Single-threaded non-blocking I/O concept in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read a file asynchronously using Node.js.

Node.js

const fs = require('fs');
fs.readFile('example.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});
console.log('Reading file...');
// This is non-blocking because [1]

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

AreadFile blocks the thread

Bconsole.log waits for readFile

CreadFile is synchronous

DreadFile runs asynchronously

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking readFile is synchronous and blocks execution

Assuming console.log waits for readFile to finish

The readFile function runs asynchronously, so the program does not wait for it to finish before moving on. This is why the console logs 'Reading file...' first.

2fill in blank

medium

Complete the code to create a simple HTTP server that handles requests without blocking.

Node.js

const http = require('http');
const server = http.createServer((req, res) => {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('Hello World');
});
server.listen(3000);
console.log('Server running at http://localhost:3000/');
// This server is non-blocking because it uses [1]

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

Awhile(true) loops

Bsynchronous file reads

Casynchronous callbacks

Dblocking loops

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking the server blocks while waiting for requests

Confusing synchronous and asynchronous code

The server uses asynchronous callbacks to handle requests, so it does not block the main thread while waiting for requests.

3fill in blank

hard

Fix the error in the code to correctly use non-blocking I/O for reading a file.

Node.js

const fs = require('fs');
const data = fs.readFileSync('data.txt', 'utf8');
console.log(data);
console.log('File read complete');
// To make this non-blocking, replace readFileSync with [1]

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

AreadFile

BreadFileSync

CwriteFileSync

DwriteFile

Attempts:

3 left

💡 Hint

Common Mistakes

Using readFileSync which blocks execution

Confusing readFile and writeFile functions

readFileSync blocks the thread until the file is read. Using readFile makes the operation asynchronous and non-blocking.

4fill in blank

hard

Fill both blanks to create a non-blocking timer that logs a message after 2 seconds.

Node.js

console.log('Start');
setTimeout(() => {
  console.log([1]);
}, [2]);
console.log('End');

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

A'Timer done'

B2000

C1000

D'Done'

Attempts:

3 left

💡 Hint

Common Mistakes

Using synchronous delay instead of setTimeout

Setting the wrong delay time

The setTimeout function schedules the callback to run after 2000 milliseconds (2 seconds) without blocking the main thread.

5fill in blank

hard

Fill all three blanks to create a Promise that resolves after 1 second and logs the result.

Node.js

const wait = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve([1]);
  }, [2]);
});
wait.then((message) => {
  console.log([3]);
});

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

A'Success'

B1000

Cmessage

D'Done'

Attempts:

3 left

💡 Hint

Common Mistakes

Not resolving the Promise with a value

Logging a string instead of the resolved message variable

The Promise resolves with the message 'Success' after 1000 milliseconds. The then method receives the message and logs it.

Practice

1. What does it mean that Node.js uses a single-threaded non-blocking I/O model?

easy

A. Node.js blocks the main thread until each task completes.

B. Node.js runs one main thread but can handle many tasks without waiting for each to finish.

C. Node.js uses multiple threads to run tasks in parallel.

D. Node.js cannot handle multiple tasks at the same time.

Solution

1. Step 1: Understand single-threaded meaning

   Node.js runs on one main thread, unlike some systems that use many threads.

2. Step 2: Understand non-blocking I/O meaning

   It does not wait for tasks like file reads to finish before moving on; it uses callbacks or events.

3. Final Answer:

   Node.js runs one main thread but can handle many tasks without waiting for each to finish. -> Option B

4. Quick Check:

   Single-threaded + non-blocking = handle many tasks without waiting [OK]

Hint: Single thread means one main path; non-blocking means no waiting [OK]

Common Mistakes:

• Thinking Node.js uses multiple threads for tasks
• Assuming Node.js waits for each task to finish before continuing
• Confusing blocking with non-blocking I/O

2. Which of the following is the correct way to write a non-blocking file read in Node.js?

easy

A. fs.readFile('file.txt', (err, data) => { if (err) throw err; console.log(data); });

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

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

D. fs.readFile('file.txt'); console.log('done');

Solution

1. Step 1: Identify non-blocking syntax

   Non-blocking file read uses fs.readFile with a callback to handle data after reading.

2. Step 2: Check options for callback usage

   fs.readFile('file.txt', (err, data) => { if (err) throw err; console.log(data); }); uses fs.readFile with a callback function correctly handling error and data.

3. Final Answer:

   fs.readFile('file.txt', (err, data) => { if (err) throw err; console.log(data); }); -> Option A

4. Quick Check:

   Non-blocking file read uses callback = fs.readFile('file.txt', (err, data) => { if (err) throw err; console.log(data); }); [OK]

Hint: Non-blocking uses callbacks, blocking uses sync functions [OK]

Common Mistakes:

• Using synchronous readFileSync for non-blocking tasks
• Calling readFile without a callback
• Expecting immediate data return from async calls

3. What will the following Node.js code output?

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

medium

A. Start\nEnd\nTimeout done

B. End\nStart\nTimeout done

C. Timeout done\nStart\nEnd

D. Start\nTimeout done\nEnd

Solution

1. Step 1: Understand synchronous logs

   console.log('Start') runs immediately and prints 'Start'.

2. Step 2: Understand setTimeout with 0 delay

   setTimeout callback runs after current code finishes, so 'Timeout done' prints last.

3. Step 3: Understand console.log('End')

   This runs immediately after 'Start', printing 'End' before the timeout callback.

4. Final Answer:

   Start End Timeout done -> Option A

5. Quick Check:

   Sync logs first, then async callback = Start\nEnd\nTimeout done [OK]

Hint: setTimeout with 0ms runs after current code finishes [OK]

Common Mistakes:

• Assuming setTimeout runs immediately
• Mixing order of synchronous and asynchronous logs
• Thinking 0ms delay means instant execution

4. Identify the error in this Node.js code snippet using non-blocking I/O:

const fs = require('fs');
let content;
fs.readFile('data.txt', (err, data) => {
  if (err) throw err;
  content = data.toString();
});
console.log(content);

medium

A. The callback function is missing the error parameter.

B. The readFile method should be readFileSync for async code.

C. The variable content is logged before the file read completes.

D. The data.toString() call is invalid.

Solution

1. Step 1: Understand async callback timing

   readFile runs asynchronously, so the callback runs after console.log(content).

2. Step 2: Check when content is logged

   console.log(content) runs immediately, before content is assigned inside the callback.

3. Final Answer:

   The variable content is logged before the file read completes. -> Option C

4. Quick Check:

   Async callback runs later, so content is undefined at log [OK]

Hint: Async callbacks run later; log inside callback to see data [OK]

Common Mistakes:

• Logging async data before callback runs
• Confusing sync and async readFile methods
• Ignoring error parameter in callback

5. You want to read two files in Node.js and then combine their contents. Which approach correctly uses non-blocking I/O to do this?

hard

A. Use fs.readFile once and then read the second file inside the first callback synchronously.

B. Use fs.readFileSync twice and then combine results synchronously.

C. Call fs.readFile twice with callbacks, combine results immediately after calling both without waiting.

D. Call fs.readFile twice with callbacks, combine results inside the second callback only after both finish.

Solution

1. Step 1: Understand non-blocking file reads

   Each fs.readFile runs asynchronously and needs a callback to get data.

2. Step 2: Combine results after both reads finish

   To combine contents, wait for both callbacks to complete, then combine inside the second callback or use coordination logic.

3. Final Answer:

   Call fs.readFile twice with callbacks, combine results inside the second callback only after both finish. -> Option D

4. Quick Check:

   Combine after both async reads finish = Call fs.readFile twice with callbacks, combine results inside the second callback only after both finish. [OK]

Hint: Combine data only after both async callbacks complete [OK]

Common Mistakes:

• Combining data before async reads finish
• Using synchronous reads in async code
• Nesting sync reads inside async callbacks incorrectly