next() function and flow control in Express - Mini Project: Build & Apply

Using the next() Function for Middleware Flow Control in Express

📖 Scenario: You are building a simple Express server that handles requests with multiple middleware functions. Middleware functions help process requests step-by-step before sending a response.Understanding how to use the next() function is important to control the flow between these middleware functions.

🎯 Goal: Build an Express server with two middleware functions. The first middleware will log a message and then call next() to pass control to the second middleware, which sends a response to the client.

📋 What You'll Learn

Create an Express app instance

Write a middleware function named logger that logs 'Request received' and calls next()

Write a middleware function named responder that sends 'Hello from Express!' as the response

Use app.use() to add both middleware functions in the correct order

💡 Why This Matters

🌍 Real World

Middleware functions are used in real Express apps to handle logging, authentication, data parsing, and more, controlling how requests flow through the server.

💼 Career

Understanding next() and middleware flow is essential for backend developers working with Express to build scalable and maintainable web servers.

1

Set up Express app

Write code to import Express and create an app instance called app.

Express

// Import Express and create app instance
// Your code here

Need a hint?

Use import express from 'express' and then const app = express() to create the app.

2

Create logger middleware

Create a middleware function named logger that takes req, res, and next as parameters. Inside it, log 'Request received' and then call next().

Express

import express from 'express';
const app = express();

// Create logger middleware
// Your code here

Need a hint?

Define logger as a function with three parameters and call next() after logging.

3

Create responder middleware

Create a middleware function named responder that takes req and res as parameters and sends the text 'Hello from Express!' as the response.

Express

import express from 'express';
const app = express();

function logger(req, res, next) {
    console.log('Request received');
    next();
}

// Create responder middleware
// Your code here

Need a hint?

Define responder as a function with req and res and use res.send() to send the response.

4

Use middleware in app

Use app.use() to add the logger middleware first, then the responder middleware, so requests flow through both.

Express

import express from 'express';
const app = express();

function logger(req, res, next) {
    console.log('Request received');
    next();
}

function responder(req, res) {
    res.send('Hello from Express!');
}

// Add middleware to app
// Your code here

Need a hint?

Call app.use() twice, first with logger, then with responder.