Building custom middleware in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this custom middleware logging request method and URL?

Consider this Express.js middleware that logs the HTTP method and URL of incoming requests. What will be printed to the console when a GET request is made to '/home'?

Node.js

function logRequest(req, res, next) {
  console.log(`${req.method} ${req.url}`);
  next();
}

// Simulated request object
const req = { method: 'GET', url: '/home' };
const res = {};
logRequest(req, res, () => console.log('Next middleware called'));

A /home GET\nNext middleware called

B Next middleware called\nGET /home

C GET /home\nNext middleware called

D GET /home

💡 Hint

Look at the order of console.log statements and the use of next()

The middleware logs the method and URL first, then calls next(), which triggers the next console.log. So the output is the method and URL line, then 'Next middleware called'.

❓ state_output

intermediate

What is the value of res.locals.user after this middleware runs?

Given this middleware that sets a user object on res.locals, what will be the value of res.locals.user after it executes?

Node.js

function setUser(req, res, next) {
  res.locals.user = { id: 42, name: 'Alice' };
  next();
}

const res = { locals: {} };
setUser({}, res, () => {});
console.log(res.locals.user);

A {}

B { id: 42, name: 'Alice' }

C undefined

D null

💡 Hint

res.locals is used to pass data between middleware and routes

The middleware assigns an object with id and name to res.locals.user, so after running, res.locals.user holds that object.

📝 Syntax

advanced

Which option correctly defines error-handling middleware in Express?

Express error-handling middleware must have a specific function signature. Which of these options correctly defines such middleware?

A function errorHandler(err, req, res, next) { res.status(500).send('Error'); }

B function errorHandler(req, res, next) { res.status(500).send('Error'); }

C function errorHandler(err, req, res) { res.status(500).send('Error'); }

D function errorHandler(req, res) { res.status(500).send('Error'); }

💡 Hint

Error middleware must have four parameters: err, req, res, next

Express recognizes error middleware by the presence of four parameters. Only option A has all four parameters.

🔧 Debug

advanced

Why does this custom middleware cause the server to hang?

This middleware logs the request but the server never responds or moves on. What is the cause?

Node.js

function logAndForget(req, res, next) {
  console.log('Request received');
  // missing next() call
}

// Usage in Express app
// app.use(logAndForget);

A Middleware does not call next(), so request never proceeds

B Middleware throws an error but does not handle it

C Middleware calls next() twice causing a crash

D Middleware modifies req object incorrectly

💡 Hint

Middleware must call next() or end the response

Without calling next() or sending a response, Express waits forever, causing the server to hang.

🧠 Conceptual

expert

Which statement best describes the order of middleware execution in Express?

Given multiple middleware functions registered in an Express app, how does Express decide the order in which to run them?

A Middleware runs only if the previous middleware sends a response

B Middleware runs in reverse order of registration, always running all middleware regardless of next()

C Middleware runs in parallel asynchronously, order does not matter

D Middleware runs in the order they are added to the app, stopping if a middleware does not call next()

💡 Hint

Think about how Express processes middleware functions sequentially

Express executes middleware in the order they are registered. Each middleware must call next() to continue the chain; otherwise, the chain stops.