Application-level middleware in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an application-level middleware that logs every request method.

Express

app.use(function(req, res, next) {
  console.log(req.[1]);
  next();
});

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

Aurl

Bmethod

Cbody

Dheaders

Attempts:

3 left

💡 Hint

Common Mistakes

Using req.url instead of req.method

Trying to log req.body which may not exist yet

Forgetting to call next() to continue middleware chain

The req.method property contains the HTTP method of the request, such as GET or POST. Logging it helps track request types.

2fill in blank

medium

Complete the code to add a middleware that sets a custom header 'X-App-Version' with value '1.0'.

Express

app.use(function(req, res, next) {
  res.setHeader('[1]', '1.0');
  next();
});

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

AAuthorization

BContent-Type

CX-App-Version

DAccept

Attempts:

3 left

💡 Hint

Common Mistakes

Using standard headers like Content-Type instead of the custom one

Misspelling the header name

Not calling next() to continue middleware

The header X-App-Version is a custom header used to indicate the app version. Setting it on the response helps clients know the version.

3fill in blank

hard

Fix the error in the middleware that tries to parse JSON body but is missing a function call.

Express

app.use(express.[1]());

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

Ajson

Burlencoded

Crouter

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using express.json without parentheses

Using express.urlencoded for JSON data

Using express.static which serves files

The express.json() middleware parses incoming JSON requests. It must be called as a function to return the middleware.

4fill in blank

hard

Fill both blanks to create middleware that logs the request URL and then calls next.

Express

app.use(function(req, res, [1]) {
  console.log(req.[2]);
  next();
});

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

Anext

Bmethod

Curl

Dres

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'res' instead of 'next' as third parameter

Logging req.method instead of req.url

Forgetting to call next()

The third parameter in middleware is next, which you call to continue. The request URL is accessed via req.url.

5fill in blank

hard

Fill all three blanks to create middleware that checks if a user is authenticated and calls next or sends 401.

Express

app.use(function(req, res, next) {
  if (req.[1] && req.[2].isAuthenticated()) {
    [3]();
  } else {
    res.status(401).send('Unauthorized');
  }
});

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

Auser

Bsession

Cnext

Dbody

Attempts:

3 left

💡 Hint

Common Mistakes

Checking req.session instead of req.user

Calling next without parentheses

Not sending 401 status on failure

The req.user object holds user info. Calling req.user.isAuthenticated() checks if logged in. If yes, call next() to continue.

Practice

1. What is the main purpose of application-level middleware in Express?

easy

A. To run code for every request or specific routes before the final handler

B. To directly send HTML files to the client

C. To connect Express with a database

D. To create new HTTP servers

Solution

1. Step 1: Understand middleware role

   Application-level middleware runs before route handlers to process requests.

2. Step 2: Identify middleware usage

   It can modify requests, responses, or decide to stop the request cycle early.

3. Final Answer:

   To run code for every request or specific routes before the final handler -> Option A

4. Quick Check:

   Middleware runs before handlers [OK]

Hint: Middleware runs before routes to process requests [OK]

Common Mistakes:

• Confusing middleware with route handlers
• Thinking middleware creates servers
• Assuming middleware connects databases directly

2. Which of the following is the correct syntax to add application-level middleware in Express?

easy

A. app.addMiddleware(function(req, res) { next(); });

B. app.middleware((req, res) => { console.log('Middleware'); });

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

D. app.routeMiddleware((req, res, next) => { next(); });

Solution

1. Step 1: Recall Express middleware syntax

   Express uses app.use() to add middleware functions.

2. Step 2: Check function parameters

   Middleware functions receive req, res, next and must call next() to continue.

3. Final Answer:

   app.use((req, res, next) => { console.log('Middleware'); next(); }); -> Option C

4. Quick Check:

   app.use with (req, res, next) and next() [OK]

Hint: Middleware uses app.use with three parameters and calls next() [OK]

Common Mistakes:

• Using non-existent methods like app.middleware
• Omitting the next parameter or call
• Confusing middleware with route handlers

3. Consider this Express code snippet:

app.use((req, res, next) => { console.log('First'); next(); });
app.use((req, res, next) => { console.log('Second'); next(); });
app.get('/', (req, res) => { res.send('Hello'); });

What will be printed in the console when a GET request is made to '/'?

medium

A. Second\nFirst

B. First\nSecond

C. Hello

D. No output

Solution

1. Step 1: Understand middleware execution order

   Middleware runs in the order they are added with app.use().

2. Step 2: Trace console logs

   First middleware logs 'First' then calls next(), second logs 'Second' then calls next(), then route sends response.

3. Final Answer:

   First\nSecond -> Option B

4. Quick Check:

   Middleware executes in order added [OK]

Hint: Middleware logs appear in order of app.use calls [OK]

Common Mistakes:

• Assuming route handler logs to console
• Thinking middleware runs after route
• Mixing order of middleware execution

4. What is the problem with this middleware code?

app.use((req, res, next) => { console.log('Middleware running'); });
app.get('/', (req, res) => { res.send('Done'); });

medium

A. Middleware should be added after routes

B. Middleware syntax is incorrect

C. Route handler is missing parameters

D. Missing call to next(), request will hang

Solution

1. Step 1: Check middleware function

   The middleware logs a message but does not call next().

2. Step 2: Understand effect of missing next()

   Without calling next(), the request does not proceed to the route handler, causing the request to hang.

3. Final Answer:

   Missing call to next(), request will hang -> Option D

4. Quick Check:

   Middleware must call next() to continue [OK]

Hint: Always call next() in middleware to avoid hanging requests [OK]

Common Mistakes:

• Forgetting to call next() in middleware
• Thinking middleware runs after routes
• Assuming middleware syntax is wrong without checking next()

5. You want to create middleware that only runs for routes starting with '/admin'. Which is the correct way to apply this middleware?

hard

A. app.use('/admin', (req, res, next) => { console.log('Admin middleware'); next(); });

B. app.use((req, res, next) => { if(req.url.startsWith('admin')) { console.log('Admin middleware'); } next(); });

C. app.get('/admin', (req, res, next) => { console.log('Admin middleware'); next(); });

D. app.use('*admin', (req, res, next) => { console.log('Admin middleware'); next(); });

Solution

1. Step 1: Understand path filtering in middleware

   Express allows specifying a path prefix in app.use(path, middleware) to run middleware only on matching routes.

2. Step 2: Evaluate options

   app.use('/admin', (req, res, next) => { console.log('Admin middleware'); next(); }); correctly uses app.use('/admin', ...) for prefix matching. app.use((req, res, next) => { if(req.url.startsWith('admin')) { console.log('Admin middleware'); } next(); }); has incorrect condition (startsWith('admin') misses leading /, never matches). app.get('/admin', (req, res, next) => { console.log('Admin middleware'); next(); }); is a route handler, not middleware. app.use('*admin', (req, res, next) => { console.log('Admin middleware'); next(); }); *admin matches paths containing admin at the end, not starting with /admin.

3. Final Answer:

   app.use('/admin', (req, res, next) => { console.log('Admin middleware'); next(); }); -> Option A

4. Quick Check:

   Use app.use with path prefix for selective middleware [OK]

Hint: Use app.use('/path', middleware) to target specific routes [OK]

Common Mistakes:

• Using route handlers instead of middleware
• Incorrect conditional checks in global middleware
• Wrong path patterns like wildcards in wrong position