Worker thread vs child process in Node.js - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new worker thread.

Node.js

const { Worker } = require('worker_threads');
const worker = new [1]('./worker.js');

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

AWorker

BChildProcess

CThread

DProcess

Attempts:

3 left

💡 Hint

Common Mistakes

Using ChildProcess instead of Worker for threads

Using lowercase 'worker' which is not a class

Confusing Process with Worker

In Node.js, Worker is used to create a new worker thread.

2fill in blank

medium

Complete the code to spawn a child process using Node.js.

Node.js

const { spawn } = require('child_process');
const child = spawn('[1]', ['-lh', '/usr']);

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

Anode

Bthread

Cworker

Dls

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'node' which runs Node.js, not a shell command

Using 'worker' or 'thread' which are not shell commands

The spawn function runs a command; here, ls lists directory contents.

3fill in blank

hard

Fix the error in the worker thread message listener.

Node.js

worker.on('message', ([1]) => {
  console.log('Received:', data);
});

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

Adata

Bmsg

Cmessage

Devent

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different parameter name than the one used inside the function

Using 'msg' or 'message' but logging 'data'

The parameter name must match the variable used inside the function. Here, data is used, so the parameter should be data.

4fill in blank

hard

Fill both blanks to send a message from the main thread to the worker.

Node.js

worker.[1]({ text: 'Hello' });
worker.[2]('exit', () => console.log('Worker exited'));

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

ApostMessage

Bsend

Con

Demit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'send' which is for child processes, not worker threads

Using 'emit' instead of 'on' to listen for events

Use postMessage to send data to a worker thread, and on to listen for events like 'exit'.

5fill in blank

hard

Fill all three blanks to create a child process and handle its output.

Node.js

const { [1] } = require('child_process');
const child = [2]('node', ['script.js']);
child.stdout.on('[3]', data => console.log(`Output: ${data}`));

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

Aspawn

Cdata

Dexec

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'exec' instead of 'spawn' which behaves differently

Listening for wrong event names like 'output' or 'message'

Import spawn from 'child_process', use it to create the child process, and listen for 'data' events on stdout.

Practice

1. Which statement best describes the difference between worker threads and child processes in Node.js?

easy

A. Worker threads run in the same process sharing memory, while child processes run in separate processes with separate memory.

B. Worker threads run separate programs, child processes share the same memory space.

C. Both worker threads and child processes run in the same process and share memory.

D. Child processes run inside worker threads to improve performance.

Solution

1. Step 1: Understand worker threads behavior

   Worker threads run JavaScript code in parallel but inside the same Node.js process and share memory.

2. Step 2: Understand child processes behavior

   Child processes run completely separate programs with their own memory space and communicate via messages.

3. Final Answer:

   Worker threads run in the same process sharing memory, while child processes run in separate processes with separate memory. -> Option A

4. Quick Check:

   Worker threads share memory, child processes do not [OK]

Hint: Remember: threads share memory, processes do not [OK]

Common Mistakes:

• Confusing memory sharing between threads and processes
• Thinking child processes share memory
• Assuming worker threads run separate programs

2. Which of the following is the correct way to create a worker thread in Node.js?

easy

A. const worker = spawn('worker.js');

B. const worker = fork('worker.js');

C. const worker = createThread('worker.js');

D. const worker = new Worker('worker.js');

Solution

1. Step 1: Recall worker thread creation syntax

   Worker threads are created using the Worker class from the worker_threads module.

2. Step 2: Identify correct constructor usage

   The correct syntax is new Worker('filename'). The fork and spawn methods are for child processes.

3. Final Answer:

   const worker = new Worker('worker.js'); -> Option D

4. Quick Check:

   Worker threads use new Worker() [OK]

Hint: Use new Worker() for threads, fork/spawn for processes [OK]

Common Mistakes:

• Using fork() to create worker threads
• Using spawn() for worker threads
• Using non-existent createThread() function

3. Consider this Node.js code snippet using a child process:

const { fork } = require('child_process');
const child = fork('child.js');
child.on('message', msg => console.log('Parent got:', msg));
child.send('Hello');

What will the parent process output if child.js sends back { reply: 'Hi' } on receiving a message?

medium

A. Parent got: Hello

B. Parent got: { reply: 'Hi' }

C. Parent got: Hi

D. No output, error occurs

Solution

1. Step 1: Understand child process communication

   The parent sends 'Hello' to the child. The child responds with an object { reply: 'Hi' } via process.send().

2. Step 2: Analyze parent's message event handler

   The parent's child.on('message') receives the object and logs it as Parent got: { reply: 'Hi' }.

3. Final Answer:

   Parent got: { reply: 'Hi' } -> Option B

4. Quick Check:

   Child sends object, parent logs object [OK]

Hint: Child sends object, parent logs exact message [OK]

Common Mistakes:

• Assuming string 'Hi' instead of object
• Expecting parent's message to be 'Hello'
• Confusing child and parent message directions

4. What is wrong with this code snippet that tries to create a worker thread?

const { Worker } = require('worker_threads');
const worker = Worker('worker.js');

medium

A. The file 'worker.js' must be a JSON file

B. Wrong module imported, should be 'child_process'

C. Missing new keyword before Worker constructor

D. Worker threads cannot be created with a filename

Solution

1. Step 1: Check Worker thread creation syntax

   The Worker class must be instantiated with the new keyword.

2. Step 2: Identify error in code

   The code calls Worker('worker.js') without new, causing a TypeError.

3. Final Answer:

   Missing new keyword before Worker constructor -> Option C

4. Quick Check:

   Use new Worker() to create threads [OK]

Hint: Always use new with Worker() constructor [OK]

Common Mistakes:

• Forgetting new keyword
• Importing wrong module for threads
• Thinking worker.js must be JSON

5. You want to perform a CPU-heavy task in Node.js without blocking the main event loop. Which approach is best and why?

hard

A. Use a child process to run the task in a separate process communicating via messages.

B. Use a worker thread to run the task sharing memory with the main thread.

C. Run the task directly in the main thread asynchronously.

D. Use setTimeout to delay the task execution in the main thread.

Solution

1. Step 1: Understand CPU-heavy task impact

   CPU-heavy tasks block the main event loop if run directly, causing app unresponsiveness.

2. Step 2: Compare worker threads and child processes for heavy tasks

   Worker threads share memory but still run in the same process, which can cause contention. Child processes run in separate processes, isolating CPU load and preventing blocking.

3. Step 3: Evaluate other options

   Running asynchronously in main thread or delaying with setTimeout does not prevent blocking for CPU-heavy tasks.

4. Final Answer:

   Use a child process to run the task in a separate process communicating via messages. -> Option A

5. Quick Check:

   Heavy CPU tasks best isolated in child processes [OK]

Hint: Heavy CPU tasks? Use child processes to avoid blocking [OK]

Common Mistakes:

• Assuming worker threads fully isolate CPU load
• Thinking async or setTimeout avoids CPU blocking
• Ignoring message communication overhead