Building custom middleware in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is middleware in Node.js 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.

beginner

How do you define a custom middleware function in Express?

A custom middleware is a function with three parameters: req, res, and next. It processes the request and calls next() to pass control to the next middleware.

beginner

What does the next() function do in middleware?

next() passes control to the next middleware or route handler. Without calling next(), the request will hang and not proceed.

intermediate

How can middleware modify the request object?

Middleware can add new properties or change existing ones on req. For example, adding req.user after authentication.

intermediate

Where should you place custom middleware in your Express app?

Place middleware before the routes that need it. This ensures the middleware runs first and can affect those routes.

What are the three parameters of an Express middleware function?

A request, response, finish

B request, response, callback

C req, res, done

D req, res, next

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

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

A The server will crash immediately.

B The request will hang and never reach the next middleware or route.

C The response will automatically be sent.

D The middleware will skip to the last route.

Without calling next(), Express does not know to continue, so the request hangs.

Which of these is a common use case for custom middleware?

A Logging requests

B Rendering HTML templates

C Connecting to a database

D Serving static files

Logging requests is a typical middleware task to track incoming traffic.

Where should you place middleware in your Express app?

A Before the routes that need it

B After all routes

C Only inside route handlers

D Anywhere, order does not matter

Middleware must be placed before routes to affect those routes.

How can middleware add data to be used later in the request cycle?

A By modifying the <code>res</code> headers only

B By sending a response immediately

C By adding properties to the <code>req</code> object

D By calling <code>next()</code> twice

Middleware can add properties to req so later middleware or routes can use that data.

Explain how to create and use a custom middleware function in Express.

Think about the function signature and where it fits in the request flow.

Describe what happens if you do not call next() inside your middleware.

Consider the flow of control in Express middleware.