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.