Receiving results from workers in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this Node.js worker thread code output?

Consider this Node.js code using worker threads. What will be printed to the console?

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('Received:', msg));
  worker.postMessage('start');
} else {
  parentPort.on('message', (msg) => {
    if (msg === 'start') {
      parentPort.postMessage('done');
    }
  });
}

A Received: done

B Received: start

C No output, program hangs

D Throws an error: parentPort is undefined

💡 Hint

Think about how the worker listens and sends messages back to the main thread.

The main thread sends 'start' to the worker. The worker listens for 'start' and replies with 'done'. The main thread logs 'Received: done'.

❓ state_output

intermediate

What is the value of 'result' after worker completes?

In this Node.js code, what will be the value of result after the worker sends its message?

Node.js

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

let result = null;

if (isMainThread) {
  const worker = new Worker(new URL(import.meta.url));
  worker.on('message', (msg) => {
    result = msg;
  });
  worker.postMessage('compute');

  setTimeout(() => {
    console.log('Result:', result);
  }, 100);
} else {
  parentPort.on('message', (msg) => {
    if (msg === 'compute') {
      parentPort.postMessage(42);
    }
  });
}

A Throws ReferenceError

B Result: null

C Result: 42

D Result: undefined

💡 Hint

The message event updates result before the timeout logs it.

The worker sends 42 back. The main thread sets result = 42. After 100ms, result is 42 and logged.

📝 Syntax

advanced

Which option causes a syntax error in worker message handling?

Which of these code snippets will cause a syntax error when used inside a Node.js worker thread?

A parentPort.on('message', msg => { if (msg === 'go') parentPort.postMessage('ok'); });

B parentPort.on('message', msg => { if (msg === 'go') { parentPort.postMessage('ok'); } });

C parentPort.on('message', function(msg) { if (msg === 'go') { parentPort.postMessage('ok'); } });

D parentPort.on('message', (msg) => { if msg === 'go' { parentPort.postMessage('ok'); } });

💡 Hint

Check the syntax of the if statement in each option.

Option D misses parentheses around the if condition, causing a syntax error.

🔧 Debug

advanced

Why does this worker code throw 'TypeError: Cannot read property "postMessage" of null'?

Given this worker code snippet, why does it throw a TypeError?

Node.js

import { parentPort } from 'worker_threads';

parentPort.postMessage('hello');

A Because parentPort is not imported correctly

B Because the code runs in the main thread where parentPort is null

C Because postMessage requires a callback function

D Because 'hello' is not a valid message format

💡 Hint

Check where the code is executed: main thread or worker thread?

parentPort is only defined inside worker threads. If this code runs in the main thread, parentPort is null, causing the error.

🧠 Conceptual

expert

Which option correctly describes how results are received from multiple workers?

You spawn multiple worker threads in Node.js to perform tasks. How do you correctly receive and aggregate their results?

A Listen to each worker's 'message' event and collect results in an array; use Promise.all to wait for all workers.

B Call worker.postMessage() with a callback to get the result synchronously.

C Use a shared global variable updated by all workers directly to store results.

D Use a single worker to handle all tasks sequentially to avoid message conflicts.

💡 Hint

Think about asynchronous message events and how to wait for all workers.

Each worker sends messages asynchronously. Listening to their 'message' events and collecting results allows aggregation. Using Promise.all with Promises wrapping each worker's completion is the correct pattern.