3.4 – Middleware in Node js

✅ What is Middleware in Node js?

Middleware in Node.js refers to functions that execute during the request-response cycle. Every HTTP request goes through a stack of middleware before reaching its final response.

You can think of middleware as interceptors that can:

• Modify the request (req) or response (res) objects

• End the request-response cycle

• Call the next middleware in the stack using next()

There are two types:

• Pre Middleware: Runs before the request is handled

• Post Middleware: Runs after some processing, or to handle things like errors

⚙️ How Middleware Works in Node.js (with Example)

Let’s create a simple Express server using TypeScript and see middleware in action.

📄 File: index.ts

import * as express from ‘express’;
const app: express.Application = express();
// ✅ This is a middleware without a specific route
app.use((req, res, next) => {
console.log(‘🚀 Global middleware called’);
next(); // Pass control to the next middleware or route
});
// ✅ Route-specific middleware example
app.get(‘/login’, (req: any, res, next) => {
const data = [{ name: ‘testUsername’ }];
req.user = data; // Attach data to the request
next(); // Move to the next middleware
}, (req: any, res, next) => {
console.log(‘✅ Login middleware called’);
res.send(req.user); // Send response
});
// ✅ A simple route with no middleware chain
app.get(‘/test’, (req, res) => {
res.send(‘This is a test request’);
});
// ✅ This middleware will be called for unmatched routes
app.use((req, res, next) => {
console.log(‘❌ No route matched, fallback middleware called’);
res.status(404).send(‘Route not found’);
});
// Start the server
app.listen(5000, () => {
console.log(‘🌐 Server running on http://localhost:5000’);
});

🔄 Middleware Execution Flow

Let’s break down what happens when a request is made:

👉 When visiting /login:

1. The global middleware is called (app.use(...))

2. The first /login middleware runs and attaches data to req.user

3. The next middleware sends the req.user response

👉 When visiting /test:

1. The global middleware is called

2. The /test route directly sends the response

👉 When visiting any unknown route:

1. Global middleware runs

2. None of the route paths match

3. Fallback middleware sends a 404 response

📌 Important Notes on Middleware in Node.js

• Always call next() to pass control; otherwise, the request will hang

• Use app.use() for global or fallback middleware

• Custom data should be added to req, not res

• The order of middleware matters: middleware runs in the order it is defined

• It’s a good practice to structure your API routes like /api/user/login rather than just /login

✅ Example with Clean Route Paths

app.get(‘/api/user/login’, (req: any, res) => {
const data = [{ name: ‘testusername’ }];
res.status(200).send(data);
});
app.get(‘/api/user/signup’, (req, res) => {
res.send(‘Signup route’);
});