Express application structure - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express route handler?

Consider this Express route handler code. What will the client receive when accessing /hello?

Express

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

app.get('/hello', (req, res) => {
  res.send('Hello, world!');
});

app.listen(3000);

A The client receives a JSON object { message: 'Hello, world!' }

B The client receives the text 'Hello, world!'

C The client receives an empty response

D The server crashes with an error

💡 Hint

Look at what the res.send method sends back.

The res.send method sends the string directly as the response body. So the client will see 'Hello, world!'.

📝 Syntax

intermediate

Which option correctly sets up a middleware in Express?

You want to add a middleware that logs every request method and URL. Which code snippet correctly does this?

A app.use((req, res) => { console.log(req.method, req.url); });

B app.addMiddleware((req, res, next) => { console.log(req.method, req.url); next(); });

C app.middleware((req, res, next) => { console.log(req.method, req.url); next(); });

D app.use((req, res, next) => { console.log(req.method, req.url); next(); });

💡 Hint

Express uses a specific method to add middleware functions.

The app.use method adds middleware. The middleware must call next() to continue the request cycle.

🔧 Debug

advanced

Why does this Express app crash on startup?

Examine the code below. Why does the server crash when you run it?

Express

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

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

app.listen(3000);
console.log('Server started');

A TypeError because res.send is called incorrectly

B No error; server starts normally

C SyntaxError due to missing closing parenthesis in app.listen call

D ReferenceError because express is not defined

💡 Hint

Check the parentheses in the app.listen line.

The app.listen call is missing a closing parenthesis, causing a syntax error and preventing the server from starting.

❓ state_output

advanced

What is the value of count after these requests?

Given this Express app, what is the value of count after three GET requests to /count?

Express

const express = require('express');
const app = express();
let count = 0;

app.get('/count', (req, res) => {
  count += 1;
  res.send(`Count is ${count}`);
});

app.listen(3000);

A 3

B 1

C 0

D Undefined

💡 Hint

Each request increases count by 1.

The variable count starts at 0 and increments by 1 on each request. After 3 requests, it becomes 3.

🧠 Conceptual

expert

Which statement best describes Express app structure?

Which of the following best describes the typical structure of an Express application?

A An Express app uses modular routing and middleware, separating concerns into different files.

B Express apps require a database connection in the main file to run.

C Express apps do not support middleware functions.

D An Express app is a single file with all routes and middleware defined inline.

💡 Hint

Think about how to keep code organized and maintainable.

Express apps are usually organized by separating routes and middleware into different files to keep code clean and manageable.

Practice

1. What is the main purpose of the app.js file in an Express application?

easy

A. To set up the Express server, middleware, and routes

B. To store database connection details only

C. To write frontend HTML code

D. To handle user authentication exclusively

Solution

1. Step 1: Understand the role of app.js

   The app.js file is the main entry point where the Express server is created and configured.

2. Step 2: Identify what app.js typically contains

   It usually sets up middleware, routes, and starts the server listening on a port.

3. Final Answer:

   To set up the Express server, middleware, and routes -> Option A

4. Quick Check:

   Main server setup = C [OK]

Hint: Remember: app.js is the main server setup file [OK]

Common Mistakes:

• Confusing app.js with frontend files
• Thinking app.js only handles database
• Assuming app.js is for authentication only

2. Which of the following is the correct way to import the Express module and create an app instance in app.js?

easy

A. import app from 'express'; const express = app();

B. import express from 'express'; const app = express();

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

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

Solution

1. Step 1: Recall common Express import syntax

   Express is commonly imported using CommonJS syntax: const express = require('express');

2. Step 2: Create app instance correctly

   The app instance is created by calling express(), so const app = express(); is correct.

3. Final Answer:

   const express = require('express'); const app = express(); -> Option D

4. Quick Check:

   CommonJS import + app creation = A [OK]

Hint: Use require() and call express() to create app [OK]

Common Mistakes:

• Mixing ES module import with CommonJS require
• Calling require('express') directly without variable
• Incorrect import syntax like importing app instead of express

3. Given this Express app structure snippet in app.js:

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

app.use('/api', require('./routes/api'));

app.listen(3000);

What happens when a user visits http://localhost:3000/api/users?

medium

A. The server returns a 404 error because no route is defined

B. The request is handled by the router defined in ./routes/api

C. The server crashes due to missing route handler

D. The request is handled by the root route /

Solution

1. Step 1: Understand app.use('/api', ...) behavior

   This mounts the router from ./routes/api at the path /api.

2. Step 2: Analyze the request URL /api/users

   The request matches the /api prefix, so it is forwarded to the router in ./routes/api which should handle /users.

3. Final Answer:

   The request is handled by the router defined in ./routes/api -> Option B

4. Quick Check:

   Mounted router handles prefixed routes = B [OK]

Hint: Routes under '/api' go to './routes/api' router [OK]

Common Mistakes:

• Assuming 404 because route not in app.js
• Thinking server crashes without explicit route in app.js
• Confusing root route with mounted router path

4. Consider this Express app snippet:

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

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

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

app.listen(3000);

Why does visiting http://localhost:3000/home return a blank page?

medium

A. Because the route handler is not properly defined or not reached

B. Because the server is not listening on port 3000

C. Because the middleware to parse requests is missing

D. Because res.send is missing a return statement

Solution

1. Step 1: Check if route handler is correct

   The route /home is defined with app.get and sends a response.

2. Step 2: Consider possible reasons for blank response

   If the page is blank, it usually means the route handler is not reached or overridden by other middleware or routes.

3. Final Answer:

   Because the route handler is not properly defined or not reached -> Option A

4. Quick Check:

   Blank page means route not reached = D [OK]

Hint: Blank page usually means route handler not reached [OK]

Common Mistakes:

• Thinking res.send needs return statement
• Assuming server not listening without error
• Confusing missing middleware with route handling

5. You want to organize your Express app by separating routes into different files. Which structure correctly shows how to use a separate users.js route file inside a routes folder and mount it in app.js?

hard

A. // In routes/users.js
const router = require('express.Router')();
router.get('/', (req, res) => res.send('Users list'));
module.exports = router;

// In app.js
app.use('/users', require('./routes/users'));

B. // In routes/users.js
module.exports = (req, res) => res.send('Users list');

// In app.js
app.use('/users', require('./routes/users'));

C. // In routes/users.js
const express = require('express');
const router = express.Router();
router.get('/', (req, res) => res.send('Users list'));
module.exports = router;

// In app.js
const usersRouter = require('./routes/users');
app.use('/users', usersRouter);

D. // In routes/users.js
const express = require('express');
const router = express.Router;
router.get('/', (req, res) => res.send('Users list'));
module.exports = router;

// In app.js
const usersRouter = require('./routes/users');
app.use('/users', usersRouter);

Solution

1. Step 1: Check correct router creation in users.js

   The router should be created with express.Router(), not express() or other calls.

2. Step 2: Verify correct export and mounting in app.js

   The router is exported with module.exports and mounted with app.use('/users', usersRouter);.

3. Final Answer:

   Using express.Router() in the route file and proper mounting -> Option C

4. Quick Check:

   Use express.Router() and app.use() = A [OK]

Hint: Use express.Router() in route files and mount with app.use() [OK]

Common Mistakes:

• Using express() instead of express.Router() for router
• Exporting a function instead of a router object
• Not mounting the router with app.use() properly