Express application structure - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Express module.

Express

const express = require([1]);

Drag options to blanks, or click blank then click option'

A"http"

B"express"

C"fs"

D"path"

Attempts:

3 left

💡 Hint

Common Mistakes

Using other module names like 'http' or 'fs' instead of 'express'.

Forgetting the quotes around the module name.

The Express module is imported using require("express"). This allows you to create an Express app.

2fill in blank

medium

Complete the code to create an Express application instance.

Express

const app = [1]();

Drag options to blanks, or click blank then click option'

Ahttp

Bserver

Crouter

Dexpress

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call 'http()' or 'server()' instead of 'express()'.

Using 'router()' which is for routing, not app creation.

You create an Express app by calling the express() function.

3fill in blank

hard

Fix the error in the code to start the server on port 3000.

Express

app.listen([1], () => {
  console.log('Server running on port 3000');
});

Drag options to blanks, or click blank then click option'

A3000

B"3000"

Capp

Dexpress

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the port as a string like '3000' instead of number 3000.

Passing the app or express variable instead of a port number.

The port number should be a number, not a string. So use 3000 without quotes.

4fill in blank

hard

Fill both blanks to define a GET route for the root URL that sends 'Hello World'.

Express

app.[1]('[2]', (req, res) => {
  res.send('Hello World');
});

Drag options to blanks, or click blank then click option'

Aget

Bpost

C/

D/home

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'post' instead of 'get' for a GET route.

Using '/home' instead of '/' for the root URL.

The get method defines a GET route. The root URL is '/'.

5fill in blank

hard

Fill all three blanks to import Express, create an app, and start the server on port 4000.

Express

const [1] = require('[2]');
const app = [3]();
app.listen(4000, () => {
  console.log('Server started on port 4000');
});

Drag options to blanks, or click blank then click option'

Aexpress

Chttp

Dexpressjs

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'http' or 'expressjs' instead of 'express' as module name.

Using different variable names for import and app creation.

You import Express with require('express'), assign it to express, then call express() to create the app.

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