Application-level middleware in Express

Introduction

Application-level middleware helps you run code for every request your app gets. It can change requests, responses, or stop the request if needed.

To log every request made to your server for monitoring.

To check if a user is logged in before allowing access to certain pages.

To add headers to all responses for security or tracking.

To parse incoming data like JSON before your routes handle it.

To handle errors or unexpected situations globally.

Syntax

Express

app.use(function (req, res, next) {
  // your code here
  next();
});

Middleware functions take three arguments: req, res, and next.

Always call next() to pass control to the next middleware or route.

Examples

This logs the HTTP method and URL for every request.

Express

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

This middleware runs only for routes starting with /admin and blocks users who are not admins.

Express

app.use('/admin', (req, res, next) => {
  if (!req.user || !req.user.isAdmin) {
    return res.status(403).send('Access denied');
  }
  next();
});

This built-in middleware parses incoming JSON data so you can use it easily in your routes.

Express

app.use(express.json());

Sample Program

This example shows an application-level middleware that logs every request method and URL. Then it responds with a greeting on the home page.

Express

import express from 'express';

const app = express();

// Application-level middleware to log requests
app.use((req, res, next) => {
  console.log(`Received ${req.method} request for ${req.url}`);
  next();
});

// Simple route
app.get('/', (req, res) => {
  res.send('Hello from Express!');
});

// Start server
app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Middleware order matters: Express runs them in the order you add them.

If you forget to call next(), the request will hang and never reach routes.

You can add middleware for specific paths by passing the path as the first argument to app.use().

Summary

Application-level middleware runs for every request or for specific paths.

It can modify requests, responses, or stop requests early.

Always call next() to continue the request cycle.

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