Permission middleware in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a middleware function that checks if the user has the required permission.

Express

function checkPermission(permission) {
  return function(req, res, next) {
    if (req.user && req.user.permissions && req.user.permissions.includes([1])) {
      next();
    } else {
      res.status(403).send('Forbidden');
    }
  };
}

Drag options to blanks, or click blank then click option'

Apermissions

Breq.permission

Cpermission

Dreq.user.permission

Attempts:

3 left

💡 Hint

Common Mistakes

Using req.permission instead of the parameter

Checking req.user.permission which is not an array

Using a wrong variable name

The middleware checks if the user's permissions include the given permission argument. So, we use the permission parameter directly.

2fill in blank

medium

Complete the code to apply the permission middleware to an Express route that requires 'admin' permission.

Express

app.get('/admin', [1]('admin'), (req, res) => {
  res.send('Welcome Admin');
});

Drag options to blanks, or click blank then click option'

AcheckPermission

BauthMiddleware

CrequirePermission

DpermissionCheck

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong middleware function name

Not calling the middleware as a function with the permission argument

The middleware function is named checkPermission, so we use it to protect the route.

3fill in blank

hard

Fix the error in the middleware to properly handle missing user or permissions.

Express

function checkPermission(permission) {
  return function(req, res, next) {
    if ([1]) {
      next();
    } else {
      res.status(403).send('Forbidden');
    }
  };
}

Drag options to blanks, or click blank then click option'

Areq.user && req.user.permissions && permission

Breq.user.permissions

Cpermission

Dreq.user?.permissions?.includes(permission)

Attempts:

3 left

💡 Hint

Common Mistakes

Not checking if req.user exists causing errors

Trying to access permissions without safety checks

Using optional chaining req.user?.permissions?.includes(permission) safely checks if user and permissions exist before calling includes.

4fill in blank

hard

Fill both blanks to create a middleware that logs the permission checked and calls next if allowed.

Express

function checkPermission(permission) {
  return function(req, res, next) {
    console.log('Checking permission:', [1]);
    if (req.user?.permissions?.includes([2])) {
      next();
    } else {
      res.status(403).send('Forbidden');
    }
  };
}

Drag options to blanks, or click blank then click option'

Apermission

Breq.user

Creq.user.permissions

Dreq.permission

Attempts:

3 left

💡 Hint

Common Mistakes

Logging req.user instead of permission

Checking wrong variable for permission

We log the permission parameter and check if the user's permissions include that same permission.

5fill in blank

hard

Fill all three blanks to create a permission middleware that sends a JSON error message when forbidden.

Express

function checkPermission([1]) {
  return function(req, res, next) {
    if (req.user?.permissions?.includes([2])) {
      next();
    } else {
      res.status(403).json({ error: [3] });
    }
  };
}

Drag options to blanks, or click blank then click option'

Apermission

C'Access denied'

D'Forbidden'

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for parameter and check

Sending plain text instead of JSON error

Using incorrect error message string

The function parameter and the check use permission. The JSON error message uses the string 'Forbidden' to clearly indicate the access issue.

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