Middleware execution flow (req, res, next) in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express middleware sequence?

Consider this Express app code with three middleware functions. What will be sent to the client when a GET request is made to '/'?

Express

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);
});

A Error: req.message is undefined

B "Hello"

C "World"

D "Hello World"

💡 Hint

Think about how middleware modifies the request object and calls next() to pass control.

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

❓ lifecycle

intermediate

Which middleware runs last before the response is sent?

Given this Express middleware setup, which middleware function executes immediately before the response is sent to the client?

Express

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('/', (req, res) => {
  console.log('Route handler');
  res.send('Done');
});

A Route handler

B Middleware 2

C No middleware runs before response

D Middleware 1

💡 Hint

Remember the order middleware and route handlers run in Express.

Middleware 1 and 2 run first, then the route handler sends the response. The route handler is last before sending.

🔧 Debug

advanced

Why does this middleware never call the next middleware?

Examine this middleware. Why does the next middleware not run after this one?

Express

app.use((req, res, next) => {
  if (!req.user) {
    res.status(401).send('Unauthorized');
  } else {
    next();
  }
});

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

A Because res.send() ends the response and next() is not called when !req.user

B Because next() is called twice causing an error

C Because the middleware is not registered properly

D Because req.user is always truthy

💡 Hint

Think about what happens when res.send() is called without next().

When res.send() is called, the response ends. The middleware does not call next() in that case, so the next middleware is skipped.

📝 Syntax

advanced

What error does this middleware code produce?

Identify the error caused by this middleware function code.

Express

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

A No error, middleware runs fine

B ReferenceError: next is not defined

C SyntaxError: Unexpected identifier

D TypeError: next is not a function

💡 Hint

Look carefully at how next is used.

The code calls next without parentheses, so next is treated as an expression without invocation, causing a SyntaxError.

❓ state_output

expert

What is the final value of req.count after this middleware chain?

Given this middleware chain, what is the value of req.count when the response is sent?

Express

app.use((req, res, next) => {
  req.count = 1;
  next();
});

app.use((req, res, next) => {
  req.count += 2;
  next();
});

app.use((req, res, next) => {
  req.count *= 3;
  next();
});

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

A "7"

B "9"

C "3"

D "6"

💡 Hint

Calculate step-by-step how req.count changes in each middleware.

Initial req.count is 1. Then add 2 → 3. Then multiply by 3 → 9. The response sends '9'.