In Node, it's a web-compatible Worker implementation atop Node's worker_threads.

In the browser (and when bundled for the browser), it's simply an alias of Worker.

Features

Here's how this is different from worker_threads:

• makes Worker code compatible across browser, Node and Deno
• only supports Module Workers ({type:'module'})
• uses DOM-style events (Event.data, Event.type, MessageEvent, etc)
• supports event handler properties (worker.onmessage=..)
• Worker() accepts a URL, Blob URL or Data URL (via loaders)
• emulates browser-style [WorkerGlobalScope] within the worker
• Worker thread can import data:, https:, blob:, file: files just fine
• Worker constructed also supports those too new Worker('https://...')

Usage Example

In its simplest form:

import Worker from 'whatwg-worker'

const worker = new Worker('data:text/javascript,postMessage("hello")')
worker.onmessage = e => console.log(e.data)  // "hello"

import Worker from 'whatwg-worker'

const url = new URL('./worker.js', import.meta.url)
const worker = new Worker(url)

worker.addEventListener('message', e => {
  console.log(e.data)  // "hiya!"
})

worker.postMessage('hello')

addEventListener('message', e => {
  if (e.data === 'hello') {
    postMessage('hiya!')
  }
})

👉 Notice how new URL('./worker.js', import.meta.url) is used above to load the worker relative to the current module instead of the application base URL. Without this, Worker URLs are relative to a document's URL, which in Node.js is interpreted to be process.cwd().

Support for this pattern in build tools and test frameworks is still limited. We are working on growing this.

Module Workers

Module Workers are supported in Node 12.8+ using this plugin, leveraging Node's native ES Modules support. In the browser, they can be used natively in Chrome 80+, or in all browsers via [worker-plugin] or [rollup-plugin-off-main-thread]. As with classic workers, there is no difference in usage between Node and the browser:

import Worker from 'whatwg-worker'

const worker = new Worker(
  new URL('./worker.js', import.meta.url),
  { type: 'module' }
)
worker.addEventListener('message', e => {
  console.log(e.data)  // "200 OK"
})
worker.postMessage('https://httpstat.us/200')

addEventListener('message', async e => {
  const url = e.data
  const res = await fetch(url)
  const text = await res.text()
  postMessage(text)
})

Data URLs

Instantiating Worker using a Data URL is supported in both module and classic workers:

import Worker from 'whatwg-worker'

const worker = new Worker(`data:application/javascript,postMessage(42)`)
worker.addEventListener('message', e => {
  console.log(e.data)  // 42
})

Blob URLs

Instantiating Worker using a Blob URL is supported

import Worker from 'whatwg-worker'

const code = 'import fs from "node:fs"'
const blob = new Blob([code], { type: 'text/javascript' })
const worker = new Worker(URL.createObjectURL(blob))

HTTP loader supported.

Worker gets added https- loader support via --loader flag

import Worker from 'whatwg-worker'

const code = 'import xyz from "https://example.com/main.js"'
const blob = new Blob([code], { type: 'text/javascript' })
const worker = new Worker(URL.createObjectURL(blob))

import Worker from 'whatwg-worker'

const url = 'https://example.com/main.js'
const worker = new Worker(url)

Worker global scope

Each time when creating a new Worker it will get assigned some new global variables including

• name
• Worker (to create worker within a worker)
• self
• postMessage
• and globalThis will be inherit EventTarget (addEventListener, remove and dispatch)