1. What is Express.js, and why is it used?

Answer:

Express.js is a lightweight, fast, and flexible web application framework for Node.js. It provides a robust set of features to build single-page, multi-page, and hybrid web applications and APIs. It's widely used due to its minimalistic design and ease of use.

Example:

const express = require('express');
const app = express();

app.get('/', (req, res) => {
  res.send('Hello, Express!');
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

2. What is middleware in Express.js?

Answer:

Middleware in Express.js functions between the request and response cycle. They can modify requests and responses or terminate the request-response cycle.

Example:

app.use((req, res, next) => {
  console.log('Middleware executed');
  next(); // Passes control to the next middleware/route handler
});

3. How do you handle different HTTP methods in Express.js?

Answer:

Express allows handling HTTP methods like GET, POST, PUT, DELETE, etc., using corresponding functions like app.get(), app.post(), etc.

Example:

app.get('/get', (req, res) => res.send('GET request'));
app.post('/post', (req, res) => res.send('POST request'));
app.put('/put', (req, res) => res.send('PUT request'));
app.delete('/delete', (req, res) => res.send('DELETE request'));

4. What is app.use() in Express.js?

Answer:

app.use() is a method to mount middleware at a specific path or globally for all routes. It's used for middleware functions that modify or handle requests and responses.

Example:

app.use(express.json()); // This parses incoming JSON requests and puts the parsed data in req.body

5. What is routing in Express.js?

Answer:

Routing refers to how an application responds to client requests for a particular endpoint (path and method). It’s the core of any web framework like Express.js.

Example:

app.get('/user', (req, res) => {
  res.send('User route');
});

app.post('/user', (req, res) => {
  res.send('User POST route');
});

6. How do you serve static files in Express.js?

Answer:

Express can serve static files (like images, CSS, and JavaScript) using the built-in middleware express.static().

Example:

app.use(express.static('public'));
// Access static files in the 'public' directory (e.g., localhost:3000/image.jpg)

7. How do you handle query parameters in Express.js?

Answer:

Query parameters are part of the URL, and you can access them via req.query.

Example:

app.get('/search', (req, res) => {
  const query = req.query.q;
  res.send(`Search query: ${query}`);
});
// Example: localhost:3000/search?q=express

8. How do you handle URL parameters in Express.js?

Answer:

URL parameters are dynamic parts of the URL. They can be accessed via req.params.

Example:

app.get('/user/:id', (req, res) => {
  const userId = req.params.id;
  res.send(`User ID: ${userId}`);
});
// Example: localhost:3000/user/123

9. How do you handle form data in Express.js?

Answer:

To handle form data (usually submitted via POST), you need to use middleware to parse application/x-www-form-urlencoded data.

Example:

app.use(express.urlencoded({ extended: true }));

app.post('/submit', (req, res) => {
  const { name, email } = req.body;
  res.send(`Name: ${name}, Email: ${email}`);
});

10. How do you redirect requests in Express.js?

Answer:

You can redirect the user to another route using res.redirect().

Example:

app.get('/old-route', (req, res) => {
  res.redirect('/new-route');
});

11. What is the difference between res.send() and res.json() in Express.js?

Answer:

• res.send() can send a response in various formats (HTML, JSON, text).
• res.json() is used to send a JSON response explicitly.

Example:

app.get('/send', (req, res) => {
  res.send('This is a text response');
});

app.get('/json', (req, res) => {
  res.json({ message: 'This is a JSON response' });
});

12. How do you create and use a custom middleware in Express.js?

Answer:

You can create custom middleware by defining a function that takes req, res, and next as parameters.

Example:

const logMiddleware = (req, res, next) => {
  console.log(`Request to: ${req.url}`);
  next(); // Pass control to the next middleware
};

app.use(logMiddleware);

13. How do you handle errors in Express.js?

Answer:

You can create error-handling middleware by adding a middleware function with four arguments: err, req, res, and next.

Example:

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something went wrong!');
});

14. What are templates in Express.js, and how do you render them?

Answer:

Templates allow you to dynamically generate HTML. Express.js supports template engines like Pug, EJS, and Handlebars.

Example (Pug template):

1. Set the view engine:

app.set('view engine', 'pug');

1. Render the template:

app.get('/template', (req, res) => {
  res.render('index', { title: 'Express', message: 'Hello, Express!' });
});

15. How do you structure an Express.js application for scalability?

Answer:

You can organize routes, middleware, and controllers in separate folders:

1. routes/: Contains route definitions.
2. controllers/: Handles business logic.
3. models/: Defines database schemas.

Example:

- app.js
- routes/
  - users.js
- controllers/
  - userController.js
- models/
  - userModel.js

16. What is next() in Express.js? Why is it important?

Answer:

next() is a function that passes control to the next middleware in the stack. It’s crucial to avoid blocking the request-response cycle.

Example:

app.use((req, res, next) => {
  console.log('Middleware 1');
  next(); // Passes control to the next middleware
});

app.use((req, res) => {
  console.log('Middleware 2');
  res.send('Request complete');
});

17. How do you implement authentication in Express.js?

Answer:

Authentication can be implemented using libraries like passport.js or JWT (JSON Web Tokens).

Example (JWT-based authentication):

1. Install jsonwebtoken:

npm install jsonwebtoken

1. Create a token:

const jwt = require('jsonwebtoken');
const token = jwt.sign({ userId: user._id }, 'secretKey', { expiresIn: '1h' });

1. Verify the token:

const verifyToken = (req, res, next) => {
  const token = req.headers['authorization'];
  if (!token) return res.status(403).send('Token is required');
  jwt.verify(token, 'secretKey', (err, decoded) => {
    if (err) return res.status(401).send('Invalid Token');
    req.user = decoded;
    next();
  });
};

18. What is CORS in Express.js, and how do you enable it?

Answer:

CORS (Cross-Origin Resource Sharing) allows servers to control which origins can access the resources. It’s often required for APIs that are accessed from different domains.

Example:

const cors = require('cors');
app.use(cors()); // Enables CORS for all routes

19. How do you handle file uploads in Express.js?

Answer:

You can handle file uploads using the multer library.

Example:

1. Install multer:

npm install multer

1. Setup multer for file upload:

const multer = require('multer');
const upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('file'), (req, res) => {
  res.send('File uploaded');
});

20. How do you handle sessions in Express.js?

Answer:

Sessions allow storing user-specific data across requests. You can manage sessions using express-session.

Example:

1. Install express-session:

npm install express-session

1. Use session middleware:

const session = require('express-session');
app.use(session({ secret: 'secret', resave: false, saveUninitialized: true }));

app.get('/', (req, res) => {
  if (!req.session.views) {
    req.session.views = 1;
  } else {
    req.session.views++;
  }
  res.send(`Views: ${req.session.views}`);
});

These questions cover a wide range of Express.js features and are commonly asked in interviews to evaluate your understanding and hands-on skills.