Built-in modules overview in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of using the 'path' module to join paths

What is the output of this Node.js code snippet using the built-in path module?

Node.js

import path from 'path';
console.log(path.join('folder', 'subfolder', 'file.txt'));

A 'folder\\subfolder\\file.txt'

B 'folder.subfolder.file.txt'

C 'folder-subfolder-file.txt'

D 'folder/subfolder/file.txt'

💡 Hint

Think about how the path.join method works on Windows systems.

The path.join method joins path segments using the platform-specific separator. On Windows, it uses backslashes \\, so the output is folder\\subfolder\\file.txt. On POSIX systems, it uses forward slashes.

❓ component_behavior

intermediate

Behavior of the 'fs' module's readFileSync method

What happens when you run this Node.js code using the fs module's readFileSync method on a non-existent file?

Node.js

import fs from 'fs';
try {
  const data = fs.readFileSync('nofile.txt', 'utf8');
  console.log(data);
} catch (err) {
  console.log('Error caught');
}

A Throws an uncaught exception and stops execution

B Prints 'undefined' if file does not exist

C Prints the file content if exists, otherwise prints 'Error caught'

D Creates an empty file named 'nofile.txt' and prints nothing

💡 Hint

Consider how readFileSync behaves when the file is missing and how the try-catch block works.

The readFileSync method throws an error if the file does not exist. The try-catch block catches this error and prints 'Error caught'.

📝 Syntax

advanced

Correct import syntax for the 'os' module in Node.js ES modules

Which option shows the correct way to import the built-in os module in a Node.js ES module environment?

A import * as os from 'os';

B const os = require('os');

C import os from 'os';

D import { os } from 'node:os';

💡 Hint

Remember that require is not used in ES modules and how to import entire modules.

In ES modules, to import the entire os module, use import * as os from 'os';. Option A uses CommonJS syntax which is not valid in ES modules. Option A tries to destructure a non-exported member.

🔧 Debug

advanced

Identify the error in this 'http' server code snippet

What error will this Node.js code produce when run, and why?

Node.js

import http from 'http';
const server = http.createServer((req, res) => {
  res.write('Hello World');
  res.end();
});
server.listen(3000);
console.log('Server running on port 3000');

// Later in code
server.listen(3000);

A Runs fine and prints 'Server running on port 3000' twice

B Server listens on two ports simultaneously without error

C Throws a SyntaxError due to duplicate listen calls

D Throws an error: 'Error: listen EADDRINUSE: address already in use 3000'

💡 Hint

Think about what happens if you try to listen on the same port twice.

Calling server.listen(3000) twice causes an error because the port is already in use by the first call. This results in an EADDRINUSE error.

🧠 Conceptual

expert

Understanding the behavior of the 'events' module with multiple listeners

Consider this Node.js code using the events module. What will be the output when the event is emitted?

Node.js

import EventEmitter from 'events';
const emitter = new EventEmitter();
emitter.on('ping', () => console.log('First listener'));
emitter.once('ping', () => console.log('Second listener'));
emitter.emit('ping');
emitter.emit('ping');

A Second listener\nFirst listener\nSecond listener

B First listener\nSecond listener\nFirst listener

C First listener\nFirst listener

D Second listener\nSecond listener

💡 Hint

Remember the difference between on and once listeners.

The on listener runs every time the event is emitted. The once listener runs only the first time. So the first emit triggers both listeners, printing 'First listener' and 'Second listener'. The second emit triggers only the on listener, printing 'First listener' again.