Express Interview Questions
Q:- What is Express JS?
  1. Express.js is a free open-source, flexible node.js web application framework.
  2. It is designed for building backend web applications and APIs, etc.
  3. It has been developed by TJ Holowaychuk in 2010 and written in JavaScript.
Express.js key features:

Following are the key features of express framework:

  1. Middlewares: Express middleare are functions that access of - request (req), response (res) and next().
  2. Routing: It is used to define routes in order to perform and handle HTTP requests/operations.
  3. Templates Engine: It has SSR renders html template used to render the HTML on the browser.
  4. High Performance: Express prepare a thin layer, therefore, the performance is adequate.
  5. Database Support: Express supports RDBMS as well as NoSQL databases - MySQL, MongoDB, etc.
  6. MVC Support: It is a web application which supports MVC architecture.
  7. ORM Support: It support various ORM/ODM - Mongoose, Sequelize, etc.
Q:- What is Scaffolding in Express.js?

Scaffolding is creating the skeleton structure of application

There are 2 way to do this, by using:

  1. Express Application Generator
  2. Yeoman
1. Express Application Generator:

Use express-generator tool to quickly create an skeleton of your application.

npm install -g express-generator express myTestExpressApp

Th above command will create your project named - myTestExpressApp with followings.

  1. bin:It is also called www is the main configuration file of the app.
  2. public:It contains static assets - javascript, css, images, etc.
  3. routes: It contains route files of the app.
  4. views: It contains the view files (HTML) of the application.
  5. app.js: It is the main file of the application which execute first.
  6. package.json: It is the manifest file. It contains all metadata info of the project.

Install all the dependencies mentioned in the package.json file:

cd myTestExpressApp npm install
Q:- How to enable debugging in express app?

In different Operating Systems, we have following commands:

On Linux:

DEBUG=express:* node app.js

On Windows:

set DEBUG=express:* node app.js
Q:- Serving static files in Express.js?
app.use(express.static('public')) app.use('/static', express.static(path.join(__dirname, 'public')));
Q:- What is routing and how routing works in Express.js?
Routing refers to determining how an application responds to a request.
Route Syntax:
app.METHOD(PATH, HANDLER);

Where:

  • app is an instance of express.
  • METHOD is an HTTP request method (get, post, put, etc.).
  • PATH is a path/endpoint on the server.
  • HANDLER is a function executed when the route is matched.

#Example: A route with path / and get method.

app.get('/', function (req, res) { res.send('Express.js Interview Questions') })
Q:- How dynamic routing works in express.js?

When someone pass parameters in URL (i.e. Parametrized URL), this routing phenomenon is called dynamic routing.

var express = require('express'), app = express(); app.get('/article/:id', function(req , res){ res.render('article' + req.params.id); })

In above example: id is a parameters, which can be different for different request.

Q:- What is Middleware in express.js?
Middleware is a function that is invoked by the express routing layer before the final request processed.
Middleware functions can perform the following tasks:
  1. Execute any code - validation, setting headers, etc.
  2. You can make changes to the request (req) and response (res) objects.
  3. You can also end the request-response cycle, if rquired.
  4. You can call the next middleware function in the stack to proceed and process the final request.

If the current middleware function does not end the request-response cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.

Types of 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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1. Application-level middleware:

This kind of middleware method is bind to the app Object using app.use() method. It applies on all routes.

//This middleware will execute for each route. app.use(function (req, res, next) { console.log('Current Time:', Date.now()) next() })
2. Router-level middleware:

Router-level middleware binds to an specific instance of express.Router()

3. Built-in middleware:

Express has the following built-in middleware functions:

  • express.static serves static assets such as HTML files, images, and so on.
  • express.json parses incoming requests with JSON payloads.
  • express.urlencoded parses incoming requests with URL-encoded payloads.
4. Third-party middleware:

There are a lots of third party middleware, such as

  • Body-parser
  • Cookie-parser
  • Mongoose
  • Sequelize
  • Cors
  • Express-validator

To handle HTTP POST request in express.js version 4 and above, you need to install a middleware module called body-parser, body-parser extract the entire body portion of an incoming request stream and exposes it on req.body, this middleware was a part of express.js earlier but now you have to install it separately.

These can be installed by using command:

>> npm install MODULE_NAME

And they can be loaded using requires and used later.

#Example:
var bodyParser = require('body-parser'); app.use(bodyParser.json()); app.use(bodyParser.urlencoded({ extended: false }))

Note: Multiple Middleware can be used as an array on a single route.

var middlewareArray = [middleware1, middleware2] app.get('/home', middlewareArray, function (req, res, next) { //Code snippets })
Q:- Database integration in express.js?
Express.js supports of many RDBMS & NoSQL databases like
  • MongoDB
  • MySQL
  • Oracle
  • PostgreSQL
  • SQL Server
  • SQLite
#Example: Install MongoDB
>>npm install mongodb var MongoClient = require('mongodb').MongoClient MongoClient.connect('mongodb://localhost:27017/test_db', function (err, db) { if (err) throw err db.collection('mammals').find().toArray(function (err, result) { if (err) throw err console.log(result) }) })
Q:- Error handling in Express.js?
var express = require('express'), app = express(); app.use(function (err, req, res, next) { console.error(err.stack) // error first callback res.status(500).send('Something went wrong!') })
var express = require('express'), app = express(); app.use(function(req, res, next) { res.status(404).json({ errorCode: 404, errorMsg: "Not found!" }); });
Q:- How to allow CORS in Express.js (Node.js)? Explain with an Example?

Cross-origin resource sharing (CORS) is a mechanism that allows restricted resources to be requested from another domain/server.

There are mainly three ways you can do this using
  1. res.setHeader() - It allow to set only single header
  2. res.header() OR res.set() - It allow to set multiple headers.
  3. express cors module
1. Enable CORS for all resources using res.setHeader().
app.use(function(req, res, next) { // Website you wish to allow to connect res.setHeader("Access-Control-Allow-Origin", "*"); // Request methods you wish to allow res.setHeader( "Access-Control-Allow-Methods", "GET, POST, OPTIONS, PUT, PATCH, DELETE" ); // Request headers you wish to allow res.setHeader( "Access-Control-Allow-Headers", "X-Requested-With,content-type" ); // Set to true if you need the website to include cookies in the requests sent // to the API (e.g. in case you use sessions) res.setHeader("Access-Control-Allow-Credentials", true); // Pass to next layer of middleware next(); });
2. Enable CORS for all resources using res.header()
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(); });
3. Enable CORS for all resources using express CORS module.

If you are using express.js you can use cors module

3.1) Enable CORS Requests for All Servers.
var express = require("express"); var cors = require("cors"); var app = express(); app.use(cors()); app.get("/", function(req, res, next) { res.json({ msg: "This is CORS-enabled for all origins!" }); }); app.listen(80, function() { console.log("CORS-enabled web server listening on port 80"); });
3.2) Enable CORS Requests for Some Specific Servers
var express = require("express"); var cors = require("cors"); var app = express(); var whitelist = ["http://server1.com", "http://server2.com"]; var corsOptions = { origin: function(origin, callback) { if (whitelist.indexOf(origin) !== -1) { callback(null, true); } else { callback(new Error("Not allowed by CORS")); } } }; app.get("/", cors(corsOptions), function(req, res, next) { res.json({ msg: "This is CORS-enabled for a whitelisted domain." }); }); app.listen(80, function() { console.log("CORS-enabled web server listening on port 80"); });
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Q:- How to implement JWT authentication in Express app ? Explain with an Example?
  • Create a folder called 'keys' inside project folder.
  • Install some dependencies as following: npm install jsonwebtoken –-save
  • Add the login router routes/index.js
    • router.post('/login, function(req, res) { // find the user User.findOne({ name: req.body.username }, function(err, res) { if (err) throw err; if (!res) { res.json({ success: false, message: Login failed.' }); } else if (res) { // check if password matches if (res.password != req.body.password) { res.json({ success: false, message: Login failed. Wrong password.' }); } else { var token = jwt.sign(res, app.get('superSecret'), { expiresInMinutes: 1600 }); // return the information including token as JSON res.json({ success: true, message: 'Valid token!', token: token }); } } }); });
  • Use the token in application
    • jwt = require("express-jwt"); app.use(function(req, res, next) { var token = req.body.token || req.query.token || req.headers['x-access-token']; if (token) { jwt.verify(token, app.get('superSecret'), function(err, decoded) { if (err) { return res.json({ success: false, message: 'Invalid token.' }); } else { req.decoded = decoded; next(); } }); } else { return res.status(403).send({ success: false, message: 'No token given.' }); } });
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