Routing in Express.js

Routing determines how an Express.js application responds when a client requests a particular URL using an HTTP method such as GET, POST, PUT, or DELETE.

1. What Is Routing?

Routing is the process of defining what happens when a user visits a specific URL on a website or API.

For example, a website may contain these routes:

Express checks the incoming request and runs the matching route handler.

2. Basic Route Syntax

The general syntax of an Express GET route is:

app.get('URL', (req, res) => {
    // Handle the request
});

• app.get() defines a route for GET requests.
• req contains information about the request.
• res is used to send the response.

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

app.get('/', (req, res) => {
    res.send('Welcome to Soopro Pathshala');
});

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

3. Create Multiple Routes

You can create multiple routes in the same Express application. Each route can return a different response.

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

app.get('/', (req, res) => {
    res.send('Home Page');
});

app.get('/about', (req, res) => {
    res.send('About Page');
});

app.get('/contact', (req, res) => {
    res.send('Contact Page');
});

app.listen(3000, () => {
    console.log('Server started');
});

4. HTTP Methods in Routing

HTTP methods tell the server what kind of operation the client wants to perform.

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

app.get('/students', (req, res) => {
    res.send('Get students');
});

app.post('/students', (req, res) => {
    res.send('Create a student');
});

app.put('/students/1', (req, res) => {
    res.send('Replace student 1');
});

app.patch('/students/1', (req, res) => {
    res.send('Update part of student 1');
});

app.delete('/students/1', (req, res) => {
    res.send('Delete student 1');
});

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

5. Route Parameters

Route parameters allow you to capture values directly from a URL. They are useful when accessing a particular student, product, or other resource.

Example URL:

http://localhost:3000/students/101

In the route /students/:id, the colon indicates that id is a route parameter.

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

app.get('/students/:id', (req, res) => {
    const studentId = req.params.id;

    res.send('Student ID: ' + studentId);
});

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

http://localhost:3000/students/101

Multiple Route Parameters

app.get('/students/:id/courses/:courseId', (req, res) => {
    res.json({
        studentId: req.params.id,
        courseId: req.params.courseId
    });
});

For a URL such as /students/101/courses/5, the response contains student ID 101 and course ID 5.

6. Query Parameters

Query parameters are values added after a question mark (?) in a URL. They are commonly used for searching, filtering, sorting, and pagination.

Example URL:

http://localhost:3000/search?name=Amit&course=Node.js

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

app.get('/search', (req, res) => {
    const name = req.query.name;
    const course = req.query.course;

    res.json({
        studentName: name,
        studentCourse: course
    });
});

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

{
  "studentName": "Amit",
  "studentCourse": "Node.js"
}

7. Route Parameters vs Query Parameters

8. Route Handlers

A route handler is a function that runs when an incoming request matches a route. You can provide multiple callback functions for a route and pass control using next().

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

function firstHandler(req, res, next) {
    console.log('First handler executed');
    next();
}

function secondHandler(req, res) {
    res.send('Second handler sent the response');
}

app.get('/example', firstHandler, secondHandler);

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

9. Organize Routes Using Express Router

In a larger application, keeping every route in one file can make the code difficult to maintain. Express Router allows you to organize routes into separate files.

Step 1: Create a Folder Structure

express-routing/
│
├── app.js
├── package.json
└── routes/
    └── studentRoutes.js

Step 2: Create routes/studentRoutes.js

const express = require('express');
const router = express.Router();

router.get('/', (req, res) => {
    res.send('All Students');
});

router.get('/:id', (req, res) => {
    res.send('Student ID: ' + req.params.id);
});

module.exports = router;

Step 3: Use the Router in app.js

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

const studentRoutes = require('./routes/studentRoutes');

app.use('/students', studentRoutes);

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

Test the routes:

• http://localhost:3000/students — All Students
• http://localhost:3000/students/101 — Student ID: 101

10. Handle Unknown Routes

If no route matches an incoming request, you can provide a custom 404 response.

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

app.get('/', (req, res) => {
    res.send('Home Page');
});

app.use((req, res) => {
    res.status(404).send('404 - Route Not Found');
});

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

The fallback middleware should appear after the routes you want to match first.

11. Common Routing Mistakes

12. Practical Assignment