Express application structure - Mini Project: Build & Apply

Express Application Structure

📖 Scenario: You are building a simple Express web server to serve a homepage and an about page.This project will guide you to create the basic structure of an Express application step-by-step.

🎯 Goal: Build a basic Express app with a main server file, a router for two routes, and start the server on port 3000.

📋 What You'll Learn

Create an Express app instance

Set up a router with two routes: '/' and '/about'

Use the router in the app

Start the server listening on port 3000

💡 Why This Matters

🌍 Real World

Express is a popular framework to build web servers and APIs in Node.js. This basic structure is the foundation for many web applications.

💼 Career

Understanding Express app structure is essential for backend development roles using Node.js, enabling you to build and maintain web servers.

1

Create the Express app instance

Write code to import express and create an Express app instance called app.

Express

// Import express and create app instance
// Your code here

Hint

Use require('express') to import Express and call it as a function to create app.

2

Create a router with two routes

Create a router called router using express.Router(). Add two routes: router.get('/', ...) that sends 'Home Page' and router.get('/about', ...) that sends 'About Page'.

Express

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

// Create router and add routes
// Your code here

Hint

Use express.Router() to create router. Use router.get to define routes and res.send to send text responses.

3

Use the router in the app

Use app.use('/', router) to connect the router to the app so it handles requests starting with '/'.

Express

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

const router = express.Router();
router.get('/', (req, res) => {
  res.send('Home Page');
});
router.get('/about', (req, res) => {
  res.send('About Page');
});

// Connect router to app
// Your code here

Hint

Use app.use with the path '/' and the router to connect routes.

4

Start the server on port 3000

Add code to start the Express app listening on port 3000 using app.listen(3000).

Express

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

const router = express.Router();
router.get('/', (req, res) => {
  res.send('Home Page');
});
router.get('/about', (req, res) => {
  res.send('About Page');
});

app.use('/', router);

// Start server on port 3000
// Your code here

Hint

Use app.listen(3000) to start the server on port 3000.

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