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Q: How do you create a basic Express.js application?

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Q: What is middleware in Express.js?

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Q: What are the different types of middleware in Express.js?

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

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Q: What is the purpose of `next()` function in middleware?

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Q: How do you serve static files in Express.js?

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Q: What is the difference between `req.params`, `req.query`, and `req.body`?

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Q: How do you enable CORS in Express.js?

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Q: What is Express Router and why use it?

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Q: How do you implement error handling in Express.js?

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How do you create a basic Express.js application?

beginner

const express = require('express');
const app = express();
const PORT = 3000;
app.get('/', (req, res) => {
  res.send('Hello World!');
});
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

This creates a basic server that listens on port 3000 and responds with "Hello World!" when accessing the root route.

What is middleware in Express.js?

beginner

Middleware functions are functions that have access to the request object (req), response object (res), and the next middleware function in the application's request-response cycle. Middleware can execute code, modify req/res objects, end the request-response cycle, or call the next middleware.

// Custom middleware
app.use((req, res, next) => {
  console.log('Time:', Date.now());
  next(); // Call next to continue to the next middleware
});

What are the different types of middleware in Express.js?

beginner

Application-level middleware - Bound to app object using app.use()
Router-level middleware - Bound to router object using router.use()
Error-handling middleware - Takes four arguments (err, req, res, next)
Built-in middleware - Provided by Express (express.static, express.json)
Third-party middleware - From npm packages (morgan, cors, helmet)

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

beginner

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

What is the purpose of `next()` function in middleware?

beginner

The next() function passes control to the next middleware function. If not called, the request will be left hanging. You can pass an error to next(err) to trigger error handling middleware.

app.use((req, res, next) => {
  if (req.headers.authorization) {
    next(); // Continue to next middleware
  } else {
    next(new Error('Unauthorized')); // Trigger error handling
  }
});

How do you serve static files in Express.js?

beginner

Use the built-in express.static middleware:

// Serve files from 'public' directory
app.use(express.static('public'));
// Serve with virtual path prefix
app.use('/static', express.static('public'));