1. Introduction to Routing in Express.js

Routing refers to how an application’s endpoints (URIs) respond to client requests. In Express.js, routes define how HTTP requests (GET, POST, PUT, DELETE, etc.) should be handled.

Basic Route Syntax:

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

2. Route Parameters

Route parameters allow capturing values from the URL and using them inside the route handler.

Example:

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

• :id is a route parameter, which can be accessed using req.params.id.

Multiple Route Parameters:

app.get('/user/:id/post/:postId', (req, res) => {  
    res.send(`User ID: ${req.params.id}, Post ID: ${req.params.postId}`);  
});

3. Query Parameters

Query parameters are passed in the URL after ? and can be accessed using req.query.

Example:

app.get('/search', (req, res) => {  
    res.send(`Search Query: ${req.query.q}`);  
});

Example request: http://localhost:3000/search?q=Node.js

4. Request Object (req)

The req object represents the HTTP request and contains:

• req.params → Route parameters
• req.query → Query parameters
• req.body → Data sent in the request body (requires express.json() middleware for JSON)
• req.headers → Request headers

5. Response Object (res)

The res object sends responses back to the client.

Common Response Methods:

res.send('Hello'); // Sends a simple response  
res.json({ message: 'Success' }); // Sends a JSON response  
res.status(404).send('Not Found'); // Sends a response with a status code  
res.redirect('/home'); // Redirects to another route  

6. Middleware for Handling Requests

To process JSON or form data, middleware is needed:

app.use(express.json()); // Parses JSON data 

app.use(express.urlencoded({ extended: true })); // Parses form data 

7. Handling POST Requests

To handle POST requests, use req.body:

app.post(‘/user’, (req, res) => { 

    res.json({ name: req.body.name, email: req.body.email }); 

});

Example Request (using Postman or Fetch API):

{
    "name": "John",
    "email": "john@example.com"
}

8. Handling 404 Errors

Define a catch-all route for undefined paths:

app.use((req, res) => { 

    res.status(404).send(‘Page Not Found’); 

});