Receiving results from workers in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to listen for messages from a worker thread.

Node.js

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

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

Asend

Bdata

Cresult

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'data' or 'result' instead of 'message' causes no event to fire.

The 'message' event is emitted when a worker sends data back to the main thread.

2fill in blank

medium

Complete the code to send a message from the main thread to the worker.

Node.js

worker.[1]({ task: 'processData' });

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

ApostMessage

Bemit

CsendMessage

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'emit' or 'sendMessage' will cause errors as they are not valid methods.

The postMessage method sends data to the worker thread.

3fill in blank

hard

Fix the error in the code to correctly receive data from the worker.

Node.js

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

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

Amsg

Bmessage

Cdata

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different parameter name than the variable inside causes a ReferenceError.

The parameter name must match the variable used inside the function to avoid reference errors.

4fill in blank

hard

Fill both blanks to correctly handle errors and exit events from a worker.

Node.js

worker.on('[1]', (error) => {
  console.error('Worker error:', error);
});
worker.on('[2]', (code) => {
  console.log('Worker exited with code', code);
});

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

Aerror

Bexit

Cclose

Ddisconnect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'close' or 'disconnect' instead of 'exit' will not detect worker termination correctly.

The 'error' event handles errors from the worker, and the 'exit' event fires when the worker stops.

5fill in blank

hard

Fill all three blanks to create a map of worker IDs to their last received messages.

Node.js

const results = {};
workers.forEach(worker => {
  worker.on('[1]', (message) => {
    results[worker.[2]] = message;
  });
});
console.log(results[[3]]);

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

Amessage

BthreadId

Cworker.threadId

DworkerId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'workerId' which does not exist, or 'worker.threadId' inside bracket notation incorrectly.

The 'message' event receives data. Each worker has a 'threadId' property used as a key. To access a specific worker's result, use its threadId.

Practice

1. In Node.js, how do you receive results from a worker thread?

easy

A. By using console.log inside the worker

B. By calling worker.getResult() method

C. By listening to the message event on the worker

D. By reading from a shared file

Solution

1. Step 1: Understand worker communication

   Workers send results back to the main thread using messages.

2. Step 2: Use the correct event listener

   The main thread listens to the message event on the worker to receive data.

3. Final Answer:

   By listening to the message event on the worker -> Option C

4. Quick Check:

   message event = D [OK]

Hint: Remember: workers send data via 'message' events [OK]

Common Mistakes:

• Trying to call a non-existent method like getResult()
• Expecting console.log output to be received
• Reading results from files instead of messages

2. Which of the following is the correct syntax to listen for messages from a worker in Node.js?

easy

A. worker.on('message', (result) => { console.log(result); });

B. worker.listen('message', (result) => { console.log(result); });

C. worker.addEventListener('message', (result) => { console.log(result); });

D. worker.receive('message', (result) => { console.log(result); });

Solution

1. Step 1: Recall Node.js worker event syntax

   Node.js workers use the on method to listen for events.

2. Step 2: Identify the correct event and method

   The event to receive data is message, and the syntax is worker.on('message', callback).

3. Final Answer:

   worker.on('message', (result) => { console.log(result); }); -> Option A

4. Quick Check:

   worker.on('message') = A [OK]

Hint: Use worker.on('message', callback) to get results [OK]

Common Mistakes:

• Using non-existent methods like listen or receive
• Confusing browser event syntax with Node.js
• Using addEventListener which is not in Node.js workers

3. What will be logged to the console when this Node.js worker code runs?

const { Worker } = require('worker_threads');
const worker = new Worker(`
  const { parentPort } = require('worker_threads');
  parentPort.postMessage('Hello from worker');
`, { eval: true });
worker.on('message', (msg) => console.log(msg));

medium

A. undefined

B. No output

C. Error: parentPort is not defined

D. Hello from worker

Solution

1. Step 1: Understand worker code execution

   The worker sends a message 'Hello from worker' using parentPort.postMessage.

2. Step 2: Check main thread message listener

   The main thread listens to the message event and logs the received message.

3. Final Answer:

   Hello from worker -> Option D

4. Quick Check:

   postMessage sends 'Hello from worker' = B [OK]

Hint: postMessage sends data, main thread logs it on 'message' event [OK]

Common Mistakes:

• Expecting no output because of missing event listener
• Confusing parentPort usage causing errors
• Assuming worker code runs in main thread

4. Identify the error in this Node.js worker communication code:

const { Worker } = require('worker_threads');
const worker = new Worker('./worker.js');
worker.on('message', (data) => {
  console.log('Received:', data);
});
parentPort.postMessage('start');

medium

A. Cannot call postMessage on the worker instance

B. postMessage should be called on parentPort inside worker, not on worker instance

C. Missing error event listener on worker

D. The worker file path is incorrect

Solution

1. Step 1: Understand where postMessage is called

   In Node.js, postMessage is called on parentPort inside the worker, not on the worker instance in main thread.

2. Step 2: Identify correct communication method

   The main thread uses worker.postMessage() to send messages to the worker, but inside the worker, parentPort.postMessage() sends messages back.

3. Final Answer:

   postMessage should be called on parentPort inside worker, not on worker instance -> Option B

4. Quick Check:

   postMessage usage inside worker = C [OK]

Hint: postMessage on worker sends to worker; inside worker use parentPort.postMessage [OK]

Common Mistakes:

• Confusing where to call postMessage
• Ignoring error event listeners
• Assuming worker file path is wrong without evidence

5. You want to receive multiple results from a worker that sends messages repeatedly. Which approach correctly handles this in Node.js?

const { Worker } = require('worker_threads');
const worker = new Worker('./worker.js');
// What should you do here to receive all messages?

hard

A. Use worker.on('message', (msg) => { console.log(msg); }); to listen continuously

B. Call worker.once('message', (msg) => { console.log(msg); }); to listen once

C. Use a loop to call worker.postMessage repeatedly

D. Use setTimeout to poll worker for messages

Solution

1. Step 1: Understand event listeners for multiple messages

   Using worker.on('message') listens continuously for all messages sent by the worker.

2. Step 2: Compare with other options

   once listens only once, loops or polling are unnecessary and inefficient for receiving messages.

3. Final Answer:

   Use worker.on('message', (msg) => { console.log(msg); }); to listen continuously -> Option A

4. Quick Check:

   Continuous message listening = A [OK]

Hint: Use worker.on('message') for all messages, not once or polling [OK]

Common Mistakes:

• Using once instead of on for multiple messages
• Trying to poll worker instead of event listening
• Confusing sending messages with receiving them