Why child processes are needed in Node.js - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

Node.js

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

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

Apath

Bfs

Chttp

Dchild_process

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'fs' or 'http' instead of 'child_process' to spawn processes.

Node.js uses the child_process module to create child processes.

2fill in blank

medium

Complete the code to listen for data from the child process's stdout.

Node.js

child.stdout.on('[1]', (data) => {
  console.log(`Output: ${data}`);
});

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

Adata

Bclose

Cexit

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'exit' or 'close' event instead of 'data' to get output.

The data event is emitted when the child process sends output.

3fill in blank

hard

Fix the error in the code to correctly handle child process exit.

Node.js

child.on('[1]', (code) => {
  console.log(`Child exited with code ${code}`);
});

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

Aclose

Bexit

Cdata

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'close' event instead of 'exit' to detect process end.

The exit event is emitted when the child process ends.

4fill in blank

hard

Fill both blanks to create a child process that runs a script and handle errors.

Node.js

const child = require('child_process').[1]('node', ['script.js']);
child.on('[2]', (err) => {
  if (err) console.error('Error:', err);
});

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

Aspawn

Bexit

Cerror

Dfork

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'exit' event to catch errors instead of 'error'.

Using 'fork' when 'spawn' is intended.

spawn starts a new process, and error event handles errors.

5fill in blank

hard

Fill all three blanks to create a child process, listen for output, and handle exit.

Node.js

const { [1] } = require('child_process');
const child = [2]('ls', ['-la']);
child.on('[3]', (code) => {
  console.log(`Process ended with code ${code}`);
});

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

Aspawn

Bchild_process

Cexit

Dfork

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'fork' and 'spawn'.

Listening for 'close' instead of 'exit' event.

Destructure spawn from the child_process module, use spawn to create the process, and listen for exit event.

Practice

1. Why do Node.js applications use child processes?

easy

A. To make the app load faster on the internet

B. To reduce the size of the app files

C. To run heavy tasks without freezing the main app

D. To automatically update Node.js version

Solution

1. Step 1: Understand Node.js single-threaded nature

   Node.js runs JavaScript in a single thread, so heavy tasks can block the app.

2. Step 2: Role of child processes

   Child processes run tasks separately, so the main app stays responsive.

3. Final Answer:

   To run heavy tasks without freezing the main app -> Option C

4. Quick Check:

   Child processes prevent freezing = B [OK]

Hint: Child processes keep main app responsive during heavy work [OK]

Common Mistakes:

• Thinking child processes speed up internet loading
• Confusing file size with process management
• Believing child processes update Node.js automatically

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

easy

A. const child = require('child_process').fork('script.js');

B. const child = require('child_process').start('script.js');

C. const child = require('child_process').run('script.js');

D. const child = require('child_process').execute('script.js');

Solution

1. Step 1: Recall Node.js child process methods

   The 'child_process' module has methods like fork(), spawn(), exec(), but not start() or run().

2. Step 2: Identify correct method for creating a child process running a script

   fork() is used to create a new Node.js process running a script file.

3. Final Answer:

   const child = require('child_process').fork('script.js'); -> Option A

4. Quick Check:

   fork() creates child process = A [OK]

Hint: Use fork() to create child Node.js processes [OK]

Common Mistakes:

• Using non-existent methods like start() or run()
• Confusing exec() with fork() for script processes
• Forgetting to require 'child_process' module

3. What will be the output of this Node.js code snippet?

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

Assuming child.js sends back the message { reply: 'Hi' } when it receives a message.

medium

A. No output because child.js is not executed

B. Message from child: Hello

C. Error: child.send is not a function

D. Message from child: { reply: 'Hi' }

Solution

1. Step 1: Understand message passing between parent and child

   The parent sends 'Hello' to child.js, which replies with { reply: 'Hi' }.

2. Step 2: Check event listener for 'message'

   The parent listens for messages from child and logs them with prefix 'Message from child:'.

3. Final Answer:

   Message from child: { reply: 'Hi' } -> Option D

4. Quick Check:

   Child replies logged correctly = D [OK]

Hint: Child sends message, parent logs with 'Message from child:' prefix [OK]

Common Mistakes:

• Confusing sent and received messages
• Expecting error from child.send() which is valid
• Assuming child.js does not run without error

4. Identify the error in this Node.js code using child processes:

const { fork } = require('child_process');
const child = fork('worker.js');
child.send('start');
child.on('message', (msg) => {
  console.log(msg);
});
child.on('error', (err) => {
  console.error('Child error:', err);
});

medium

A. Calling child.send() before setting up 'message' event listener

B. No error; code is correct

C. Not handling 'exit' event of child process

D. Missing require statement for 'child_process' module

Solution

1. Step 1: Check order of send() and event listeners

   It's valid to call send() before setting up 'message' listener; messages will queue.

2. Step 2: Verify required event handlers and module import

   Module is required correctly; 'error' event is handled; 'exit' event is optional.

3. Final Answer:

   No error; code is correct -> Option B

4. Quick Check:

   Code follows child process patterns = C [OK]

Hint: send() can be called anytime; event listeners catch messages/errors [OK]

Common Mistakes:

• Thinking send() must come after 'message' listener
• Expecting mandatory 'exit' event handling
• Missing module import (not in this code)

5. You want to perform a CPU-heavy task in Node.js without blocking the main event loop. Which approach best uses child processes to achieve this?

hard

A. Use fork() to run the heavy task in a separate process and communicate results via messages

B. Run the heavy task directly in the main thread and use setTimeout to delay it

C. Use exec() to run a shell command that blocks the main thread

D. Use require() to load the heavy task module synchronously

Solution

1. Step 1: Identify how to avoid blocking main event loop

   Heavy CPU tasks block the single-threaded main loop, causing freezes.

2. Step 2: Use child processes to run heavy tasks separately

   fork() creates a separate Node.js process to run the task without blocking.

3. Step 3: Communicate results safely

   Use message passing between main and child process to get results asynchronously.

4. Final Answer:

   Use fork() to run the heavy task in a separate process and communicate results via messages -> Option A

5. Quick Check:

   fork() isolates heavy tasks = A [OK]

Hint: fork() runs heavy tasks separately, keeping main loop free [OK]

Common Mistakes:

• Using setTimeout to delay heavy tasks (does not prevent blocking)
• Using exec() which can block main thread
• Loading heavy tasks synchronously with require()