Permission middleware in Express

Introduction

Permission middleware helps control who can access certain parts of a web app. It checks if a user has the right permissions before allowing them to continue.

When you want to restrict access to admin-only pages.

When users should only see their own data, not others'.

When certain actions like deleting or editing need special rights.

When you want to protect sensitive routes from unauthorized users.

Syntax

Express

function permissionMiddleware(requiredPermission) {
  return function (req, res, next) {
    const userPermissions = req.user?.permissions || [];
    if (userPermissions.includes(requiredPermission)) {
      next();
    } else {
      res.status(403).send('Access denied');
    }
  };
}

The middleware is a function that returns another function to use in routes.

It checks if the user's permissions include the required one, then calls next() to continue.

Examples

This route only allows users with 'admin' permission to access the admin page.

Express

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

This route allows users with 'edit' permission to submit changes.

Express

app.post('/edit', permissionMiddleware('edit'), (req, res) => {
  res.send('Edit allowed');
});

Sample Program

This example sets up a simple Express server. It adds a mock user with 'read' and 'edit' permissions. The permission middleware checks if the user can access each route. The '/delete' route will deny access because the user lacks 'delete' permission.

Express

import express from 'express';

const app = express();

// Mock user data middleware
app.use((req, res, next) => {
  req.user = { permissions: ['read', 'edit'] };
  next();
});

function permissionMiddleware(requiredPermission) {
  return (req, res, next) => {
    const userPermissions = req.user?.permissions || [];
    if (userPermissions.includes(requiredPermission)) {
      next();
    } else {
      res.status(403).send('Access denied');
    }
  };
}

app.get('/read', permissionMiddleware('read'), (req, res) => {
  res.send('Reading content');
});

app.post('/edit', permissionMiddleware('edit'), (req, res) => {
  res.send('Editing content');
});

app.delete('/delete', permissionMiddleware('delete'), (req, res) => {
  res.status(403).send('Access denied');
});

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

Important Notes

Always attach user info to req.user before using permission middleware.

Return a 403 status code for forbidden access to follow web standards.

Middleware can be reused for different permissions by passing different strings.

Summary

Permission middleware controls access based on user rights.

It checks user permissions before allowing route handlers to run.

Use it to protect sensitive or restricted parts of your app.

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