Application-level middleware in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express middleware chain?

Consider the following Express app code. What will the client receive when making a GET request to '/'?

Express

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

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

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

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

A "Hello"

B "Hello World"

C "World"

D Error: req.customValue is undefined

💡 Hint

Think about how middleware modifies the request object before the route handler.

The first middleware adds 'Hello' to req.customValue. The second appends ' World'. The route sends the combined string.

❓ lifecycle

intermediate

In what order are these middlewares executed?

Given this Express app setup, what is the order of console logs when a GET request to '/' is made?

Express

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

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

app.get('/', (req, res, next) => {
  console.log('Route handler');
  next();
});

app.use((req, res) => {
  console.log('Last middleware');
  res.send('Done');
});

A First middleware -> Route handler -> Last middleware

B First middleware -> Last middleware -> Route handler

C Route handler -> First middleware -> Last middleware

D Last middleware -> First middleware -> Route handler

💡 Hint

Remember that app.use middlewares run in the order they are added.

Middleware and route handlers run in the order they are registered. The last app.use acts as a catch-all after the route.

🔧 Debug

advanced

Why does this cause a 'headers already sent' error?

Examine this Express app. Why does the server throw an error when making a GET request to '/stop'?

Express

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

app.use((req, res, next) => {
  if (req.path === '/stop') {
    res.send('Stopped here');
    return;
  }
  next();
});

app.get('/stop', (req, res) => {
  res.send('This will never run');
});

A Because next() is called after res.send(), causing headers to be sent twice

B Because res.send() is missing inside the if block

C Because the middleware does not call next() when path is '/stop'

D Because the route handler is missing for '/stop'

💡 Hint

Think about what happens when next() is called after sending a response.

Calling next() after res.send() causes Express to continue processing, leading to errors because headers are already sent.

📝 Syntax

advanced

Which middleware definition is syntactically correct?

Identify the correct syntax for an Express application-level middleware function.

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

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

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

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

💡 Hint

Middleware functions must have three parameters and call next() properly if not ending response.

Option D correctly defines middleware with three parameters and sends a response. Option D misses next parameter usage. Option D calls next without parentheses. Option D misses semicolon causing syntax error.

❓ state_output

expert

What is the final value of req.processed after all middleware run?

Given this Express app, what is the value of req.processed when the client requests '/'?

Express

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

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

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

app.get('/', (req, res) => {
  req.processed *= 3;
  res.send(req.processed.toString());
});

A "3"

B "1"

C "9"

D "6"

💡 Hint

Track the changes to req.processed step by step through middleware and route.

First middleware sets req.processed = 1. Second adds 2 making it 3. Route multiplies by 3 resulting in 9.