Middleware concept and execution flow in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express middleware chain?

Consider this Express.js middleware setup. What will the server respond with when a GET request is made to '/'?

Node.js

const express = require('express');
const app = express();

app.use((req, res, next) => {
  req.message = 'Hello';
  next();
});

app.use((req, res, next) => {
  req.message += ' World';
  next();
});

app.get('/', (req, res) => {
  res.send(req.message);
});

app.listen(3000);

A "Hello World"

B "Hello"

C "World"

D Error: req.message is undefined

💡 Hint

Think about how each middleware modifies the request object before the final handler.

The first middleware sets req.message to 'Hello'. The second appends ' World'. The final handler sends the combined string.

❓ lifecycle

intermediate

Which middleware runs first in this Express app?

Given this Express app, which middleware function executes first when a GET request to '/test' is received?

Node.js

const express = require('express');
const app = express();

app.use((req, res, next) => {
  console.log('Middleware 1');
  next();
});

app.use((req, res, next) => {
  console.log('Middleware 2');
  next();
});

app.get('/test', (req, res, next) => {
  console.log('Handler 1');
  next();
});

app.listen(3000);

A Middleware 2

B Handler 1

C Middleware 1

D No middleware runs before handler

💡 Hint

Remember that app.use middlewares run before route handlers unless path or method restricts them.

app.use middlewares run before route handlers. Middleware 1 logs first, then Middleware 2, then the handler.

📝 Syntax

advanced

What error does this middleware code produce?

Examine this middleware function. What error will occur when a request is made?

Node.js

app.use((req, res, next) => {
  res.send('Hello');
  next();
});

app.use((req, res, next) => {
  res.send('World');
});

A No error, works fine

B SyntaxError: Unexpected token

C TypeError: next is not a function

D Error: Cannot set headers after they are sent to the client

💡 Hint

Think about what happens if you call next() after sending a response.

Calling next() after res.send() causes Express to try to send another response, which is not allowed.

🔧 Debug

advanced

Why does this middleware not modify the request as expected?

This middleware is supposed to add a user property to req, but it doesn't work. Why?

Node.js

app.use('/api', (req, res, next) => {
  req.user = { id: 1 };
  // missing next call
});

app.get('/api/data', (req, res) => {
  res.json(req.user);
});

A Because req.user is overwritten later

B Because next() is not called, the request hangs and never reaches the handler

C Because the path '/api' does not match '/api/data'

D Because res.json cannot send objects

💡 Hint

Middleware must call next() to continue the chain.

Without calling next(), Express does not proceed to the next middleware or route handler, so req.user is never sent.

🧠 Conceptual

expert

What is the correct order of middleware execution in Express?

Arrange these middleware and route handler steps in the order Express executes them for a matching request.

A 2,3,1,4

B 3,2,1,4

C 2,1,3,4

D 3,1,2,4

💡 Hint

Global middleware runs before route-specific middleware and handlers. Error middleware runs last if needed.

Express runs global middleware first, then route-specific middleware, then the route handler. Error middleware runs only if an error is passed.