What is Middleware? | Lecture 11

 

 

1. What is Middleware?

Middleware is a function that runs between the client request and the server response.

Flow:

Client Request → Middleware → Route → Response

Middleware can:

• Execute code
• Modify request or response objects
• End the request–response cycle
• Call the next middleware or route

2. Why Do We Need Middleware?

Without middleware:

• Code becomes repetitive
• Logic is mixed inside routes
• Difficult to manage security and validation

With middleware:

• Clean and reusable code
• Better security
• Easier debugging and maintenance

3. Middleware Syntax

function middlewareName(req, res, next) {

    // logic here

    next();

}

Key Points:

• req → request object
• res → response object
• next() → moves control to the next middleware or route

If next() is not called, the request will stop.

4. Simple Middleware Example

Step 1: Create Server

const express = require('express');

const app = express();

Step 2: Create Middleware

function myMiddleware(req, res, next) {

    console.log("Middleware executed");

    next();

}

Step 3: Use Middleware

app.use(myMiddleware);

Step 4: Create Route

app.get('/', (req, res) => {

    res.send("Home Page");

});

 

app.listen(3000);

5. Types of Middleware in Express.js

1. Application-Level Middleware

Used for all routes.

app.use((req, res, next) => {

    console.log("Application level middleware");

    next();

});

Runs on every request.

2. Route-Level Middleware

Used for specific routes only.

function checkLogin(req, res, next) {

    console.log("User authenticated");

    next();

}

 

app.get('/dashboard', checkLogin, (req, res) => {

    res.send("Dashboard Page");

});

Runs only for the /dashboard route.

3. Built-in Middleware

Provided by Express.

Example: Parsing JSON data

app.use(express.json());

Used to read JSON data from the request body.

4. Third-Party Middleware

Installed using npm.

Example: Morgan (logger)

npm install morgan

const morgan = require('morgan');

app.use(morgan('dev'));

Logs HTTP requests automatically.

5. Error-Handling Middleware

Used to handle application errors.

Must contain four parameters.

app.use((err, req, res, next) => {

    console.log(err.message);

    res.status(500).send("Something went wrong");

});

6. Middleware Execution Order

Middleware executes in the order it is defined.

app.use((req, res, next) => {

    console.log("First middleware");

    next();

});

 

app.use((req, res, next) => {

    console.log("Second middleware");

    next();

});

Output:

First middleware

Second middleware

7. Authentication Middleware Example

function auth(req, res, next) {

    const isLoggedIn = true;

 

    if (isLoggedIn) {

        next();

    } else {

        res.send("Access Denied");

    }

}

 

app.get('/profile', auth, (req, res) => {

    res.send("Welcome to Profile");

});

Only authorized users can access the profile route.

8. Middleware Without next()

app.use((req, res) => {

    res.send("Request stopped here");

});

Routes defined after this middleware will never execute.

9. Real-Life Example

Airport security process:

Passenger → Security Check → Boarding → Flight

If security stops the passenger, they cannot board the flight.
Middleware works in the same way in Express.js.

10. Summary Table

Feature	Description
Middleware	Function between request and response
next()	Passes control to next middleware
app.use()	Applies middleware
Route middleware	Runs on specific routes
Built-in middleware	express.json(), express.static()
Third-party middleware	morgan, cors
Error middleware	Handles errors