Conditional middleware execution in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Middleware execution based on request path

Consider this Express middleware setup. What will be the output when a GET request is made to /api/data?

Express

const express = require('express');
const app = express();

function logMiddleware(req, res, next) {
  console.log('Middleware triggered');
  next();
}

app.use((req, res, next) => {
  if (req.path.startsWith('/api')) {
    logMiddleware(req, res, next);
  } else {
    next();
  }
});

app.get('/api/data', (req, res) => {
  res.send('Data response');
});

app.listen(3000);

A Console logs nothing and client receives 404 Not Found.

B Console logs nothing and client receives 'Data response'.

C Console logs 'Middleware triggered' but client hangs without response.

D Console logs 'Middleware triggered' and client receives 'Data response'.

💡 Hint

Think about when the middleware function is called based on the request path.

The middleware checks if the request path starts with '/api'. For '/api/data', it does, so logMiddleware runs, logging the message. Then it calls next() to continue to the route handler, which sends the response.

📝 Syntax

intermediate

Correct conditional middleware syntax

Which option correctly applies middleware only for POST requests to '/submit' in Express?

A app.post('/submit', (req, res, next) => { next(); });

B app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') next(); else next(); });

C app.use('/submit', (req, res, next) => { if (req.method === 'POST') next(); else res.sendStatus(405); });

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💡 Hint

Remember the difference between app.use and app.post for method-specific middleware.

Option A uses app.post which applies middleware only for POST requests to '/submit'. Option A uses app.use but does not handle the response properly. Option A has an assignment (=) instead of comparison (===), causing a bug.

🔧 Debug

advanced

Middleware not executing as expected

Given this code, why does the middleware never log 'Middleware active' when accessing '/dashboard'?

Express

const express = require('express');
const app = express();

app.use('/dashboard', (req, res, next) => {
  console.log('Middleware active');
  next();
});

app.get('/dashboard', (req, res) => {
  res.send('Dashboard page');
});

app.listen(3000);

A Because the middleware is applied after the route handler, so it never runs.

B Because app.use with a path only matches requests starting with that path, so it should work and the problem is elsewhere.

C Because the middleware is applied to '/dashboard' but the request path includes a trailing slash '/dashboard/', so it doesn't match.

D Because the middleware is synchronous and blocks the route handler.

💡 Hint

Check how Express matches paths with and without trailing slashes.

Express matches paths as prefixes for app.use with a path. If the request URL is '/dashboard/' (with trailing slash), it still matches '/dashboard' prefix, so the middleware runs. The problem is likely elsewhere.

❓ state_output

advanced

Middleware modifying request state conditionally

What will be the value of req.user in the route handler after this middleware runs for a request to '/profile' with header Authorization: Bearer token123?

Express

const express = require('express');
const app = express();

app.use((req, res, next) => {
  if (req.headers.authorization?.startsWith('Bearer ')) {
    req.user = { id: 1, name: 'Alice' };
  }
  next();
});

app.get('/profile', (req, res) => {
  res.json(req.user || null);
});

app.listen(3000);

A {"id":1,"name":"Alice"}

B null

C undefined

D Throws a TypeError

💡 Hint

Look at how the middleware sets req.user based on the Authorization header.

The middleware checks if the Authorization header starts with 'Bearer '. Since the request has 'Bearer token123', it sets req.user to the user object. The route handler then returns this object as JSON.

🧠 Conceptual

expert

Understanding conditional middleware order and effect

In Express, if you want to run a logging middleware only for routes starting with '/admin' and ensure it runs before any route handler, which setup is correct?

A Place <code>app.use(loggerMiddleware)</code> after all '/admin' routes are defined and check path inside middleware.

B Define <code>app.get('/admin/*', loggerMiddleware, handler)</code> for each route individually.

C Place <code>app.use('/admin', loggerMiddleware)</code> before defining any '/admin' routes.

D Use <code>app.use((req, res, next) => { if (req.path.startsWith('/admin')) loggerMiddleware(req, res, next); else next(); })</code> after routes.

💡 Hint

Middleware order matters in Express; earlier middleware runs before later routes.

Middleware must be registered before the routes it affects to run first. Using app.use('/admin', loggerMiddleware) before defining '/admin' routes ensures it runs only for those routes and before their handlers.

Practice

1. What is the main purpose of conditional middleware in Express?

easy

A. To run middleware only when certain conditions are met

B. To run all middleware for every request without exception

C. To replace route handlers completely

D. To stop the server from accepting requests

Solution

1. Step 1: Understand middleware role

   Middleware functions process requests and can modify them or respond.

2. Step 2: Identify conditional middleware purpose

   Conditional middleware runs only when a specific condition is true, saving resources and controlling flow.

3. Final Answer:

   To run middleware only when certain conditions are met -> Option A

4. Quick Check:

   Conditional middleware = run only if condition true [OK]

Hint: Think: middleware runs only if condition matches [OK]

Common Mistakes:

• Assuming middleware always runs for every request
• Confusing middleware with route handlers
• Thinking middleware stops server requests

2. Which of the following is the correct way to conditionally execute middleware based on the request path?

easy

A. app.use((req, res, next) => { if (req.path === '/admin') res.send('OK'); });

B. app.use((req, res, next) => { if (req.method === 'POST') res.send('Allowed'); else next(); });

C. app.use((req, res, next) => { if (req.path = '/admin') next(); else next(); });

D. app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); });

Solution

1. Step 1: Check condition syntax

   app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); }); correctly uses '===' to compare req.path to '/admin'.

2. Step 2: Verify middleware flow

   app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); }); calls next() only if path matches; otherwise, it sends a response, stopping further middleware.

3. Final Answer:

   app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); }); -> Option D

4. Quick Check:

   Correct condition and flow = app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); }); [OK]

Hint: Use '===' for comparison and call next() properly [OK]

Common Mistakes:

• Using single '=' instead of '===' for comparison
• Not calling next() to continue middleware chain
• Sending response without ending middleware properly

3. Consider this middleware code snippet:

app.use((req, res, next) => {
  if (req.query.token === '123') {
    console.log('Token valid');
    next();
  } else {
    res.status(401).send('Unauthorized');
  }
});
app.get('/', (req, res) => res.send('Welcome!'));

What will be the output when a GET request is made to '/' with query ?token=123?

medium

A. Console logs 'Token valid' but response is 'Unauthorized'

B. Console logs 'Token valid' and response is 'Welcome!'

C. No response, request hangs

D. Response is 'Unauthorized' with status 401

Solution

1. Step 1: Check token condition

   Request query token is '123', so condition is true and 'Token valid' logs.

2. Step 2: Middleware calls next()

   Calling next() passes control to next handler, which sends 'Welcome!'.

3. Final Answer:

   Console logs 'Token valid' and response is 'Welcome!' -> Option B

4. Quick Check:

   Valid token triggers next() and welcome message [OK]

Hint: If condition true, next() runs next handler [OK]

Common Mistakes:

• Assuming response is 'Unauthorized' despite valid token
• Thinking middleware always sends response
• Missing that next() continues to route handler

4. Identify the error in this conditional middleware:

app.use((req, res, next) => {
  if (req.headers['x-auth']) {
    next();
  }
  res.status(403).send('Forbidden');
});

medium

A. Middleware does not check request method

B. next() is called without arguments

C. Missing else block causes response sent twice

D. Headers property is misspelled

Solution

1. Step 1: Analyze middleware flow

   If header exists, next() is called but code continues to send 403 response anyway.

2. Step 2: Identify missing else

   Without else, response is sent after next(), causing errors or double responses.

3. Final Answer:

   Missing else block causes response sent twice -> Option C

4. Quick Check:

   Always use else or return after next() [OK]

Hint: Use else or return after next() to avoid double response [OK]

Common Mistakes:

• Not using else or return after next()
• Assuming next() stops code execution
• Ignoring middleware flow after next()

5. You want to create middleware that only runs for POST requests to '/submit' and logs 'Processing submission'. Which code correctly implements this conditional middleware?

hard

A. app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') { console.log('Processing submission'); } next(); });

B. app.use('/submit', (req, res, next) => { if (req.method === 'POST') { console.log('Processing submission'); } next(); });

C. app.post('/submit', (req, res, next) => { console.log('Processing submission'); next(); });

D. app.use('/submit', (req, res, next) => { if (req.method = 'POST') { console.log('Processing submission'); next(); } });

Solution

1. Step 1: Check condition for path and method

   app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') { console.log('Processing submission'); } next(); }); correctly checks both req.path and req.method inside middleware.

2. Step 2: Confirm middleware calls next()

   app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') { console.log('Processing submission'); } next(); }); calls next() after logging, ensuring request continues.

3. Step 3: Evaluate other options

   app.use('/submit', (req, res, next) => { if (req.method === 'POST') { console.log('Processing submission'); } next(); }); runs on all methods for '/submit', app.post('/submit', (req, res, next) => { console.log('Processing submission'); next(); }); is a route handler, app.use('/submit', (req, res, next) => { if (req.method = 'POST') { console.log('Processing submission'); next(); } }); uses '=' instead of '===' causing assignment error.

4. Final Answer:

   app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') { console.log('Processing submission'); } next(); }); -> Option A

5. Quick Check:

   Check both path and method, call next() [OK]

Hint: Check both path and method inside middleware, then call next() [OK]

Common Mistakes:

• Using assignment '=' instead of comparison '==='
• Not calling next() after middleware logic
• Confusing route handlers with middleware