When would you use worker threads in Node.js?

A worker is a real thread with its own V8 isolate, event loop and memory heap. It can run JavaScript in parallel with the main thread. Communication happens through postMessage, which uses structured cloning, or through transferable objects and SharedArrayBuffer when you need to avoid copying.

import { Worker } from "node:worker_threads";

const worker = new Worker(new URL("./hash-worker.js", import.meta.url));

worker.postMessage({ path: "large-file.bin" });

worker.on("message", (result) => {
  console.log("hash", result.digest);
});

worker.on("error", (error) => {
  console.error("worker failed", error);
});

Trade-offs

• Each worker has its own isolate, so startup and memory costs are real
• postMessage copies data unless you transfer an ArrayBuffer or use shared memory
• Workers share the process, so a crash can still affect the whole service
• Too many workers cause contention and memory pressure; size the pool deliberately
• Workers are not a fix for blocking I/O, which is already asynchronous

Prefer a fixed pool of workers over creating one per request. Reuse workers for repeated jobs, keep messages small, and make the protocol explicit with request IDs so responses can be matched. For CPU work that needs strong isolation or separate scaling, a child process or a separate service is often simpler.