Permission middleware in Express - Mini Project: Build & Apply

Permission middleware

📖 Scenario: You are building a simple Express server that controls access to certain routes based on user roles.Users have roles like 'admin' or 'user'. You want to create a middleware function that checks if the user has permission to access a route.

🎯 Goal: Create a permission middleware function that checks if the logged-in user has the required role to access a route. Then apply this middleware to protect a route.

📋 What You'll Learn

Create an Express app with a sample user object

Define a required role variable for permission checking

Write a middleware function called checkPermission that checks the user's role

Use the checkPermission middleware on a protected route

💡 Why This Matters

🌍 Real World

Permission middleware is used in web servers to control access to routes based on user roles or permissions, ensuring security and proper authorization.

💼 Career

Understanding middleware and permission checks is essential for backend developers working with Express or similar frameworks to build secure APIs.

1

Create a sample user object

Create a variable called user that is an object with these exact properties: name set to 'Alice' and role set to 'user'.

Express

# Create the user object with name 'Alice' and role 'user'
# Your code here

Hint

Use const user = { name: 'Alice', role: 'user' };

2

Define the required role variable

Create a variable called requiredRole and set it to the string 'admin'.

Express

const user = { name: 'Alice', role: 'user' };
// Define the required role as 'admin'
// Your code here

Hint

Use const requiredRole = 'admin';

3

Write the permission middleware function

Write a middleware function called checkPermission that takes req, res, and next as parameters. Inside it, check if user.role is equal to requiredRole. If yes, call next(). Otherwise, respond with status 403 and JSON message { error: 'Access denied' }.

Express

const user = { name: 'Alice', role: 'user' };
const requiredRole = 'admin';

// Write the checkPermission middleware function
// Your code here

Hint

Check the user's role and call next() if allowed, else send a 403 response.

4

Use the middleware on a protected route

Create an Express app by requiring express and calling express(). Then create a GET route at '/admin' that uses the checkPermission middleware. The route handler should respond with JSON { message: 'Welcome admin' }.

Express

const user = { name: 'Alice', role: 'user' };
const requiredRole = 'admin';

function checkPermission(req, res, next) {
    if (user.role === requiredRole) {
        next();
    } else {
        res.status(403).json({ error: 'Access denied' });
    }
}

// Create an Express app and add a GET '/admin' route using checkPermission middleware
// Your code here

Hint

Use app.get('/admin', checkPermission, (req, res) => { ... }) to protect the route.

Practice

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

easy

A. To check if a user has rights to access a route before running its handler

B. To format the response data before sending it to the client

C. To log every request made to the server

D. To handle errors thrown by route handlers

Solution

1. Step 1: Understand middleware role

   Middleware runs before route handlers to control flow or check conditions.

2. Step 2: Identify permission middleware function

   Permission middleware specifically checks user rights to allow or deny access.

3. Final Answer:

   To check if a user has rights to access a route before running its handler -> Option A

4. Quick Check:

   Permission middleware controls access = A [OK]

Hint: Permission middleware controls access before route runs [OK]

Common Mistakes:

• Confusing permission middleware with logging middleware
• Thinking it formats response data
• Assuming it handles errors

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

easy

A. function checkPermission(req, res, next) { if (!req.user) next('No user'); else next(); }

B. function checkPermission(req, res) { if (!req.user) res.send('No user'); else next(); }

C. function checkPermission(req, res, next) { if (!req.user) res.send('No user'); else next(); }

D. function checkPermission(req, res, next) { if (!req.user) return; else next(); }

Solution

1. Step 1: Check middleware signature

   Express middleware must have three parameters: req, res, next.

2. Step 2: Verify correct usage of next()

   If permission fails, respond or send error; else call next() to continue.

3. Final Answer:

   function checkPermission(req, res, next) { if (!req.user) res.send('No user'); else next(); } -> Option C

4. Quick Check:

   Middleware needs (req, res, next) and calls next() [OK]

Hint: Middleware needs three params: req, res, next [OK]

Common Mistakes:

• Missing next parameter
• Calling next() without parentheses
• Not sending response or calling next() properly

3. Given this middleware and route, what will be the response if req.user.role is 'guest'?

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

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

medium

A. Forbidden

B. Welcome Admin

C. Internal Server Error

D. No response (timeout)

Solution

1. Step 1: Check user role in middleware

   The middleware checks if req.user.role is not 'admin'. Here it is 'guest', so condition is true.

2. Step 2: Middleware response on failed permission

   It sends status 403 with 'Forbidden' and does not call next(), so route handler is skipped.

3. Final Answer:

   Forbidden -> Option A

4. Quick Check:

   Role not admin = 403 Forbidden [OK]

Hint: If role not admin, middleware sends 403 and stops [OK]

Common Mistakes:

• Assuming route handler runs anyway
• Confusing status codes
• Missing optional chaining on req.user

4. Identify the error in this permission middleware code:

function checkPermission(req, res, next) {
  if (!req.user.permissions.includes('edit')) {
    res.status(401).send('Unauthorized');
  }
  next();
}

medium

A. Middleware should not call next() at all

B. Wrong status code for unauthorized access

C. Incorrect property name 'permissions' on req.user

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

Solution

1. Step 1: Analyze flow after sending response

   After res.status(401).send(), the code continues and calls next(), allowing next middleware or route to run.

2. Step 2: Fix by adding return to stop execution

   Adding 'return' before res.status(401).send() prevents next() from running when unauthorized.

3. Final Answer:

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

4. Quick Check:

   Send response must return to stop next() [OK]

Hint: Return after res.send() to prevent next() running [OK]

Common Mistakes:

• Not returning after sending response
• Using wrong HTTP status code
• Assuming next() should never be called

5. You want to create a permission middleware that allows access only if the user has at least one role from an array of allowed roles. Which code correctly implements this?

hard

A. function permitRoles(allowedRoles) {
return (req, res, next) => {
if (!allowedRoles.includes(req.user.role)) {
res.status(403).send('Forbidden');
}
next();
};
}

B. function permitRoles(allowedRoles) {
return (req, res, next) => {
if (allowedRoles.some(role => role === req.user.role)) {
next();
} else {
res.status(403).send('Forbidden');
}
};
}

C. function permitRoles(allowedRoles) {
return (req, res, next) => {
if (allowedRoles.indexOf(req.user.role) === -1) {
res.status(403).send('Forbidden');
}
next();
};
}

D. function permitRoles(allowedRoles) {
return (req, res, next) => {
if (allowedRoles.every(role => role !== req.user.role)) {
next();
} else {
res.status(403).send('Forbidden');
}
};
}

Solution

1. Step 1: Understand the requirement

   Access allowed if user role matches any role in allowedRoles array.

2. Step 2: Check each option logic

   function permitRoles(allowedRoles) { return (req, res, next) => { if (!allowedRoles.includes(req.user.role)) { res.status(403).send('Forbidden'); } next(); }; } uses includes but misses return before res.send(), so next() runs anyway. function permitRoles(allowedRoles) { return (req, res, next) => { if (allowedRoles.indexOf(req.user.role) === -1) { res.status(403).send('Forbidden'); } next(); }; } misses return before res.send(), so next() runs anyway. function permitRoles(allowedRoles) { return (req, res, next) => { if (allowedRoles.some(role => role === req.user.role)) { next(); } else { res.status(403).send('Forbidden'); } }; } uses some() to check if any role matches, then calls next() or sends 403 correctly. function permitRoles(allowedRoles) { return (req, res, next) => { if (allowedRoles.every(role => role !== req.user.role)) { next(); } else { res.status(403).send('Forbidden'); } }; } reverses logic, allowing access if no match, which is wrong.

3. Step 3: Choose best correct code

   function permitRoles(allowedRoles) { return (req, res, next) => { if (allowedRoles.some(role => role === req.user.role)) { next(); } else { res.status(403).send('Forbidden'); } }; } correctly implements the logic with proper flow control.

4. Final Answer:

   Uses some() to allow access if any role matches, else sends 403 -> Option B

5. Quick Check:

   Use some() to check roles and control flow correctly [OK]

Hint: Use some() to check if user role is in allowed roles [OK]

Common Mistakes:

• Not returning after sending response
• Using every() incorrectly
• Calling next() even after forbidden response