0.0.20 • Public • Published


webpack-worker is a tiny library that greatly simplifies running CPU intensive work on another thread in the browser.

For a detailed walk-through of using webpack-worker, see the blog post here.

A live example of a graph filtering system made with webpack-worker and Plotly.js can be found here


npm i --save webpack-worker


webpack-worker provides two different styles of workers - processes and APIs.


The process is intended to represent long running processes with a defined start and end. Your worker file will look something like:

var process = require('webpack-worker/process')
var data = require('my-data-library')
process((params, emit) => {
  emit('Starting process')
  return data.fetch(params.query)
    .then(result => {
      emit('Loaded data')
      // ... do CPU intensive operation...
      return processedData

This file will be run through webpack as an entry point (more on this later). To execute the process and monitor for any emitted events:

var client = require('webpack-worker/client')
var worker = new Worker('process.bundle.js')
var process = client(worker, { query: 'my query' })
  .subscribe(message => console.log(message))
  .then(processedData => {
    // ... do stuff with your data!

Once the promise returned from the worker process resolves, the worker will terminate.

webpack-worker handles the marshalling of ArrayBuf and other transferrable objects. This is particularly useful for passing the results of FileReader operations.


API workers, on the other hand, are intended to be a persistent process that commands can be issued to. An example worker definition:

var api = require('webpack-worker/api')
var data = require('my-data-library')
var bigCalculation = require('big-calculation-library')
// we can perform initialization that returns a promise
// then return the API structure you want to expose
api(baseValue => data.load(baseValue).then(data => ({
  multiply: value => baseValue * value,
  power: value => Math.pow(baseValue, value),
  bigCalculation: value => bigCalculation(data, value)

To call our API:

var client = require('webpack-worker/client')
var worker = new Worker('api.bundle.js')
client(worker, 123).then(api => {
  api.multiply(32).then(result => console.log(result /* 4224 */))
  api.bigCalculation({ data: { a: 1 } }).then(result => /* do stuff with result */)

Throttling API Calls

Applying throttling functions from other libraries such as lodash.throttle is as simple as applying the throttling function to client API functions:

var client = require('webpack-worker/client')
var worker = new Worker('api.bundle.js')
var throttle = require('lodash.throttle')
client(worker).then(api => {
  // ensure the bigCalculation API function is executed at most once every 100ms
  return Object.assign(api, { bigCalculation: throttle(api.bigCalculation, 100) })

Throttling User Generated UI Feedback

A built in throttling function is provided that provides effective throttling for events such as mouse move / drag.

The API wraps a function that is expected to return promises. If multiple invocations are received while the promise is still pending, only the most recent is queued for execution, others are dropped.

Dropped invocations are indicated by the promise associated with the invocation being rejected. To identify dropped invocations, the passed Error object has a property attached named dropped that is set to true.

Individual functions can be throttled:

var throttle = require('webpack-worker/throttle.mostRecent')
module.exports = throttle(delay => Promise.delay(delay || 1000))

An additional helper function applyTo is attached to the exported function that applies the throttle to any function member of the supplied object. It can be used to easily wrap worker client API objects:

var client = require('webpack-worker/client')
var throttle = require('webpack-worker/throttle.mostRecent')
client(new Worker('api.bundle.js')).then(api => {
  return throttle.applyTo(api)

Configuring webpack

Configuring webpack is as simple as adding an entry point to the webpack configuration:

module.exports = {
  entry: {
    app: require.resolve('index.js'),
    process: require.resolve('process.worker.js'),
    api: require.resolve('api.worker.js')
  output: {
    path: 'build',
    filename: '[name].bundle.js'

Configuring a create-react-app Application

There are a few extra steps involved in configuring react applications created with create-react-app. Unfortunately, create-react-app does not support customising the webpack configuration, so we'll need to eject by running npm run eject. This leaves us with a config folder that contains webpack configuration.

After we've ejected, we need to add the entry point to the configuration by changing the entry node to a hash containing the original array, similar to above:

  entry: {
    app: [
      // previous entry node here
    worker: require.resolve('../src/worker')

Then, we need to include the bundle name in the the output filename:

  output: {
    // ...
    filename: 'static/js/[name].bundle.js',
    // ...

create-react-app uses a webpack plugin called html-webpack-plugin that generates an index.html that loads all entry points when the page is rendered. We need to prevent the worker bundle from being loaded on page render as it's loaded at a later point when the worker is created.

Under the plugins node, you'll find an entry that looks like the following. Add an excludeChunks property that matches the name of the entry point:

    new HtmlWebpackPlugin({
      inject: true,
      template: paths.appHtml,
      excludeChunks: ['worker']




npm i webpack-worker

DownloadsWeekly Downloads






Unpacked Size

15.2 kB

Total Files


Last publish


  • danderson00