A simple wrapper to handle pools and multiples jobs in Node worker_threads module.

Usage no npm install needed!

<script type="module">
  import workerThreadManager from '';



Helps to manage pool of tasks into worker_threads of Node.js

This package requires worker_threads support, bundled with Node 10 with --experimental-worker flag or with Node 12 natively.

Getting started

Install the package using npm or yarn.

npm i worker-thread-manager
# or
yarn add worker-thread-manager

You can import the package using CommonJS or ECMAScript native imports. The whole package is written in TypeScript and provide type definitions.

This documentation will use TypeScript typings, remove them in order to use native JavaScript.

// CommonJS
const { WorkerThreadManager } = require('worker-thread-manager');

// ECMAScript
import { WorkerThreadManager } from 'worker-thread-manager';
// or (this is the default export)
import WorkerThreadManager from 'worker-thread-manager';


Node.js's worker_threads package allow users to spread tasks across multiple threads to do intensive jobs without blocking its event loop.

Unfortunatly, defined API provides only a few tools to communicate through main thread and its children, all event based.

This package handles the boilerplate introduced by this biais of communication and wraps tasks into Promises objects.

  • Create multiple pools of workers to distribute your tasks at will
  • Auto-kill children processes that are not used after a certain period of time to save memory
  • With one call, dispatch a task to a pool of worker by providing custom data, and await the result
  • Easily await end of a group of tasks
  • Workers are only spawned when necessary to avoid useless memory usage


Communications between main thread and workers are limited.

Values sent and received by workers are limited to supported values in HTML structured clone algorithm.

In particular, the significant differences to JSON are:

  • Value may contain circular references.
  • Value may contain instances of builtin JS types such as RegExps, BigInts, Maps, Sets, etc.
  • Value may contain typed arrays, both using ArrayBuffers and SharedArrayBuffers.
  • Value may contain WebAssembly.Module instances.
  • Value may not contain native (C++-backed) objects other than MessagePorts.

Those limitations apply to initial worker values (workerData option), input values (first parameter of .run() method of WorkerPool), and results of workers (return type of WorkerChild's onTask handler).

Basic usage

Main thread

Create a pool with the .spawn() method and start a job with .run().

The best worker to use (the less used) will automatically be selected in order to run new jobs. You can adjust the threshold needed to force spawning of a new worker instead of using a started one with spawnerThreshold option.

You can await a single job by awaiting result of .run(), or await all the started job in the worker pool by using .join() method.

// main.js / main.ts
import WorkerThreadManager, { WorkerPool } from 'worker-thread-manager';

// Create the pool that take items of type number in input,
// and returns a result of type string.
const worker_pool = WorkerThreadManager.spawn<number, string>(
  // Filename to worker JS file
    // Maximum of 4 instances of worker.js (default: 1)
    poolLength: 4,
    // Before instanciate a new worker, wait for every
    // started worker to have 5 tasks currently running at least
    // (default: 0 (aggressive spawning if a worker is unused))
    spawnerThreshold: 5,

// In order to use await, 
// wrap script into a async function

async function main(pool: WorkerPool) {
  // Start 10,000 jobs distributed to pool.
  for (let i = 1; i <= 10_000; i++) {
    // Start a task on pool, then register it
    const task =;

    task.then(data => {
      console.log(`Job #${task.uuid} says: ${data}`);

  // Await for every job to end
  await pool.join();

Child process

The child must import WorkerChild from the package, instanciate it and listen for events.

// worker.js / worker.ts
import { WorkerChild } from 'worker-thread-manager';

async function onTask(data: number, job_id: string) {
  // This function must return a string, as defined in WorkerChild types.
  let current_number = 0;

  // Each task will sleep between 1 and 1000 ms, 
  // then generate 50K random numbers
  for (let i = 0; i < 50_000; i++) {
    current_number += Math.random();

  return `Job ${job_id} finished well with number ${current_number}.`;

function onStartup() {
  // This function is executed when worker starts.
  // You can return a Promise, it will be awaited.
  // As worker can be automatically killed, this function 
  // can be executed multiple times.

// Starts the child
// It take number as input, returns a string,
// and take nothing as startup argument (workerData is empty).
new WorkerChild<number, string, void>({
  onTask: onTask,
  onStartup: onStartup,

Advanced usage

Options of WorkerThreadManager.spawn method

First parameter is script filename. Prefer using absolute filename, it's more safe. If you're using TypeScript, this path must be the compiled version of script, always the JavaScript version (unless you're using ts-node)!

Second parameter is a optional WorkerThreadManagerOptions object, that extends the native WorkerOptions object from worker_threads module (see this).

Every following parameter is optional.

  • stopOnNoTask: number. Timeout started after worker ends every handled task. If the worker gets no task during given time (in ms), it is killed. If a new task is started, this timeout is stopped. Default: Infinity (disable autokill).

  • poolLength: number. Number of start-able workers in the pool. Default: 1.

  • spawnerThreshold: number. Define minimum occupation in started workers needed to force starting of a stopped worker. Default: 0 (every time a worker is available, it will be used, stopped or not).

  • lazy: boolean. On WorkerPool instancation, do not spawn workers immediately. It let workers instanciate when they receive their first task. You can set this parameter to false to enforce worker start at the pool's creation, for example if startup task is heavy and should not happen during runtime. Default: true.

  • workerData: any. Data to give to worker at instanciation. It will be directly given in parameter of onStartup parameter function of WorkerChild.

  • eval: boolean. If true, interpret the first argument (initially, filename) as a script that is executed once the worker is online.

Options of WorkerChild constructor

You can enable type checking when you instanciate WorkerChild object by using generics.

new WorkerChild<
  /* Type of data received by worker child */
  /* Type of data sent by worker child at the end of a task */
  /* Type of data received by worker at its startup */

A WorkerChild takes an object with two properties, one required and one optional.

interface WorkerChildOptions {
  onTask: (data: TaskData, job_uuid: string) => TaskResult | Promise<TaskResult>;
  onStartup?: (data?: StartupData) => any | Promise<any>;

Methods of a WorkerPool instance

  • .run(data: TaskData, transferList?: Array<ArrayBuffer | MessagePort>): ThreadPromise<TaskResult>: Start a new task distributed to workers. data is given to worker and must respect rules defined in Limitations. transferList is an optional parameter where you should define ArrayBuffers or MessagePorts given to worker through data.

    Returned value, a ThreadPromise object, is a custom Promise with additionnal properties:

    • .uuid: string: Job ID.
    • .worker: Worker: Worker associated to this task (useful to get stdin and stdout streams). To send and receive data from one task, it is more recommanded to use MessagePort objects instead of std streams.
    • .stop(): Send a stop message for this task. This be manually handled in your child's task handler.
  • .init(): Promise<void>: Force every worker in the pool to start and await their online status.

  • .stats(): PoolStats: Get statistics about worker occupation.

interface PoolStats {
 /** Number of workers in the pool */
 worker_count: number;
 /** Number of started workers */
 active: number;
 /** Number of stopped workers */
 stopped: number;
 /** Number of jobs currently handled for every worker */
 job_counts: number[];
 /** Number of jobs currently handled by the least loaded worker */
 minimum_load: number;
 /** Number of jobs currently handled by most loaded worker */
 maximum_load: number;
 /** Average number of jobs per worker */
 average_load: number;
  • .join(): Promise<(TaskResult | undefined)[]>: Await every dispatched task (failed or not) into worker pool to end. Returns their results into a arbitary order. If a task failed, undefined is present in result array.

  • .terminate(): void: Kill every worker in the pool.

  • .joinAndTerminate(): Promise<(TaskResult | undefined)[]>: Await every task end like .join(), then terminate the started workers.

  • .exists(id: string): boolean: Tells if job id is currently started.

  • .get(id: string): ThreadPromise<TaskResult>: Get the ThreadPromise associated to the given job ID. The job must exists.

Methods of a WorkerChild instance

  • .listen(): Listen for tasks sent by main thread. This method must be called only one time.

  • .isStarted(id: string): boolean: Tells if task id is still considered as started. If user requested task end, this function will return false.