Middleware composition for auth layers in Express

Introduction

Middleware composition helps organize multiple small functions that check user identity and permissions. It keeps your code clean and easy to manage.

You want to check if a user is logged in before accessing a page.

You need to verify user roles like admin or editor before allowing actions.

You want to add multiple checks like token validation and permission checks in order.

You want to reuse authentication steps across different routes.

You want to handle errors or redirects if authentication fails.

Syntax

Express

app.use(middleware1, middleware2, middleware3);

// or for a single route
app.get('/path', middleware1, middleware2, handler);

Each middleware function receives req, res, and next to pass control.

Order matters: middleware runs in the order you list them.

Examples

This example checks if the user is logged in, then if they are an admin, before showing the admin page.

Express

function checkLoggedIn(req, res, next) {
  if (req.user) next();
  else res.status(401).send('Not logged in');
}

function checkAdmin(req, res, next) {
  if (req.user && req.user.role === 'admin') next();
  else res.status(403).send('Admin only');
}

app.get('/admin', checkLoggedIn, checkAdmin, (req, res) => {
  res.send('Welcome Admin');
});

You can group middleware in an array and spread it for cleaner code.

Express

const authMiddleware = [checkLoggedIn, checkAdmin];

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

Sample Program

This Express app sets a fake user, then uses two middleware functions to check if the user is logged in and is an admin before showing a welcome message.

Express

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

// Simulate user data
app.use((req, res, next) => {
  req.user = { name: 'Alice', role: 'admin' };
  next();
});

function checkLoggedIn(req, res, next) {
  if (req.user) next();
  else res.status(401).send('Not logged in');
}

function checkAdmin(req, res, next) {
  if (req.user && req.user.role === 'admin') next();
  else res.status(403).send('Admin only');
}

app.get('/admin', checkLoggedIn, checkAdmin, (req, res) => {
  res.send(`Welcome Admin ${req.user.name}`);
});

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

Important Notes

Middleware functions must call next() to continue the chain.

Always handle errors or send responses to avoid hanging requests.

Use middleware composition to keep your auth logic reusable and clear.

Summary

Middleware composition lets you run multiple auth checks in order.

Use small middleware functions for each auth step.

Group middleware for cleaner route definitions.

Practice

1. What is the main purpose of composing multiple middleware functions for authentication in Express?

easy

A. To run several small auth checks in order before allowing access

B. To combine all auth logic into one big function

C. To skip authentication for faster response

D. To handle database queries inside middleware

Solution

1. Step 1: Understand middleware composition

   Middleware composition means running multiple middleware functions one after another.

2. Step 2: Purpose in auth layers

   Using multiple small auth checks in order helps keep code clean and checks each condition separately.

3. Final Answer:

   To run several small auth checks in order before allowing access -> Option A

4. Quick Check:

   Middleware composition = multiple small auth checks [OK]

Hint: Think of middleware as a chain of small checks [OK]

Common Mistakes:

• Thinking all auth logic must be in one function
• Believing middleware skips auth
• Confusing middleware with database queries

2. Which of the following is the correct way to apply two middleware functions checkToken and checkRole to an Express route using an array?

easy

A. app.get('/admin', checkToken, checkRole, (req, res) => res.send('OK'))

B. app.get('/admin', checkToken && checkRole, (req, res) => res.send('OK'))

C. app.get('/admin', [checkToken, checkRole], (req, res) => res.send('OK'))

D. app.get('/admin', checkToken || checkRole, (req, res) => res.send('OK'))

Solution

1. Step 1: Understand Express middleware syntax

   Express accepts multiple middleware as an array or separate arguments before the handler. This question specifies using an array.

2. Step 2: Check each option

   A uses separate arguments. B and D use logical operators which are invalid here. C correctly uses an array.

3. Final Answer:

   app.get('/admin', [checkToken, checkRole], (req, res) => res.send('OK')) -> Option C

4. Quick Check:

   Middleware array syntax = app.get('/admin', [checkToken, checkRole], (req, res) => res.send('OK')) [OK]

Hint: Use arrays to group middleware in routes [OK]

Common Mistakes:

• Using logical operators instead of arrays
• Passing middleware as a single combined expression
• Forgetting to include middleware before handler

3. Given the middleware functions below, what will be the response when a request with req.user = { role: 'user' } hits the route?

function checkToken(req, res, next) {
  if (!req.user) return res.status(401).send('No token');
  next();
}

function checkAdmin(req, res, next) {
  if (req.user.role !== 'admin') return res.status(403).send('Forbidden');
  next();
}

app.get('/secure', [checkToken, checkAdmin], (req, res) => res.send('Welcome admin'));

medium

A. Welcome admin

B. Forbidden

C. No token

D. Internal Server Error

Solution

1. Step 1: Analyze checkToken middleware

   It checks if req.user exists. Here req.user is { role: 'user' }, so it passes and calls next().

2. Step 2: Analyze checkAdmin middleware

   It checks if req.user.role is 'admin'. Here it is 'user', so it returns 403 Forbidden response.

3. Final Answer:

   Forbidden -> Option B

4. Quick Check:

   Role check fails = Forbidden [OK]

Hint: Check middleware order and conditions carefully [OK]

Common Mistakes:

• Assuming role 'user' passes admin check
• Ignoring middleware that sends response early
• Confusing status codes

4. Identify the error in this middleware composition code:

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

app.get('/data', auth, (req, res) => res.send('Data'));

medium

A. Missing return after sending 401 response, so next() runs anyway

B. Middleware should be async function

C. Route handler missing res.end() call

D. Authorization header check should be in route handler

Solution

1. Step 1: Check middleware flow

   If authorization header is missing, it sends 401 but does not stop execution.

2. Step 2: Identify missing return

   Without return after res.status(401).send(), next() is called anyway, causing route handler to run incorrectly.

3. Final Answer:

   Missing return after sending 401 response, so next() runs anyway -> Option A

4. Quick Check:

   Send response must stop middleware with return [OK]

Hint: Always return after sending response in middleware [OK]

Common Mistakes:

• Calling next() after sending response
• Thinking async needed for simple middleware
• Putting auth logic in route handler

5. You want to create a reusable middleware group for routes that require both token validation and admin role check. Which is the best way to compose and apply these middlewares in Express?

hard

A. Use a global app.use() for all routes regardless of auth needs

B. Create a single middleware combining both checks and use it in routes

C. Call each middleware manually inside the route handler function

D. Use an array of separate middlewares and apply the array to routes

Solution

1. Step 1: Understand middleware grouping

   Grouping middlewares as an array keeps each check separate and reusable.

2. Step 2: Compare options

   Use an array of separate middlewares and apply the array to routes uses an array of middlewares applied to routes, which is clean and composable. Create a single middleware combining both checks and use it in routes merges checks into one, losing modularity. Call each middleware manually inside the route handler function is manual and error-prone. Use a global app.use() for all routes regardless of auth needs applies auth globally, which is not selective.

3. Final Answer:

   Use an array of separate middlewares and apply the array to routes -> Option D

4. Quick Check:

   Middleware arrays = reusable and clean [OK]

Hint: Group middlewares in arrays for reuse [OK]

Common Mistakes:

• Combining all logic into one middleware
• Calling middleware inside handlers manually
• Applying auth globally without route control