Routing in Express.js

# CHAPTER 7

Routing in Express.js #

1. Introduction #

2. Learning Objectives #

• Define distinct GET, POST, PUT, and DELETE routes.

• Capture dynamic data from URLs using Route Parameters.

• Extract query strings from the URL (e.g., ?sort=asc).

• Modularize your code using express.Router().

3. Beginner-Friendly Explanation #

4. Route Methods (CRUD) #

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

// READ (GET)
app.get('/api/users', (req, res) => {
    res.json({ message: "Fetching all users" });
});

// CREATE (POST)
app.post('/api/users', (req, res) => {
    res.json({ message: "Creating a new user" });
});

// UPDATE (PUT)
app.put('/api/users', (req, res) => {
    res.json({ message: "Updating a user" });
});

// DELETE (DELETE)
app.delete('/api/users', (req, res) => {
    res.json({ message: "Deleting a user" });
});

5. Dynamic Route Parameters (req.params) #

// The ':id' acts as a wild-card placeholder
app.get('/api/users/:id', (req, res) => {
    
    // Extract the dynamic ID from the URL
    const userId = req.params.id; 
    
    res.json({ message: `Fetching data for User ID: ${userId}` });
});

*If a user visits /api/users/99, the API returns "Fetching data for User ID: 99".*

6. Query Strings (req.query) #

app.get('/api/search', (req, res) => {
    const sortBy = req.query.sort; // equals "asc"
    const role = req.query.role;   // equals "admin"
    
    res.json({ message: `Searching for ${role}s, sorted by ${sortBy}` });
});

7. Modular Routing (express.Router) #

Step 1: Create routes/userRoutes.js

const express = require('express');
const router = express.Router(); // Create a mini-app

router.get('/', (req, res) => {
    res.json({ message: "All users" });
});

router.get('/:id', (req, res) => {
    res.json({ message: `User ${req.params.id}` });
});

module.exports = router; // Export the mini-app

Step 2: Connect it in app.js

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

// Import the Router file
const userRoutes = require('./routes/userRoutes');

// Tell Express: "Any URL that starts with /api/users should be handled by userRoutes"
app.use('/api/users', userRoutes);

app.listen(3000);

*Now, app.js is perfectly clean, acting only as the main traffic director!*

8. Backend Workflow: Designing an API #

• GET /api/posts (All posts)

• POST /api/posts (Create post)

• GET /api/posts/:id (One post)

• GET /api/posts/:id/comments (Get comments for a specific post)

9. Best Practices #

• Route Ordering Matters: In Express, routes are evaluated top-to-bottom. If you define app.get('/api/users/:id') BEFORE app.get('/api/users/profile'), and someone visits /api/users/profile, Express will think the word "profile" is actually the dynamic :id! Always put specific static routes above dynamic wildcard routes.

10. Common Mistakes #

• Mixing up Params and Query: Beginners often try to access req.params.sort when the URL is ?sort=asc. Remember:

• /users/5 -> Uses req.params.id

• /users?id=5 -> Uses req.query.id

11. Exercises #

1. 1. Explain the architectural benefit of using express.Router() instead of defining all routes in app.js.

12. Coding Challenges #

• Challenge: Write a single Express route that captures both a dynamic route parameter and a query string. The URL format is /api/products/:category?limit=10. Extract both the category name and the limit number, and return them in a JSON response.

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the difference between req.params and req.query. Give an example URL for each, demonstrating when a frontend developer would choose to use one over the other.

• Q: Why is the physical top-to-bottom placement of route definitions critical in an Express application? What happens if a dynamic wildcard route is placed above a static route?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #