Quick start

Quick start

This is how to spawn a simple worker managed using threads.js. The worker will hash passwords, lifting the main CPU load off the master thread.

// master.js
import { spawn, Thread, Worker } from "threads"

async function main() {
  const auth = await spawn(new Worker("./workers/auth"))
  const hashed = await auth.hashPassword("Super secret password", "1234")

  console.log("Hashed password:", hashed)

  await Thread.terminate(auth)

// workers/auth.js - will be run in worker thread
import sha256 from "js-sha256"
import { expose } from "threads/worker"

  hashPassword(password, salt) {
    return sha256(password + salt)

Moving parts

The interesting bits in the sample code above are

  • spawn() to create a new worker
  • expose() to declare what functionality you want your worker to expose
  • Thread.terminate() to kill the worker once you don’t need it anymore

Also note that we imported Worker from threads.js. This is an important detail as you would usually use the global Worker on the window in browsers or import Worker from worker_threads in node.js.

Importing the Worker from threads.js allows us not only to run the same code in browsers and node, but the threads.js Worker transparently provides additional functionality, too, to make using it as easy as possible.

Learn more about it on the Basic usage page.


npm install threads tiny-worker

You only need to install the tiny-worker package to support node.js < 12. It’s an optional dependency and used as a fallback if worker_threads are not available.

Platform setup

Run using node.js

Running code using threads.js in node works out of the box.

Note that we wrap the native Worker, so new Worker("./foo/bar") will resolve the path relative to the module that calls it, not relative to the current working directory.

That aligns it with the behavior when bundling the code with webpack or parcel.

Build with webpack

Webpack config

Use with the threads-plugin.

It will transparently detect all new Worker("./unbundled-path") expressions, bundles the worker code and replaces the new Worker(...) path with the worker bundle path, so you don’t need to explicitly use the worker-loader or define extra entry points.

  npm install -D threads-plugin

Then add it to your webpack.config.js:

+ const ThreadsPlugin = require('threads-plugin')

  module.exports = {
    // ...
    plugins: [
+     new ThreadsPlugin()
    // ...

Node.js bundles

If you are using webpack to create a bundle that will be run in node (webpack config target: "node"), you also need to specify that the tiny-worker package used for node < 12 should not be bundled:

  module.exports = {
    // ...
+   externals: {
+     "tiny-worker": "tiny-worker"
+   }
    // ...

Make sure that tiny-worker is listed in your package.json dependencies in that case.

When using TypeScript

Make sure the TypeScript compiler keeps the import / export statements intact, so webpack resolves them. Otherwise the threads-plugin won’t be able to do its job.

  module.exports = {
    // ...
    module: {
      rules: [
          test: /\.ts$/,
          loader: "ts-loader",
+         options: {
+           compilerOptions: {
+             module: "esnext"
+           }
+         }
    // ...

Electron & webpack

In case you are using electron-webpack for your electron application and your bundle does not work, you probably need to add threads to whiteListedModules. Add this to your webpackElectron field in your package.json:

  "electronWebpack": {
    "whiteListedModules": [
+     "threads"

Build with parcel bundler

You need to import threads/register once at the beginning of your application code (in the master code, not in the workers):

  import { spawn } from "threads"
+ import "threads/register"

  // ...

  const work = await spawn(new Worker("./worker"))

This registers the library’s Worker implementation for your platform as the global Worker. This is necessary, since you cannot import { Worker } from "threads" or Parcel won’t recognize new Worker() as a web worker anymore.

Be aware that this might affect any code that tries to instantiate a normal web worker Worker and now instead instantiates a threads.js Worker. The threads.js Worker is just a web worker with some sugar on top, but that sugar might have unexpected side effects on third-party libraries.

Everything else should work out of the box.


When building an Electron application you probably want to enable ASAR packaging – it’s usually enabled by default. Your JavaScript files will then be packaged into an ASAR archive which can help reducing the executable size and time to launch.

The problem is that you can require() / import JavaScript modules from within the ASAR archive, but you cannot spawn workers packaged in the archive as easily. In order to spawn workers, you can use the asarUnpack option to unpack the archive when the app launches. threads.js will automatically look for the worker in the unpacked archive directory.

The following sample snippet shows how to set that option in your package.json file. You will have to use the right paths for your application’s files.

+ "asarUnpack": {
+   "dist/main/0.bundle.worker.js",
+   "dist/main/0.bundle.worker.js.map"
+ }


Learn about the details and all the other features of the threads.js API, like

  • Exposing more than one function
  • Writing stateful workers
  • Using thread pools
  • Using observables to stream data
  • and more…