Conditional middleware execution in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add middleware that runs only for POST requests.

Express

app.use((req, res, next) => {
  if (req.method === '[1]') {
    console.log('POST request detected');
  }
  next();
});

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

AGET

BDELETE

CPUT

DPOST

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'GET' instead of 'POST' will run middleware on GET requests.

Forgetting to call next() will block the request.

The middleware checks if the request method is 'POST' to run the code only for POST requests.

2fill in blank

medium

Complete the code to apply middleware only on routes starting with '/admin'.

Express

app.use('[1]', (req, res, next) => {
  console.log('Admin route accessed');
  next();
});

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

A/user

B/api

C/admin

D/public

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/user' will not affect admin routes.

Leaving the path empty applies middleware to all routes.

Middleware with path '/admin' runs only on routes that start with '/admin'.

3fill in blank

hard

Fix the error in the middleware to run only if query parameter 'debug' equals 'true'.

Express

app.use((req, res, next) => {
  if (req.query.debug [1] 'true') {
    console.log('Debug mode active');
  }
  next();
});

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

A===

B==

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' causes assignment error.

Using '==' may cause type coercion issues.

Use '===' for strict equality check in JavaScript to compare query parameter to string 'true'.

4fill in blank

hard

Fill both blanks to create middleware that runs only for PUT requests on '/update' path.

Express

app.[1]('/update', (req, res, next) => {
  if (req.method === '[2]') {
    console.log('Update request received');
  }
  next();
});

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

Aput

Bpost

CGET

DPUT

Attempts:

3 left

💡 Hint

Common Mistakes

Using app.post() will not handle PUT requests.

Checking req.method === 'GET' will never be true for PUT requests.

Use app.put() to handle PUT requests on '/update' path and check method 'PUT' inside middleware.

5fill in blank

hard

Fill all three blanks to create middleware that runs only if the user is authenticated and the request method is DELETE on '/remove' path.

Express

app.[1]('/remove', (req, res, next) => {
  if (req.user && req.method === '[2]') {
    console.log('Authenticated DELETE request');
    [3]();
  } else {
    res.status(403).send('Forbidden');
  }
});

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

Adelete

BDELETE

Cnext

Dpost

Attempts:

3 left

💡 Hint

Common Mistakes

Using app.post() instead of app.delete().

Not calling next() causes request to hang.

Checking req.method === 'delete' (lowercase) fails.

Use app.delete() for DELETE requests on '/remove'. Check req.method === 'DELETE'. Call next() to continue middleware chain.

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