', function(req, res, next){
 });

req and res which represent the request and response objects 

nextIt passes control to the next matching route.

 
26) What Function Arguments Are Available To Express.js Route Handlers? 

The arguments available to an Express.js route handler function are:

• req - the request object
• res - the response object
• next (optional) - a function to pass control to one of the subsequent route handlers

The third argument may be omitted, but is useful in cases where you have a chain of handlers and you would like to pass control to one of the subsequent route handlers, and skip the current one.

 
27) How To Config Properties In Express Application? 

In an ExpressJS Application, we can config properties in following two ways:

With Process.ENV:

• Create a file with name ‘.env’ inside the project folder. 
• Add all the properties in ‘.env’ file. 
• In server.js any of the properties can be used as:
  
  var host = process.env.APP_HOST
  app.set('host', host);
  logger.info('Express server listening on http://' + app.get('host'));

With RequireJs:

• Create a file called  ‘config.json’ inside a folder called ‘config’ inside the project folder.
• Add config properties in config.json.
  
  {
   "env":"development",  "apiurl":"http://localhost:9080/api/v1/"
  }

• Use require to access the config.json file.
  
  var config = require('./config/config.json');

 
28) How To Allow Cors In Expressjs? Explain With An Example? 

In order to allow CORS in Express.js,  add the following code in server.js:

app.all('*', function(req, res, next) {
res.set('Access-Control-Allow-Origin', '*');
res.set('Access-Control-Allow-Methods', 'GET, POST, DELETE, PUT');
res.set('Access-Control-Allow-Headers', 'X-Requested-With, Content-Type');
if ('OPTIONS' == req.method) return res.send(200);
next();
});

29) How To Redirect 404 Errors To A Page In Expressjs? 

In server.js add the following code to redirect 404 errors back to a page in our ExpressJS App:

/* Define fallback route */
app.use(function(req, res, next) {
    res.status(404).json({errorCode: 404, errorMsg: "route not found"});
});

30) Explain Error Handling In Express.js Using An Example? 

From Express 4.0 Error handling is much easier. The steps are as following:

• Create an express.js application and as there is no built-in middleware like errorhandler in express 4.0, therefore, the middleware need to be either installed or need to create a custom one.

Create a Middleware:

• Create a middleware as following:
  
  // error handler
  app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};
  // render the error page
  res.status(err.status || 500);
  res.render('error');
  });

Install Error Handler Middleware:

• Install errorhandler.
  npm install errorhandler --save
• Create a variable.
  var errorhandler = require('errorhandler')
• Use the middleware as following:
  
  if (process.env.NODE_ENV === 'development') {
   // only use in development
    app.use(errorhandler({log: errorNotification}))
  }
  function errorNotification(err, str, req) {
   var title = 'Error in ' + req.method + ' ' + req.url
   notifier.notify({
     title: title,
     message: str
   })
  }

 
31) How To Enable Debugging In Express App? 

In different Operating Systems, we have following commands:

On Linux the command would be as follows:

  $ DEBUG=express:* node index.js

On Windows the command would be:

  set DEBUG=express:* & node index.js

32) How Should I Structure My Application?

There is no definitive answer to this question. The answer depends on the scale of your application and the team that is involved. To be as flexible as possible, Express makes no assumptions in terms of structure.

Routes and other application-specific logic can live in as many files as you wish, in any directory structure you prefer. View the following examples for inspiration:

• Route listings
• Route map
• MVC style controllers

Also, there are third-party extensions for Express, which simplify some of these patterns:

• Resourceful routing

33) How Do I Define Models? 

Express has no notion of a database. This concept is left up to third-party Node modules, allowing you to interface with nearly any database.
 

34) How Can I Authenticate Users? 

Authentication is another opinionated area that Express does not venture into. You may use any authentication scheme you wish.
 

35) Which Template Engines Does Express Support? 

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

36) What are events? 

An event is an action or occurrence recognized by software/app that is handled by event handler by writing a code that will be executed when the event fired.
Mouse move, Click, file copied or deleted are some examples of events.
In Node Js there are two types of events.
1)System Events: The event that comes from the C++ side.
2)Custom Events: Custom events are user-defined events.
 

37) Explain event loop in Node Js? 

In Node Js processes are single threaded, to supports concurrency it uses events and callbacks. An event loop is a mechanism that allows Node.js to perform non-blocking I/O operations.
 

38) How to create a simple server in Node js that returns Hello World? 

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');

 
39) How can plain HTML be rendered in express JS? 

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.
 

40) Explain the difference between readFile and createReadStream in Node js? 

• 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.