What are worker threads and when would you use them in Node.js?

Worker threads in Node.js are a way to run JavaScript code in parallel on multiple threads, allowing CPU-intensive tasks to be executed without blocking the main event loop. This is particularly important in Node.js because it is single-threaded by default, and long-running computations can block the entire server if not offloaded properly.

What Are Worker Threads?

• Introduced in Node.js v10.5.0 (stable in v12+).

• Part of the worker_threads module.

• Each worker thread runs in its own isolated V8 instance.

• Communication is done via messages or shared memory.

When to Use Worker Threads:

Use them when:

1. You have CPU-intensive tasks: like data processing, image/video manipulation, cryptography, or large JSON parsing.

2. You want to avoid blocking the event loop: which can degrade performance for other incoming requests.

3. You need true parallelism: not just asynchronous I/O.

4. You want better performance over spawning separate processes: since threads are more lightweight than child processes.

Example Use Case:

const { Worker } = require('worker_threads');

function runService(workerData) {
  return new Promise((resolve, reject) => {
    const worker = new Worker('./worker.js', { workerData });
    worker.on('message', resolve);
    worker.on('error', reject);
    worker.on('exit', (code) => {
      if (code !== 0)
        reject(new Error(`Worker stopped with exit code ${code}`));
    });
  });
}

And in worker.js:

const { parentPort, workerData } = require('worker_threads');

// Perform some heavy computation
const result = heavyComputation(workerData);
parentPort.postMessage(result);