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    1.0.0-rc2 • Public • Published


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    Offload CPU-intensive tasks to worker threads in node.js, web browsers and electron using one uniform API.

    Uses web workers in the browser, worker_threads in node 12+ and tiny-worker in node 8 to 11.

    What is different from the original threads?

    • Add fromScript function into the Worker class (andywer/threads.js #196)
      const work = await spawn(Worker.fromScript(`
      // javascript worker code...
    • Support threads-loader


    • First-class support for async functions & observables
    • Write code once, run it on all platforms
    • Manage bulk task executions with thread pools
    • Use require() and import/export in workers
    • Works great with webpack

    Version 0.x

    You can find the old version 0.12 of threads.js on the v0 branch. All the content on this page refers to version 1.0 which is a rewrite of the library with a whole new API.


    npm install @jc-lab/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 support

    Run on 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.

    Webpack build setup

    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"
    +           }
    +         }
        // ...
    Parcel bundler setup

    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.

    Getting Started


    // master.js
    import { spawn, Thread, Worker } from "@jc-lab/threads"
    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
    import sha256 from "js-sha256"
    import { expose } from "threads/worker"
      hashPassword(password, salt) {
        return sha256(password + salt)


    The hashPassword() function of the auth object in the master code proxies the call to the hashPassword() function in the worker:

    If the worker's function returns a promise or an observable then you can just use the return value as such in the master code. If the function returns a primitive value, expect the master function to return a promise resolving to that value.


    Use expose() to make a function or an object containing methods callable from the master thread.

    In case of exposing an object, spawn() will asynchronously return an object exposing all the object's functions. If you expose() a function, spawn will also return a callable function, not an object.


    Find the full documentation on the website:


    Threads.js works with webpack. Usually all you need to do is adding the threads-plugin.

    See Build with webpack on the website for details.


    We are using the debug package to provide opt-in debug logging. All the package's debug messages have a scope starting with threads:, with different sub-scopes:

    • threads:master:messages
    • threads:master:spawn
    • threads:master:thread-utils
    • threads:pool:${poolName || poolID}

    Set it to DEBUG=threads:* to enable all the library's debug logging. To run its tests with full debug logging, for instance:

    DEBUG=threads:* npm test




    npm i @jc-lab/threads

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