Application-level middleware in Express - Mini Project: Build & Apply

Building Application-Level Middleware in Express

📖 Scenario: You are creating a simple Express server that logs request details for every incoming request. This helps you monitor what requests your server receives.

🎯 Goal: Build an Express app that uses application-level middleware to log the HTTP method and URL of each request before sending a response.

📋 What You'll Learn

Create an Express app instance

Add application-level middleware that logs request method and URL

Add a route handler for GET requests to '/'

Send a simple text response from the route handler

💡 Why This Matters

🌍 Real World

Logging requests helps developers monitor and debug web servers in real time.

💼 Career

Understanding middleware is essential for backend developers working with Express to build scalable web applications.

1

Create Express app instance

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

Express

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

Hint

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

2

Add application-level middleware

Add application-level middleware to app using app.use that logs the HTTP method and URL of each request. Use parameters req, res, and next. Call next() after logging.

Express

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

// Add middleware to log method and URL
// Your code here

Hint

Use app.use with a function that logs req.method and req.url, then calls next().

3

Add GET route handler for '/'

Add a GET route handler on app for path '/' that sends the text response 'Hello, world!'. Use parameters req and res.

Express

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

app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
});

// Add GET route handler for '/' here
// Your code here

Hint

Use app.get with path '/' and send 'Hello, world!' in the response.

4

Start the server on port 3000

Add code to start the Express server by calling app.listen on port 3000. Provide a callback that logs 'Server running on port 3000'.

Express

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

app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
});

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

// Start server on port 3000
// Your code here

Hint

Use app.listen(3000, () => { console.log(...) }) to start the server.

Practice

1. What is the main purpose of application-level middleware in Express?

easy

A. To run code for every request or specific routes before the final handler

B. To directly send HTML files to the client

C. To connect Express with a database

D. To create new HTTP servers

Solution

1. Step 1: Understand middleware role

   Application-level middleware runs before route handlers to process requests.

2. Step 2: Identify middleware usage

   It can modify requests, responses, or decide to stop the request cycle early.

3. Final Answer:

   To run code for every request or specific routes before the final handler -> Option A

4. Quick Check:

   Middleware runs before handlers [OK]

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

Common Mistakes:

• Confusing middleware with route handlers
• Thinking middleware creates servers
• Assuming middleware connects databases directly

2. Which of the following is the correct syntax to add application-level middleware in Express?

easy

A. app.addMiddleware(function(req, res) { next(); });

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

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

D. app.routeMiddleware((req, res, next) => { next(); });

Solution

1. Step 1: Recall Express middleware syntax

   Express uses app.use() to add middleware functions.

2. Step 2: Check function parameters

   Middleware functions receive req, res, next and must call next() to continue.

3. Final Answer:

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

4. Quick Check:

   app.use with (req, res, next) and next() [OK]

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

Common Mistakes:

• Using non-existent methods like app.middleware
• Omitting the next parameter or call
• Confusing middleware with route handlers

3. Consider this Express code snippet:

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

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

medium

A. Second\nFirst

B. First\nSecond

C. Hello

D. No output

Solution

1. Step 1: Understand middleware execution order

   Middleware runs in the order they are added with app.use().

2. Step 2: Trace console logs

   First middleware logs 'First' then calls next(), second logs 'Second' then calls next(), then route sends response.

3. Final Answer:

   First\nSecond -> Option B

4. Quick Check:

   Middleware executes in order added [OK]

Hint: Middleware logs appear in order of app.use calls [OK]

Common Mistakes:

• Assuming route handler logs to console
• Thinking middleware runs after route
• Mixing order of middleware execution

4. What is the problem with this middleware code?

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

medium

A. Middleware should be added after routes

B. Middleware syntax is incorrect

C. Route handler is missing parameters

D. Missing call to next(), request will hang

Solution

1. Step 1: Check middleware function

   The middleware logs a message but does not call next().

2. Step 2: Understand effect of missing next()

   Without calling next(), the request does not proceed to the route handler, causing the request to hang.

3. Final Answer:

   Missing call to next(), request will hang -> Option D

4. Quick Check:

   Middleware must call next() to continue [OK]

Hint: Always call next() in middleware to avoid hanging requests [OK]

Common Mistakes:

• Forgetting to call next() in middleware
• Thinking middleware runs after routes
• Assuming middleware syntax is wrong without checking next()

5. You want to create middleware that only runs for routes starting with '/admin'. Which is the correct way to apply this middleware?

hard

A. app.use('/admin', (req, res, next) => { console.log('Admin middleware'); next(); });

B. app.use((req, res, next) => { if(req.url.startsWith('admin')) { console.log('Admin middleware'); } next(); });

C. app.get('/admin', (req, res, next) => { console.log('Admin middleware'); next(); });

D. app.use('*admin', (req, res, next) => { console.log('Admin middleware'); next(); });

Solution

1. Step 1: Understand path filtering in middleware

   Express allows specifying a path prefix in app.use(path, middleware) to run middleware only on matching routes.

2. Step 2: Evaluate options

   app.use('/admin', (req, res, next) => { console.log('Admin middleware'); next(); }); correctly uses app.use('/admin', ...) for prefix matching. app.use((req, res, next) => { if(req.url.startsWith('admin')) { console.log('Admin middleware'); } next(); }); has incorrect condition (startsWith('admin') misses leading /, never matches). app.get('/admin', (req, res, next) => { console.log('Admin middleware'); next(); }); is a route handler, not middleware. app.use('*admin', (req, res, next) => { console.log('Admin middleware'); next(); }); *admin matches paths containing admin at the end, not starting with /admin.

3. Final Answer:

   app.use('/admin', (req, res, next) => { console.log('Admin middleware'); next(); }); -> Option A

4. Quick Check:

   Use app.use with path prefix for selective middleware [OK]

Hint: Use app.use('/path', middleware) to target specific routes [OK]

Common Mistakes:

• Using route handlers instead of middleware
• Incorrect conditional checks in global middleware
• Wrong path patterns like wildcards in wrong position