web-worker/README.md

Consistent Web Workers in browser and Node.

<h1 align="center">
  web-worker
  <a href="https://www.npmjs.org/package/web-worker"><img src="https://img.shields.io/npm/v/web-worker.svg?style=flat-square" alt="npm"></a>
</h1>
<p align="center">
  Native cross-platform Web Workers. Works in published npm modules.
</p>
<p align="center">
  <img src="https://user-images.githubusercontent.com/105127/79602228-1998bf00-80b8-11ea-91e4-26b212aabaa2.png" width="1000" alt="">
</p>

**In Node**, it's a web-compatible Worker implementation atop Node's [worker_threads](https://nodejs.org/api/worker_threads.html).

**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 and Node
- supports Module Workers (`{type:'module'}`) natively in Node 12.8+
- uses DOM-style events (`Event.data`, `Event.type`, etc)
- supports event handler properties (`worker.onmessage=..`)
- `Worker()` accepts a module URL, Blob URL or Data URL
- emulates browser-style [WorkerGlobalScope] within the worker

### Usage Example

In its simplest form:

```js
import Worker from 'web-worker';

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

<table>
<thead><tr><th><strong>main.js</strong></th><th><strong>worker.js</strong></th></tr></thead>
<tbody><tr><td>

```js
import Worker from 'web-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');
```

</td><td valign="top">

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

</td></tr></tbody>
</table>

👉 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](https://github.com/developit/web-worker/issues/4)._

### 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:

<table>
<thead><tr><th><strong>main.mjs</strong></th><th><strong>worker.mjs</strong></th></tr></thead>
<tbody><tr><td>

```js
import Worker from 'web-worker';

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

</td><td valign="top">

```js
import fetch from 'isomorphic-fetch';

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

</td></tr></tbody>
</table>


### Data URLs

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

```js
import Worker from 'web-worker';

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

### Special Thanks

This module aims to provide a simple and forgettable piece of infrastructure,
and as such it needed an obvious and descriptive name.
[@calvinmetcalf](https://github.com/calvinmetcalf), who you may recognize as the author of [Lie](https://github.com/calvinmetcalf/lie) and other fine modules, gratiously offered up the name from his `web-worker` package.
Thanks Calvin!


[worker-plugin]: https://github.com/googlechromelabs/worker-plugin
[rollup-plugin-off-main-thread]: https://github.com/surma/rollup-plugin-off-main-thread
[WorkerGlobalScope]: https://developer.mozilla.org/en-US/docs/Web/API/WorkerGlobalScope