Express.js Answers

Basic Express.js Answers

#	Question	Answer	Examples
1	What is Express.js?	A minimal and flexible Node.js web application framework	Provides features for building web and mobile applications
2	What is the purpose of Express.js?	Building web servers, APIs, and web applications using Node.js	Handling HTTP requests and responses, routing, middleware
3	How do you install Express.js?	Using npm install express	npm install express
4	How do you create a basic Express app?	Import Express, create an app instance, and start the server	const express = require('express'); const app = express(); app.listen(3000, ...);
5	What is a route?	A specific endpoint (URL path and HTTP method) that the application responds to	app.get('/', (req, res) => { ... });
6	How do you define a GET route?	Using app.get()	app.get('/users', (req, res) => { res.send('Users'); });
7	How do you define a POST route?	Using app.post()	app.post('/users', (req, res) => { ... });
8	What is the req object?	Represents the incoming HTTP request	req.params, req.query, req.body, req.headers
9	What is the res object?	Represents the HTTP response sent back to the client	res.send(), res.json(), res.status(), res.render()
10	How do you send a text response?	Using res.send()	res.send('Hello World!');
11	How do you send a JSON response?	Using res.json()	res.json({ message: 'Success' });
12	How do you set the HTTP status code?	Using res.status()	res.status(404).send('Not Found');
13	What is middleware?	Functions that have access to req, res, and the next function	app.use((req, res, next) => { ... });
14	How do you use middleware?	Using app.use()	app.use(express.json());
15	What is express.json() middleware?	Built-in middleware to parse incoming requests with JSON payloads	app.use(express.json());

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Intermediate Express.js Answers

#	Question	Answer	Examples
1	What are route parameters?	Placeholders in a route path that capture values	app.get('/users/:id', (req, res) => { res.send(req.params.id); });
2	What are query parameters?	Key-value pairs appended to the URL after a ?	app.get('/search', (req, res) => { res.send(req.query.q); });
3	What is express.Router()?	A way to create modular, mountable route handlers	const router = express.Router(); router.get('/', ...); app.use('/users', router);
4	What is error handling middleware?	Middleware functions with four arguments ((err, req, res, next)) to catch errors	app.use((err, req, res, next) => { console.error(err.stack); res.status(500).send('Something broke!'); });
5	What is express.static() middleware?	Built-in middleware to serve static assets (CSS, JS, images) from a directory	app.use(express.static('public'));
6	What is a template engine in Express?	Allows you to render dynamic HTML pages (e.g., EJS, Pug, Handlebars)	app.set('view engine', 'ejs'); res.render('index', { name: 'User' });
7	How do you configure a template engine?	Using app.set('view engine', 'engine-name')	app.set('view engine', 'ejs');
8	What is CORS (Cross-Origin Resource Sharing)?	A mechanism that allows a web page to request resources from a different domain (origin)	Using cors middleware: app.use(cors());
9	What is the cors middleware?	A third-party middleware to enable CORS	const cors = require('cors'); app.use(cors());
10	How do you handle form submissions (POST data)?	Using express.urlencoded() middleware	app.use(express.urlencoded({ extended: true }));
11	What is express.urlencoded() middleware?	Parses incoming requests with URL-encoded payloads (like application/x-www-form-urlencoded)	app.use(express.urlencoded({ extended: true }));
12	What is the difference between app.use() and app.METHOD()?	app.use() applies middleware globally or to specific paths; app.METHOD() defines a specific route	app.use() vs app.get()
13	How do you chain middleware?	Apply multiple middleware functions sequentially	app.get('/', middleware1, middleware2, routeHandler);
14	What is the purpose of next() in middleware?	Passes control to the next middleware function in the stack	app.use((req, res, next) => { ... next(); });
15	How do you handle environment variables?	Using process.env and often libraries like dotenv	require('dotenv').config(); const port = process.env.PORT || 3000;

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Advanced Express.js Answers

#	Question	Answer	Examples
1	What is error handling middleware in Express.js?	A special middleware function with four arguments ((err, req, res, next)) to catch errors	app.use((err, req, res, next) => { console.error(err.stack); res.status(500).send('Something broke!'); });
2	What is rate limiting?	Restricting the number of requests a client can make to an API within a specific time frame	Using middleware libraries like express-rate-limit to prevent abuse.
3	What is input validation?	Ensuring incoming data meets specific format and type requirements	Using libraries like Joi, Zod, express-validator
4	How do you handle authentication and authorization?	Verifying a user's identity (authentication) and determining what they are allowed to do (authorization)	Using JWT (JSON Web Tokens), OAuth, session-based authentication.
5	What is the concept of caching in Node.js?	Storing frequently accessed data in a faster store (e.g., Redis, Memcached) to improve performance	Using libraries like node-cache or integrating with Redis.
6	What is the purpose of WebSockets?	Enables real-time, bidirectional communication between client and server over a single connection	Real-time chat applications, live updates (e.g., using Socket.IO).
7	What is the concept of GraphQL?	A query language for APIs that allows clients to request exactly the data they need	Using Apollo Server, Express-GraphQL
8	What is a message queue?	A system for asynchronous communication between application components (e.g., RabbitMQ, Kafka)	Using libraries like bull, amqplib
9	How do you handle testing in Node.js?	Using frameworks like Jest, Mocha, Chai, Supertest	Unit tests, integration tests, end-to-end tests.
10	What is the purpose of helmet middleware?	Secures Express apps by setting various HTTP headers	app.use(helmet());
11	What is the purpose of compression middleware?	Compresses responses using Gzip or Brotli algorithms to reduce transfer size	app.use(compression());
12	What is the purpose of morgan middleware?	HTTP request logger middleware	app.use(morgan('dev'));
13	How do you manage sessions in Express.js?	Using middleware like express-session to store user session data on the server	app.use(session({ secret: 'keyboard cat', resave: false, saveUninitialized: false }));
14	What is the difference between RESTful APIs and GraphQL?	REST uses resources and HTTP methods for CRUD; GraphQL allows clients to specify data needs precisely	GraphQL can be more efficient for complex data requirements, avoiding over-fetching/under-fetching.
15	How do you handle asynchronous middleware in Express?	Using async/await within middleware functions	app.use(async (req, res, next) => { await someAsyncTask(); next(); });

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