Passing data to workers in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this worker output?

Consider this Node.js worker code snippet. What will the worker print to the console?

Node.js

import { Worker, isMainThread, parentPort, workerData } from 'node:worker_threads';

if (isMainThread) {
  const worker = new Worker(new URL(import.meta.url));
  worker.postMessage({ greeting: 'hello' });
} else {
  parentPort.on('message', (msg) => {
    console.log(msg.greeting);
  });
}

A undefined

B hello

C ReferenceError

D No output

💡 Hint

Think about how the main thread sends data to the worker using postMessage and how the worker listens for messages.

The main thread sends an object with a greeting property to the worker using postMessage. The worker listens for 'message' events and logs the greeting property, so it prints 'hello'.

📝 Syntax

intermediate

Which option correctly passes initial data to a worker?

You want to pass initial data to a worker thread at creation. Which code snippet correctly does this?

A new Worker('worker.js', { data: { user: 'Alice' } })

B new Worker(new URL('worker.js', import.meta.url), { message: { user: 'Alice' } })

C new Worker('worker.js', { initialData: { user: 'Alice' } })

D new Worker(new URL('worker.js', import.meta.url), { workerData: { user: 'Alice' } })

💡 Hint

Check the official property name used to pass data at worker creation.

The correct option uses the 'workerData' property to pass initial data to the worker thread. Other options use invalid property names.

❓ state_output

advanced

What is the value of 'workerData' inside the worker?

Given this worker creation code, what will be the value of 'workerData' inside the worker thread?

Node.js

import { Worker, isMainThread, workerData } from 'node:worker_threads';

if (isMainThread) {
  new Worker(new URL(import.meta.url), { workerData: { count: 5, active: true } });
} else {
  console.log(workerData);
}

A {}

B undefined

C { count: 5, active: true }

D null

💡 Hint

Remember that 'workerData' contains the data passed at worker creation.

The workerData object inside the worker contains exactly what was passed in the main thread's Worker constructor.

🔧 Debug

advanced

Why does this worker not receive data?

This code intends to send data to the worker, but the worker never logs it. Why?

Node.js

import { Worker, isMainThread, parentPort, workerData } from 'node:worker_threads';

if (isMainThread) {
  const worker = new Worker(new URL(import.meta.url));
  worker.postMessage('data');
} else {
  console.log(workerData);
}

A workerData is undefined because data was sent via postMessage, not workerData option

B postMessage is not a function on worker

C parentPort is undefined in the worker

D The worker script URL is incorrect

💡 Hint

Check how data is accessed inside the worker and how it was sent.

workerData only contains data passed at creation. postMessage sends data asynchronously and must be received via parentPort.on('message').

🧠 Conceptual

expert

Which method correctly shares a large buffer efficiently between main and worker?

You want to share a large ArrayBuffer between the main thread and a worker without copying it. Which approach is correct?

A Pass the ArrayBuffer via workerData with a TransferList containing the buffer

B Send the ArrayBuffer via postMessage without a TransferList

C Serialize the ArrayBuffer to JSON and send as a string

D Write the buffer to a file and read it in the worker

💡 Hint

Think about zero-copy transfer of buffers in worker_threads.

Passing the buffer with a TransferList transfers ownership without copying, making it efficient. Sending without TransferList copies the data. JSON serialization is slow and lossy. File I/O is inefficient.