Protecting routes with auth middleware in Express

Introduction

Protecting routes with auth middleware helps keep parts of your app safe. It checks if a user is allowed before they see certain pages.

When you want only logged-in users to access their profile page.

When you need to hide admin pages from regular users.

When you want to check if a user has permission before showing sensitive data.

When you want to stop visitors from accessing private routes without signing in.

Syntax

Express

function authMiddleware(req, res, next) {
  if (req.user) {
    next(); // allow access
  } else {
    res.status(401).send('Not authorized');
  }
}

app.get('/protected-route', authMiddleware, (req, res) => {
  res.send('This is protected');
});

The middleware function runs before the route handler.

Call next() to continue if the user is allowed.

Examples

Checks if req.user exists to allow access.

Express

function authMiddleware(req, res, next) {
  if (req.user) {
    next();
  } else {
    res.status(401).send('Please log in');
  }
}

Protects the dashboard route so only authorized users can see it.

Express

app.get('/dashboard', authMiddleware, (req, res) => {
  res.send('Welcome to your dashboard');
});

Protects all routes under /admin with the auth middleware.

Express

app.use('/admin', authMiddleware);

app.get('/admin/settings', (req, res) => {
  res.send('Admin settings page');
});

Sample Program

This example shows a simple Express app with a public route and a protected route. The authMiddleware checks if the request has a header x-user. If yes, it allows access to the profile page. Otherwise, it sends a 401 error.

Express

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

// Simple auth middleware
function authMiddleware(req, res, next) {
  // Simulate user logged in if header 'x-user' exists
  if (req.headers['x-user']) {
    next();
  } else {
    res.status(401).send('Not authorized');
  }
}

// Public route
app.get('/', (req, res) => {
  res.send('Welcome to the public page');
});

// Protected route
app.get('/profile', authMiddleware, (req, res) => {
  res.send('This is your profile');
});

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

Important Notes

Middleware runs in the order you add it, so place auth middleware before protected routes.

In real apps, auth checks usually look at tokens or sessions, not just headers.

Always send a clear message or redirect when access is denied.

Summary

Auth middleware checks if a user can access a route.

Use next() to allow access, or send an error to block.

Protect routes by adding middleware before route handlers.

Practice

1. What is the main purpose of auth middleware in an Express app?

easy

A. To check if a user is allowed to access a route

B. To format the response data before sending

C. To log every request made to the server

D. To serve static files like images and CSS

Solution

1. Step 1: Understand middleware role

   Middleware runs before route handlers to process requests.

2. Step 2: Identify auth middleware function

   Auth middleware specifically checks user permissions to allow or deny access.

3. Final Answer:

   To check if a user is allowed to access a route -> Option A

4. Quick Check:

   Auth middleware = Access control [OK]

Hint: Auth middleware controls access to routes [OK]

Common Mistakes:

• Confusing auth middleware with logging middleware
• Thinking middleware serves static files
• Assuming middleware formats response data

2. Which of the following is the correct way to use auth middleware for a route in Express?

easy

A. app.get('/profile', authMiddleware, (req, res) => { res.send('Profile'); });

B. app.get(authMiddleware, '/profile', (req, res) => { res.send('Profile'); });

C. app.get('/profile', (req, res) => { authMiddleware(); res.send('Profile'); });

D. app.get('/profile', (req, res) => { res.send('Profile'); }, authMiddleware);

Solution

1. Step 1: Recall Express route syntax

   Middleware functions come before the final route handler in the argument list.

2. Step 2: Check each option's order

   Only app.get('/profile', authMiddleware, (req, res) => { res.send('Profile'); }); places authMiddleware correctly before the handler function.

3. Final Answer:

   app.get('/profile', authMiddleware, (req, res) => { res.send('Profile'); }); -> Option A

4. Quick Check:

   Middleware before handler = app.get('/profile', authMiddleware, (req, res) => { res.send('Profile'); }); [OK]

Hint: Middleware goes before the route handler function [OK]

Common Mistakes:

• Placing middleware after the handler
• Passing middleware as the first argument instead of path
• Calling middleware inside the handler instead of passing it

3. Given this auth middleware, what will happen when a request without a valid token hits the protected route?

function authMiddleware(req, res, next) {
  if (req.headers.authorization === 'valid-token') {
    next();
  } else {
    res.status(401).send('Unauthorized');
  }
}

app.get('/dashboard', authMiddleware, (req, res) => {
  res.send('Welcome to dashboard');
});

medium

A. The user sees 'Welcome to dashboard' regardless of token

B. The server crashes due to missing next() call

C. The user gets a 401 Unauthorized response if token is missing or invalid

D. The user gets a 404 Not Found error

Solution

1. Step 1: Analyze authMiddleware logic

   If the authorization header equals 'valid-token', next() is called to continue.

2. Step 2: Check behavior when token is missing or invalid

   Else block sends 401 Unauthorized response and does not call next(), blocking access.

3. Final Answer:

   The user gets a 401 Unauthorized response if token is missing or invalid -> Option C

4. Quick Check:

   Invalid token = 401 Unauthorized [OK]

Hint: Middleware sends 401 if token invalid, else calls next() [OK]

Common Mistakes:

• Assuming next() is always called
• Thinking user always sees dashboard
• Confusing 401 with 404 errors

4. Identify the error in this auth middleware code:

function authMiddleware(req, res, next) {
  if (!req.user) {
    res.status(403).send('Forbidden');
  }
  next();
}

medium

A. Missing call to next() inside the if block

B. next() is called even after sending a response, causing an error

C. Status code 403 is incorrect for unauthorized access

D. req.user should be checked with req.auth instead

Solution

1. Step 1: Understand middleware flow

   If !req.user is true, response is sent with status 403.

2. Step 2: Check what happens after sending response

   next() is called unconditionally after the if block, so it runs even after response sent, causing errors.

3. Final Answer:

   next() is called even after sending a response, causing an error -> Option B

4. Quick Check:

   Call next() only if no response sent [OK]

Hint: Do not call next() after sending a response [OK]

Common Mistakes:

• Calling next() after res.send()
• Not stopping middleware after response
• Using wrong status codes for auth errors

5. You want to protect multiple routes with the same auth middleware and also log the user ID if authenticated. Which is the best way to do this?

function authMiddleware(req, res, next) {
  if (!req.headers.authorization) {
    return res.status(401).send('Unauthorized');
  }
  req.userId = req.headers.authorization;
  next();
}

// How to apply this middleware and log userId for routes '/profile' and '/settings'?

hard

A. Apply authMiddleware after route handlers to log userId

B. Add authMiddleware only to '/profile' route and log userId in '/settings' without middleware

C. Call authMiddleware inside each route handler manually before logging userId

D. Use app.use(authMiddleware) before defining both routes, then log req.userId inside each route handler

Solution

1. Step 1: Understand middleware application

   app.use(authMiddleware) applies middleware to all routes defined after it, protecting multiple routes easily.

2. Step 2: Logging userId in route handlers

   Since authMiddleware sets req.userId, route handlers can access and log it safely after middleware runs.

3. Final Answer:

   Use app.use(authMiddleware) before defining both routes, then log req.userId inside each route handler -> Option D

4. Quick Check:

   Use app.use for shared middleware [OK]

Hint: Use app.use(authMiddleware) to protect many routes [OK]

Common Mistakes:

• Applying middleware only to some routes inconsistently
• Calling middleware inside handlers manually
• Applying middleware after route handlers