Middleware execution flow (req, res, next) in Express - Mini Project: Build & Apply

Middleware Execution Flow in Express

📖 Scenario: You are building a simple Express server that uses middleware functions to process requests step-by-step. Middleware functions help you handle tasks like logging, authentication, and sending responses in a clear order.

🎯 Goal: Create an Express app with three middleware functions that run in sequence. Each middleware should log a message and call next() to pass control to the next middleware. The last middleware sends a response to the client.

📋 What You'll Learn

Create an Express app instance called app

Add a middleware function called logger that logs 'Logger middleware' and calls next()

Add a middleware function called auth that logs 'Auth middleware' and calls next()

Add a middleware function called finalHandler that logs 'Final handler' and sends 'Hello from Express!' as the response

Use app.use() to add the middleware functions in the order: logger, auth, finalHandler

Start the server on port 3000

💡 Why This Matters

🌍 Real World

Middleware is used in real web servers to handle logging, authentication, data parsing, and more in a clean, organized way.

💼 Career

Understanding middleware flow is essential for backend developers working with Express or similar web frameworks to build scalable and maintainable web applications.

1

Create Express app instance

Write code to import Express and create an app instance called app using express().

Express

const express = require('express');
// Your code here

Need a hint?

Use require('express') to import Express and then call express() to create the app.

2

Add logger middleware

Create a middleware function called logger that takes req, res, and next as parameters. Inside it, log 'Logger middleware' and call next().

Express

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

// Your code here

Need a hint?

Middleware functions always have three parameters: req, res, and next. Call next() to continue.

3

Add auth and finalHandler middleware and use them

Create two middleware functions: auth that logs 'Auth middleware' and calls next(), and finalHandler that logs 'Final handler' and sends 'Hello from Express!' as the response using res.send(). Then add all three middleware to app using app.use() in this order: logger, auth, finalHandler.

Express

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

function logger(req, res, next) {
    console.log('Logger middleware');
    next();
}

// Your code here

Need a hint?

Remember to call next() in auth to continue. The last middleware sends the response and does not call next().

4

Start the Express server

Add code to start the Express server on port 3000 using app.listen(). Inside the listen callback, log 'Server running on port 3000'.

Express

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

function logger(req, res, next) {
    console.log('Logger middleware');
    next();
}

function auth(req, res, next) {
    console.log('Auth middleware');
    next();
}

function finalHandler(req, res) {
    console.log('Final handler');
    res.send('Hello from Express!');
}

app.use(logger);
app.use(auth);
app.use(finalHandler);

// Your code here

Need a hint?

Use app.listen(3000, () => { ... }) to start the server and log a message inside the callback.