# Multithreading

With [Web Workers][web-workers], it is possible to run JavaScript in OS-level
threads.

## Multi-threaded Node.js

It is possible to use Node.js features in Electron's Web Workers, to do
so the `nodeIntegrationInWorker` option should be set to `true` in
`webPreferences`.

```javascript
const win = new BrowserWindow({
  webPreferences: {
    nodeIntegrationInWorker: true
  }
})
```

The `nodeIntegrationInWorker` can be used independent of `nodeIntegration`, but
`sandbox` must not be set to `true`.

**Note:** This option is not available in [`SharedWorker`s](https://developer.mozilla.org/en-US/docs/Web/API/SharedWorker) or [`Service Worker`s](https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker) owing to incompatibilities in sandboxing policies.

## Available APIs

All built-in modules of Node.js are supported in Web Workers, and `asar`
archives can still be read with Node.js APIs. However none of Electron's
built-in modules can be used in a multi-threaded environment.

## Native Node.js modules

Any native Node.js module can be loaded directly in Web Workers, but it is
strongly recommended not to do so. Most existing native modules have been
written assuming single-threaded environment, using them in Web Workers will
lead to crashes and memory corruptions.

Note that even if a native Node.js module is thread-safe it's still not safe to
load it in a Web Worker because the `process.dlopen` function is not thread
safe.

The only way to load a native module safely for now, is to make sure the app
loads no native modules after the Web Workers get started.

```javascript
process.dlopen = () => {
  throw new Error('Load native module is not safe')
}
const worker = new Worker('script.js')
```

[web-workers]: https://developer.mozilla.org/en/docs/Web/API/Web_Workers_API/Using_web_workers