Single-threaded non-blocking I/O concept in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does Node.js handle multiple I/O operations?

Node.js uses a single thread for JavaScript execution. How does it manage to handle multiple I/O operations without blocking the thread?

A It uses multiple threads internally to run JavaScript code in parallel.

B It delegates I/O operations to the system kernel and uses callbacks to handle results asynchronously.

C It queues all I/O operations and runs them one by one synchronously.

D It pauses JavaScript execution until each I/O operation completes.

💡 Hint

Think about how Node.js can keep running code while waiting for files or network responses.

Node.js uses the system kernel to perform I/O operations asynchronously. It registers callbacks that run when the operation finishes, allowing the single JavaScript thread to continue running other code.

❓ component_behavior

intermediate

What is the output order of this Node.js code?

Consider this code snippet: console.log('Start'); setTimeout(() => console.log('Timeout'), 0); console.log('End'); What will be the order of the printed lines?

Node.js

console.log('Start');
setTimeout(() => console.log('Timeout'), 0);
console.log('End');

A Start, Timeout, End

B Timeout, Start, End

C Start, End, Timeout

D End, Start, Timeout

💡 Hint

Remember that setTimeout with 0 delay still runs after the current code finishes.

The synchronous console.log('Start') runs first, then console.log('End'). The setTimeout callback runs last, even with 0 delay, because it is queued in the event loop.

🔧 Debug

advanced

Why does this Node.js code block the event loop?

Look at this code: const fs = require('fs'); const data = fs.readFileSync('file.txt'); console.log('File read'); What is the problem with this code in terms of Node.js single-threaded non-blocking I/O?

Node.js

const fs = require('fs');

const data = fs.readFileSync('file.txt');
console.log('File read');

A readFileSync blocks the event loop until the file is read, stopping other code from running.

B readFileSync runs asynchronously and does not block the event loop.

C The code will throw an error because readFileSync requires a callback.

D The console.log will run before the file is read.

💡 Hint

Check if readFileSync is synchronous or asynchronous.

readFileSync is a synchronous function that blocks the event loop until the file is fully read. This stops other JavaScript code from running during that time.

📝 Syntax

advanced

Which option correctly uses a Promise to read a file asynchronously in Node.js?

You want to read a file asynchronously using Promises. Which code snippet is correct?

A

const fs = require('fs');
const data = fs.readFile('file.txt').then(console.log);

B

const fs = require('fs/promises');
const data = fs.readFileSync('file.txt');
console.log(data);

C

const fs = require('fs');
fs.readFile('file.txt', (err, data) => { if (!err) console.log(data); });

D

const fs = require('fs/promises');
fs.readFile('file.txt').then(data => console.log(data.toString()));

💡 Hint

Look for the correct module and method that returns a Promise.

fs/promises module provides Promise-based versions of fs functions. Using fs.readFile from fs/promises returns a Promise that can be handled with .then().

❓ state_output

expert

What is the output of this Node.js event loop example?

Analyze this code: console.log('1'); setTimeout(() => console.log('2'), 0); Promise.resolve().then(() => console.log('3')); console.log('4');

Node.js

console.log('1');
setTimeout(() => console.log('2'), 0);
Promise.resolve().then(() => console.log('3'));
console.log('4');

A 1, 4, 3, 2

B 1, 3, 4, 2

C 1, 2, 3, 4

D 1, 4, 2, 3

💡 Hint

Remember microtasks (Promises) run before timers in the event loop.

First, synchronous logs '1' and '4' run. Then Promise callbacks (microtasks) run, logging '3'. Finally, setTimeout callbacks (macrotasks) run, logging '2'.