Creating a basic Express server - Quick Revision & Summary

Recall & Review

beginner

What is Express in Node.js?

Express is a simple and flexible web framework for Node.js that helps you build web servers and APIs easily.

beginner

Which method creates an Express application?

The method express() creates an Express application instance.

beginner

How do you make an Express server listen on port 3000?

Use app.listen(3000, () => { console.log('Server running on port 3000') }) to start the server on port 3000.

beginner

What does app.get('/', (req, res) => { ... }) do?

It defines a route handler for GET requests to the root URL '/' and sends a response back to the client.

beginner

Why do we use res.send() in Express?

res.send() sends a response back to the client, like text or HTML, to show in the browser.

Which line creates a new Express app?

A const app = express();

B const app = new Express();

C const app = require('express');

D const app = express.createApp();

The correct way to create an Express app is by calling express() as a function.

How do you define a route for GET requests to '/'?

A app.post('/', (req, res) => { ... });

B app.listen('/', () => { ... });

C app.route('/', 'GET', () => { ... });

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

app.get() defines a handler for GET requests to the specified path.

What does app.listen(3000) do?

A Sends a response to port 3000

B Stops the server on port 3000

C Starts the server on port 3000

D Creates a new route on port 3000

app.listen(3000) starts the server and listens for requests on port 3000.

Which object represents the incoming request in a route handler?

A req

B res

C app

D server

The 'req' object holds information about the incoming request.

Which method sends a response back to the client?

A server.send()

B res.send()

C app.send()

D req.send()

res.send() is used to send a response to the client.

Explain how to create a basic Express server that responds with 'Hello World' on the root URL.

Think about the steps from importing Express to starting the server.

Describe the roles of the 'req' and 'res' objects in an Express route handler.

One is for input, the other for output.

Practice

1. What is the main purpose of app.listen() in an Express server?

easy

A. To define a route for GET requests

B. To start the server and listen for incoming requests

C. To import the Express library

D. To send a response to the client

Solution

1. Step 1: Understand the role of app.listen()

   This function starts the Express server and makes it listen on a specified port for incoming requests.

2. Step 2: Compare with other functions

   Functions like app.get() define routes, but do not start the server. Importing Express or sending responses are done elsewhere.

3. Final Answer:

   To start the server and listen for incoming requests -> Option B

4. Quick Check:

   app.listen() starts server [OK]

Hint: Remember: listen means server waits for requests [OK]

Common Mistakes:

• Confusing route definition with server start
• Thinking app.listen() sends responses
• Mixing import statements with server functions

2. Which of the following is the correct way to import Express in a modern Node.js project?

easy

A. const express = require('express');

B. load express from 'express';

C. var express = import('express');

D. import express from 'express';

Solution

1. Step 1: Identify modern import syntax

   In modern Node.js with ES modules, the correct syntax is import express from 'express';.

2. Step 2: Check other options

   require() is CommonJS style, not modern ES modules. var express = import('express'); is invalid syntax. load is not a valid keyword.

3. Final Answer:

   import express from 'express'; -> Option D

4. Quick Check:

   Modern Node.js uses import [OK]

Hint: Use 'import' for modern Node.js, not 'require' [OK]

Common Mistakes:

• Using require() in ES module projects
• Trying to use 'load' keyword
• Incorrect use of dynamic import syntax

3. What will be the output when you visit http://localhost:3000/hello given this Express code?

import express from 'express';
const app = express();
app.get('/hello', (req, res) => {
  res.send('Hello World!');
});
app.listen(3000);

medium

A. Hello World!

B. Cannot GET /hello

C. 404 Not Found error

D. Server error

Solution

1. Step 1: Analyze the route definition

   The code defines a GET route at '/hello' that sends 'Hello World!' as a response.

2. Step 2: Understand server behavior on request

   When visiting /hello, the server matches the route and sends the defined response.

3. Final Answer:

   Hello World! -> Option A

4. Quick Check:

   Defined route sends 'Hello World!' [OK]

Hint: Check if route matches URL path exactly [OK]

Common Mistakes:

• Visiting a route not defined causes 404
• Confusing response text with error messages
• Forgetting to start server with listen()

4. Identify the error in this Express server code:

import express from 'express';
const app = express();
app.get('/', (req, res) => {
  res.send('Welcome!');
});
app.listen();

medium

A. Missing port number in app.listen()

B. Incorrect route path in app.get()

C. res.send() should be res.sendResponse()

D. Express not imported correctly

Solution

1. Step 1: Check app.listen() usage

   The app.listen() method requires a port number to start the server. Here, no port is given.

2. Step 2: Verify other parts

   The route path '/' is valid, res.send() is correct, and Express is imported properly.

3. Final Answer:

   Missing port number in app.listen() -> Option A

4. Quick Check:

   app.listen() needs port [OK]

Hint: Always specify a port in app.listen(port) [OK]

Common Mistakes:

• Omitting port number in app.listen()
• Using wrong method names for response
• Incorrect route paths syntax

5. You want to create an Express server that responds with JSON at /data and listens on port 8080. Which code snippet correctly does this?

hard

A. import express from 'express';
const app = express();
app.get('/data', (req, res) => {
res.send('Hello JSON');
});
app.listen(3000);

B. import express from 'express';
const app = express();
app.post('/data', (req, res) => {
res.send({ message: 'Hello JSON' });
});
app.listen(8080);

C. import express from 'express';
const app = express();
app.get('/data', (req, res) => {
res.json({ message: 'Hello JSON' });
});
app.listen(8080);

D. import express from 'express';
const app = express();
app.get('/data', (req, res) => {
res.json('Hello JSON');
});
app.listen(8080);

Solution

1. Step 1: Check route method and path

   The question asks for a GET route at '/data', so app.get('/data', ...) is correct.

2. Step 2: Verify response and port

   Response should be JSON, so res.json({ message: 'Hello JSON' }) is correct. The server must listen on port 8080.

3. Step 3: Eliminate alternatives with errors

   Alternatives fail due to using POST instead of GET, wrong port like 3000, res.send('string') instead of res.json(object), or res.json('string') instead of object.

4. Final Answer:

   The snippet with app.get('/data'), res.json({ message: 'Hello JSON' }), app.listen(8080) -> Option C

5. Quick Check:

   GET /data + res.json + port 8080 [OK]

Hint: Use app.get + res.json + correct port [OK]

Common Mistakes:

• Using POST instead of GET for route
• Sending string instead of JSON object
• Listening on wrong port number