Middleware execution flow (req, res, next) in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is middleware in Express?

Middleware is a function that has access to the request (req), response (res), and next middleware function in the app's request-response cycle. It can modify req and res or end the request-response cycle.

beginner

What does the 'next' function do in Express middleware?

The 'next' function passes control to the next middleware function in the stack. Without calling next(), the request will hang and not proceed.

beginner

In what order are middleware functions executed?

Middleware functions are executed in the order they are added to the Express app or router. Each calls next() to pass control to the next middleware.

intermediate

What happens if a middleware does not call next() or end the response?

The request will hang because Express waits for the middleware to either call next() or send a response. This causes the client to wait indefinitely.

intermediate

How can middleware modify the request and response objects?

Middleware can add, change, or remove properties on req and res objects. For example, adding user info to req or setting headers on res before sending.

What are the three parameters of an Express middleware function?

A req, res, next

B request, response, callback

C req, res, done

D request, response, nextMiddleware

Express middleware functions always receive req, res, and next as parameters.

What happens if you forget to call next() in a middleware?

A The next middleware runs automatically

B The request hangs and never completes

C Express throws an error immediately

D The response is sent automatically

Without calling next() or ending the response, the request will hang waiting.

Which middleware runs first in Express?

A The last middleware added

B Middleware added in random order

C Middleware added first

D Middleware with the shortest name

Middleware executes in the order it is added to the app or router.

How can middleware modify the response before sending it?

A By calling next() twice

B By changing req properties only

C Middleware cannot modify response

D By changing res properties or headers

Middleware can set headers or data on res before sending the response.

What is the purpose of calling next() in middleware?

A To pass control to the next middleware

B To end the request-response cycle

C To restart the middleware stack

D To send an error response

Calling next() passes control to the next middleware function.

Explain how middleware functions work together in Express using req, res, and next.

Think about how a relay race passes the baton from one runner to the next.

Describe what happens if a middleware neither calls next() nor sends a response.

Imagine stopping in the middle of a line without telling anyone.