Middleware concept and execution flow in Node.js - Mini Project: Build & Apply

Middleware Concept and Execution Flow in Node.js

📖 Scenario: You are building a simple Node.js server that handles requests step-by-step using middleware functions. Middleware are like helpers that process requests one after another, like a line of workers passing a package along.

🎯 Goal: Build a Node.js server using Express that demonstrates how middleware functions run in order and modify the request and response objects.

📋 What You'll Learn

Create an Express app with a basic route

Add two middleware functions that modify the request object

Modify the route handler to send a response using the modified request data

Understand the order of middleware execution and how next() controls flow

💡 Why This Matters

🌍 Real World

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

💼 Career

Understanding middleware is essential for backend developers working with Node.js and Express, as it is a core pattern for building scalable web applications.

1

Set up Express app and initial route

Create a variable called express that requires the 'express' module. Then create a variable called app by calling express(). Finally, add a GET route on '/' that takes req, res and sends the text 'Hello' as the response.

Node.js

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

// Your code here

Hint

Use require('express') to import Express. Then call express() to create the app. Use app.get to add a route.

2

Add first middleware to add a property to request

Add a middleware function using app.use that takes req, res, and next. Inside it, add a property req.firstName with the value 'Alice'. Then call next() to pass control to the next middleware.

Node.js

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

app.use((req, res, next) => {
  // Your code here
});

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

Hint

Middleware functions get req, res, and next. Add the property to req and call next().

3

Add second middleware to add lastName to request

Add another middleware function using app.use that takes req, res, and next. Inside it, add a property req.lastName with the value 'Smith'. Then call next() to continue the flow.

Node.js

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

app.use((req, res, next) => {
  req.firstName = 'Alice';
  next();
});

app.use((req, res, next) => {
  // Your code here
});

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

Hint

Just like the first middleware, add req.lastName and call next().

4

Modify route to send full name using middleware data

Change the GET route on '/' so that it sends a response with the text `Hello ${req.firstName} ${req.lastName}` using template literals. This shows how middleware data is used in the final response.

Node.js

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

app.use((req, res, next) => {
  req.firstName = 'Alice';
  next();
});

app.use((req, res, next) => {
  req.lastName = 'Smith';
  next();
});

app.get('/', (req, res) => {
  // Your code here
});

Hint

Use template literals with backticks to include req.firstName and req.lastName in the response.

Practice

1. What is the main purpose of middleware in a Node.js Express app?

easy

A. To style the HTML pages

B. To directly send responses to the client without routes

C. To define database schemas

D. To run code on requests before they reach the route handlers

Solution

1. Step 1: Understand middleware role and compare options

   Middleware functions run before route handlers to process requests. Only To run code on requests before they reach the route handlers describes middleware's role correctly; others describe unrelated tasks.

2. Final Answer:

   To run code on requests before they reach the route handlers -> Option D

3. Quick Check:

   Middleware = Pre-route code [OK]

Hint: Middleware runs before routes to handle requests [OK]

Common Mistakes:

• Thinking middleware sends responses directly
• Confusing middleware with database logic
• Assuming middleware styles pages

2. Which of the following is the correct way to define a middleware function in Express?

easy

A. app.get((req, res) => { console.log('Middleware'); });

B. app.use((req, res, next) => { console.log('Middleware'); next(); });

C. app.middleware((req, res, next) => { next(); });

D. app.route('/').middleware((req, res) => { next(); });

Solution

1. Step 1: Recall Express middleware syntax and check options

   Middleware uses app.use() with (req, res, next) parameters. Only app.use((req, res, next) => { console.log('Middleware'); next(); }); uses app.use() and calls next(), which is correct middleware syntax.

2. Final Answer:

   app.use((req, res, next) => { console.log('Middleware'); next(); }); -> Option B

3. Quick Check:

   Middleware = app.use + next() [OK]

Hint: Middleware uses app.use and calls next() [OK]

Common Mistakes:

• Using app.get instead of app.use for middleware
• Missing next() call inside middleware
• Using non-existent app.middleware method

3. Consider this Express code:

app.use((req, res, next) => { console.log('First'); next(); });
app.use((req, res, next) => { console.log('Second'); next(); });
app.get('/', (req, res) => { res.send('Done'); });

What will be printed in the console when a GET request is made to '/'?

medium

A. First\nSecond

B. Second\nFirst

C. Done

D. No output

Solution

1. Step 1: Understand middleware execution order and route handler

   Middleware runs in the order added: first prints 'First', then calls next(), second prints 'Second'. After middleware, route handler sends response but does not print to console.

2. Final Answer:

   First\nSecond -> Option A

3. Quick Check:

   Middleware order = console logs order [OK]

Hint: Middleware logs appear in order added before route runs [OK]

Common Mistakes:

• Assuming route handler logs to console
• Thinking middleware runs in reverse order
• Missing next() causes no further logs

4. What is the problem with this middleware code?

app.use((req, res, next) => { console.log('Start'); });
app.get('/', (req, res) => { res.send('Hello'); });

medium

A. It calls next() twice, causing an error

B. It sends response twice, causing an error

C. It never calls next(), so the request hangs

D. It uses wrong parameters in middleware function

Solution

1. Step 1: Check middleware function and request flow effect

   The middleware logs 'Start' but does not call next(), so it blocks further processing. Without next(), the route handler never runs, so the request hangs.

2. Final Answer:

   It never calls next(), so the request hangs -> Option C

3. Quick Check:

   Missing next() blocks request [OK]

Hint: Always call next() in middleware unless sending response [OK]

Common Mistakes:

• Forgetting to call next() in middleware
• Calling next() multiple times
• Confusing middleware parameters

5. You want to add two middleware functions to log request time and check authentication before handling routes. Which code correctly ensures both run in order and the route handler executes after?

hard

A. app.use((req, res, next) => { console.log('Time:', Date.now()); next(); });\napp.use((req, res, next) => { if(req.user) next(); else res.status(401).send('Unauthorized'); });\napp.get('/', (req, res) => { res.send('Welcome'); });

B. app.use((req, res) => { console.log('Time:', Date.now()); });\napp.use((req, res, next) => { if(req.user) next(); else res.status(401).send('Unauthorized'); });\napp.get('/', (req, res) => { res.send('Welcome'); });

C. app.use((req, res, next) => { if(req.user) next(); else res.status(401).send('Unauthorized'); });\napp.use((req, res, next) => { console.log('Time:', Date.now()); next(); });\napp.get('/', (req, res) => { res.send('Welcome'); });

D. app.get('/', (req, res) => { console.log('Time:', Date.now()); });\napp.use((req, res, next) => { if(req.user) next(); else res.status(401).send('Unauthorized'); });

Solution

1. Step 1: Check middleware order, next() calls, and route handler execution

   app.use((req, res, next) => { console.log('Time:', Date.now()); next(); });\napp.use((req, res, next) => { if(req.user) next(); else res.status(401).send('Unauthorized'); });\napp.get('/', (req, res) => { res.send('Welcome'); }); runs time logger first, calls next(), then auth check middleware, which calls next() or sends response. Only if both middleware call next(), the route handler runs and sends 'Welcome'.

2. Final Answer:

   app.use((req, res, next) => { console.log('Time:', Date.now()); next(); });\napp.use((req, res, next) => { if(req.user) next(); else res.status(401).send('Unauthorized'); });\napp.get('/', (req, res) => { res.send('Welcome'); }); -> Option A

3. Quick Check:

   Middleware order + next() = correct flow [OK]

Hint: Middleware must call next() to continue chain in correct order [OK]

Common Mistakes:

• Missing next() in first middleware
• Reversing middleware order
• Using app.get instead of app.use for middleware