Middleware concept and execution flow in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is middleware in Node.js?

Middleware is a function that runs during the request-response cycle. It can modify the request or response objects, end the request, or pass control to the next middleware.

beginner

How does middleware execution flow work in Express.js?

Middleware functions run in the order they are added. Each middleware calls next() to pass control to the next one. If next() is not called and the response is not ended, the request stops there.

intermediate

What happens if a middleware does not call next()?

The request-response cycle stops at that middleware. The server will not continue to the next middleware or route handler, possibly causing the request to hang.

beginner

What are common uses of middleware in Node.js?

Middleware is used for logging, authentication, parsing request bodies, error handling, and serving static files.

intermediate

Explain the difference between application-level and error-handling middleware.

Application-level middleware handles normal requests and calls next() to continue. Error-handling middleware has four parameters (err, req, res, next) and handles errors passed down the chain.

What does the next() function do in middleware?

A Ends the request-response cycle

B Throws an error

C Sends a response to the client

D Passes control to the next middleware

Calling next() passes control to the next middleware or route handler in the stack.

Which of these is NOT a typical use of middleware?

A Rendering HTML templates

B Parsing JSON bodies

C Handling authentication

D Logging requests

Rendering HTML is usually done in route handlers or view engines, not middleware.

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

A The request continues normally

B The request hangs and never completes

C The server crashes

D An error is automatically thrown

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

How many parameters does an error-handling middleware function have?

A 2

B 3

C 4

D 1

Error-handling middleware has four parameters: (err, req, res, next).

In Express.js, middleware functions are executed in:

A The order they are added

B Random order

C Alphabetical order

D Reverse order

Middleware runs in the order they are added to the app or router.

Describe what middleware is and how it controls the flow of requests in a Node.js app.

Think about how functions can be chained to handle requests step-by-step.

Explain the difference between normal middleware and error-handling middleware in Express.js.

Focus on the special signature and role of error middleware.