Application-level middleware in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is application-level middleware in Express?

Application-level middleware is a function that runs during the request-response cycle for all or specific routes in an Express app. It can modify requests, responses, or end the request.

beginner

How do you add application-level middleware in Express?

Use app.use() to add middleware that applies to all routes, or add middleware as additional arguments to specific route handlers like app.get('/path', middleware, handler).

beginner

What are the three arguments of an Express middleware function?

The middleware function receives req (request), res (response), and next (a function to pass control to the next middleware).

intermediate

What happens if you don’t call next() in middleware?

The request will hang because Express won’t move to the next middleware or route handler. You must call next() unless you send a response.

intermediate

Can application-level middleware modify the request or response objects?

Yes, middleware can add properties or change req and res objects to share data or customize behavior for later middleware or routes.

Which method adds application-level middleware to all routes in Express?

A app.listen()

B app.use()

C app.route()

D app.get()

app.use() adds middleware that runs for all routes or specific paths.

What is the purpose of the next argument in middleware?

A To send a response to the client

B To log the request

C To pass control to the next middleware or route handler

D To end the server

next() moves the request to the next middleware or route handler.

If middleware does not call next() or send a response, what happens?

A The request hangs and never finishes

B The server crashes

C The request completes successfully

D The middleware runs twice

Without calling next() or sending a response, the request stays open and hangs.

How can you apply middleware only to a specific route?

A Use <code>app.listen()</code> with middleware

B Use <code>app.use()</code> with the route path

C Middleware cannot be route-specific

D Add middleware as an argument before the route handler, e.g., <code>app.get('/path', middleware, handler)</code>

You can add middleware as an argument before the route handler to apply it only to that route.

Which of these is NOT a typical use of application-level middleware?

A Handling database connections

B Parsing JSON bodies

C Logging requests

D Serving static files

Handling database connections is typically managed outside of middleware, such as at application startup with connection pools.

Explain how application-level middleware works in Express and how you add it to your app.

Think about how middleware fits between request and response.

Describe what happens if you forget to call next() in your middleware function.

Imagine the request is stuck waiting for something.

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