Creating worker threads in Node.js - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple worker thread example

What will be printed to the console when this Node.js code runs?

Node.js

import { Worker, isMainThread, parentPort } from 'worker_threads';

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

A undefined

B Hello from worker

C Error: Cannot find module

D No output

💡 Hint

Remember that the worker sends a message back to the main thread.

The worker thread sends the string 'Hello from worker' to the main thread, which logs it.

❓ component_behavior

intermediate

Behavior of worker thread with data passed

What will be the output of this code snippet?

Node.js

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

if (isMainThread) {
  const worker = new Worker(new URL(import.meta.url), { workerData: 5 });
  worker.on('message', (msg) => console.log(msg));
} else {
  parentPort.postMessage(workerData * 2);
}

A 5

B NaN

C 10

D undefined

💡 Hint

Check how workerData is used inside the worker thread.

The main thread passes 5 as workerData. The worker multiplies it by 2 and sends 10 back.

📝 Syntax

advanced

Identify the syntax error in worker thread creation

Which option contains a syntax error that prevents the worker from starting?

Node.js

import { Worker, isMainThread } from 'worker_threads';

if (isMainThread) {
  const worker = new Worker('worker.js');
}

A const worker = new Worker(new URL('worker.js'));

B const worker = new Worker(new URL('worker.js', import.meta.url));

C const worker = new Worker(new URL(import.meta.url));

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

💡 Hint

Check how new URL() is used with relative paths in ES modules.

new URL('worker.js') alone is invalid because it lacks a base URL, causing a TypeError.

🔧 Debug

advanced

Why does this worker thread code throw an error?

This code throws an error when run. What is the cause?

Node.js

import { Worker, isMainThread } from 'worker_threads';

if (isMainThread) {
  const worker = new Worker('./worker.js');
  worker.on('message', (msg) => console.log(msg));
} else {
  parentPort.postMessage('Hi');
}

A Cannot use relative path string without URL in ES modules

B parentPort is undefined in main thread

C Missing import of parentPort in worker thread

D Worker file './worker.js' does not exist

💡 Hint

Consider how worker threads are created in ES modules with relative paths.

In ES modules, Worker constructor needs a URL object for relative paths, not a plain string.

❓ lifecycle

expert

Order of events in worker thread lifecycle

What is the correct order of these events when a worker thread is created and communicates back?

A 1,3,2,4

B 2,1,3,4

C 3,1,2,4

D 1,2,3,4

💡 Hint

Think about what happens first: creation, running, messaging, then receiving.

First main thread creates the worker, then worker runs, sends message, and main thread receives it.