Middleware composition for auth layers in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is middleware in Express?

Middleware is a function that runs during the request-response cycle. It can modify the request or response, or end the cycle by sending a response. Middleware helps organize code into reusable steps.

beginner

Why compose multiple middleware functions for authentication?

Composing middleware lets you separate concerns like checking tokens, verifying user roles, and logging. This makes code easier to read, test, and reuse.

beginner

How does Express know to move from one middleware to the next?

Express moves to the next middleware when the current middleware calls the next() function. If next() is not called, the request stops there.

intermediate

What happens if an authentication middleware detects an invalid token?

It usually sends a response with an error status (like 401 Unauthorized) and does not call next(), stopping the request from reaching protected routes.

intermediate

Give an example of composing two middleware functions for auth layers.

You can create one middleware to check if a token exists and another to verify user roles. Then use them together like app.use(checkToken, verifyRole) to protect routes.

What does the next() function do in Express middleware?

A Ends the request and sends a response

B Moves to the next middleware in the stack

C Restarts the server

D Logs the request details

Calling next() tells Express to continue to the next middleware or route handler.

Which status code is commonly sent when authentication fails?

A 500 Internal Server Error

B 200 OK

C 404 Not Found

D 401 Unauthorized

401 Unauthorized means the user is not authenticated or token is invalid.

Why use multiple middleware functions for auth instead of one big function?

A To make code modular and easier to maintain

B To slow down the server

C To confuse developers

D To avoid using next()

Modular middleware separates concerns and improves code clarity.

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

A The server crashes

B Express automatically calls next()

C The request hangs and never finishes

D The browser reloads

Without next() or a response, the request stays open and the client waits indefinitely.

How do you apply multiple middleware functions to a route in Express?

A app.use(middleware1, middleware2)

B app.get(middleware1 + middleware2)

C app.route(middleware1, middleware2)

D app.listen(middleware1, middleware2)

You pass multiple middleware functions as arguments to app.use or route methods.

Explain how middleware composition helps in building authentication layers in Express.

Think about breaking down auth steps into small functions.

Describe what happens when an Express middleware detects an invalid token during authentication.

Focus on the flow control and response.