279 lines
12 KiB
Markdown
279 lines
12 KiB
Markdown
# Comlink
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Comlink makes [WebWorkers][webworker] enjoyable. Comlink is a **tiny library (1.1kB)**, that removes the mental barrier of thinking about `postMessage` and hides the fact that you are working with workers.
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At a more abstract level it is an RPC implementation for `postMessage` and [ES6 Proxies][es6 proxy].
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```
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$ npm install --save comlink
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```
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## Browsers support & bundle size
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Browsers without [ES6 Proxy] support can use the [proxy-polyfill].
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**Size**: ~2.5k, ~1.2k gzip’d, ~1.1k brotli’d
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## Introduction
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On mobile phones, and especially on low-end mobile phones, it is important to keep the main thread as idle as possible so it can respond to user interactions quickly and provide a jank-free experience. **The UI thread ought to be for UI work only**. WebWorkers are a web API that allow you to run code in a separate thread. To communicate with another thread, WebWorkers offer the `postMessage` API. You can send JavaScript objects as messages using `myWorker.postMessage(someObject)`, triggering a `message` event inside the worker.
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Comlink turns this messaged-based API into a something more developer-friendly by providing an RPC implementation: Values from one thread can be used within the other thread (and vice versa) just like local values.
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## Examples
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### [Running a simple function](./docs/examples/01-simple-example)
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**main.js**
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```javascript
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import * as Comlink from "https://unpkg.com/comlink/dist/esm/comlink.mjs";
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async function init() {
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const worker = new Worker("worker.js");
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// WebWorkers use `postMessage` and therefore work with Comlink.
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const obj = Comlink.wrap(worker);
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alert(`Counter: ${await obj.counter}`);
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await obj.inc();
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alert(`Counter: ${await obj.counter}`);
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}
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init();
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```
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**worker.js**
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```javascript
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importScripts("https://unpkg.com/comlink/dist/umd/comlink.js");
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// importScripts("../../../dist/umd/comlink.js");
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const obj = {
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counter: 0,
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inc() {
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this.counter++;
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},
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};
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Comlink.expose(obj);
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```
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### [Callbacks](./docs/examples/02-callback-example)
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**main.js**
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```javascript
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import * as Comlink from "https://unpkg.com/comlink/dist/esm/comlink.mjs";
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// import * as Comlink from "../../../dist/esm/comlink.mjs";
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function callback(value) {
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alert(`Result: ${value}`);
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}
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async function init() {
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const remoteFunction = Comlink.wrap(new Worker("worker.js"));
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await remoteFunction(Comlink.proxy(callback));
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}
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init();
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```
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**worker.js**
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```javascript
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importScripts("https://unpkg.com/comlink/dist/umd/comlink.js");
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// importScripts("../../../dist/umd/comlink.js");
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async function remoteFunction(cb) {
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await cb("A string from a worker");
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}
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Comlink.expose(remoteFunction);
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```
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### [`SharedWorker`](./docs/examples/07-sharedworker-example)
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When using Comlink with a [`SharedWorker`](https://developer.mozilla.org/en-US/docs/Web/API/SharedWorker) you have to:
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1. Use the [`port`](https://developer.mozilla.org/en-US/docs/Web/API/SharedWorker/port) property, of the `SharedWorker` instance, when calling `Comlink.wrap`.
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2. Call `Comlink.expose` within the [`onconnect`](https://developer.mozilla.org/en-US/docs/Web/API/SharedWorkerGlobalScope/onconnect) callback of the shared worker.
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**Pro tip:** You can access DevTools for any shared worker currently running in Chrome by going to: **chrome://inspect/#workers**
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**main.js**
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```javascript
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import * as Comlink from "https://unpkg.com/comlink/dist/esm/comlink.mjs";
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async function init() {
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const worker = new SharedWorker("worker.js");
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/**
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* SharedWorkers communicate via the `postMessage` function in their `port` property.
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* Therefore you must use the SharedWorker's `port` property when calling `Comlink.wrap`.
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*/
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const obj = Comlink.wrap(worker.port);
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alert(`Counter: ${await obj.counter}`);
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await obj.inc();
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alert(`Counter: ${await obj.counter}`);
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}
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init();
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```
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**worker.js**
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```javascript
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importScripts("https://unpkg.com/comlink/dist/umd/comlink.js");
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// importScripts("../../../dist/umd/comlink.js");
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const obj = {
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counter: 0,
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inc() {
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this.counter++;
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},
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};
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/**
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* When a connection is made into this shared worker, expose `obj`
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* via the connection `port`.
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*/
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onconnect = function (event) {
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const port = event.ports[0];
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Comlink.expose(obj, port);
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};
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// Single line alternative:
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// onconnect = (e) => Comlink.expose(obj, e.ports[0]);
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```
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**For additional examples, please see the [docs/examples](./docs/examples) directory in the project.**
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## API
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### `Comlink.wrap(endpoint)` and `Comlink.expose(value, endpoint?, allowedOrigins?)`
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Comlink’s goal is to make _exposed_ values from one thread available in the other. `expose` exposes `value` on `endpoint`, where `endpoint` is a [`postMessage`-like interface][endpoint] and `allowedOrigins` is an array of
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RegExp or strings defining which origins should be allowed access (defaults to special case of `['*']` for all origins).
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`wrap` wraps the _other_ end of the message channel and returns a proxy. The proxy will have all properties and functions of the exposed value, but access and invocations are inherently asynchronous. This means that a function that returns a number will now return _a promise_ for a number. **As a rule of thumb: If you are using the proxy, put `await` in front of it.** Exceptions will be caught and re-thrown on the other side.
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### `Comlink.transfer(value, transferables)` and `Comlink.proxy(value)`
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By default, every function parameter, return value and object property value is copied, in the sense of [structured cloning]. Structured cloning can be thought of as deep copying, but has some limitations. See [this table][structured clone table] for details.
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If you want a value to be transferred rather than copied — provided the value is or contains a [`Transferable`][transferable] — you can wrap the value in a `transfer()` call and provide a list of transferable values:
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```js
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const data = new Uint8Array([1, 2, 3, 4, 5]);
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await myProxy.someFunction(Comlink.transfer(data, [data.buffer]));
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```
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Lastly, you can use `Comlink.proxy(value)`. When using this Comlink will neither copy nor transfer the value, but instead send a proxy. Both threads now work on the same value. This is useful for callbacks, for example, as functions are neither structured cloneable nor transferable.
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```js
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myProxy.onready = Comlink.proxy((data) => {
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/* ... */
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});
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```
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### Transfer handlers and event listeners
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It is common that you want to use Comlink to add an event listener, where the event source is on another thread:
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```js
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button.addEventListener("click", myProxy.onClick.bind(myProxy));
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```
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While this won’t throw immediately, `onClick` will never actually be called. This is because [`Event`][event] is neither structured cloneable nor transferable. As a workaround, Comlink offers transfer handlers.
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Each function parameter and return value is given to _all_ registered transfer handlers. If one of the event handler signals that it can process the value by returning `true` from `canHandle()`, it is now responsible for serializing the value to structured cloneable data and for deserializing the value. A transfer handler has be set up on _both sides_ of the message channel. Here’s an example transfer handler for events:
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```js
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Comlink.transferHandlers.set("EVENT", {
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canHandle: (obj) => obj instanceof Event,
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serialize: (ev) => {
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return [
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{
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target: {
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id: ev.target.id,
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classList: [...ev.target.classList],
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},
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},
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[],
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];
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},
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deserialize: (obj) => obj,
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});
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```
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Note that this particular transfer handler won’t create an actual `Event`, but just an object that has the `event.target.id` and `event.target.classList` property. Often, this is enough. If not, the transfer handler can be easily augmented to provide all necessary data.
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### `Comlink.releaseProxy`
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Every proxy created by Comlink has the `[releaseProxy]()` method.
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Calling it will detach the proxy and the exposed object from the message channel, allowing both ends to be garbage collected.
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```js
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const proxy = Comlink.wrap(port);
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// ... use the proxy ...
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proxy[Comlink.releaseProxy]();
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```
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If the browser supports the [WeakRef proposal], `[releaseProxy]()` will be called automatically when the proxy created by `wrap()` gets garbage collected.
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### `Comlink.finalizer`
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If an exposed object has a property `[Comlink.finalizer]`, the property will be invoked as a function when the proxy is being released. This can happen either through a manual invocation of `[releaseProxy]()` or automatically during garbage collection if the runtime supports the [WeakRef proposal] (see `Comlink.releaseProxy` above). Note that when the finalizer function is invoked, the endpoint is closed and no more communication can happen.
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### `Comlink.createEndpoint`
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Every proxy created by Comlink has the `[createEndpoint]()` method.
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Calling it will return a new `MessagePort`, that has been hooked up to the same object as the proxy that `[createEndpoint]()` has been called on.
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```js
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const port = myProxy[Comlink.createEndpoint]();
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const newProxy = Comlink.wrap(port);
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```
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### `Comlink.windowEndpoint(window, context = self, targetOrigin = "*")`
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Windows and Web Workers have a slightly different variants of `postMessage`. If you want to use Comlink to communicate with an iframe or another window, you need to wrap it with `windowEndpoint()`.
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`window` is the window that should be communicate with. `context` is the `EventTarget` on which messages _from_ the `window` can be received (often `self`). `targetOrigin` is passed through to `postMessage` and allows to filter messages by origin. For details, see the documentation for [`Window.postMessage`](https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage).
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For a usage example, take a look at the non-worker examples in the `docs` folder.
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## TypeScript
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Comlink does provide TypeScript types. When you `expose()` something of type `T`, the corresponding `wrap()` call will return something of type `Comlink.Remote<T>`. While this type has been battle-tested over some time now, it is implemented on a best-effort basis. There are some nuances that are incredibly hard if not impossible to encode correctly in TypeScript’s type system. It _may_ sometimes be necessary to force a certain type using `as unknown as <type>`.
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## Node
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Comlink works with Node’s [`worker_threads`][worker_threads] module. Take a look at the example in the `docs` folder.
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[webworker]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API
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[umd]: https://github.com/umdjs/umd
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[transferable]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Transferable_objects
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[messageport]: https://developer.mozilla.org/en-US/docs/Web/API/MessagePort
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[examples]: https://github.com/GoogleChromeLabs/comlink/tree/master/docs/examples
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[dist]: https://github.com/GoogleChromeLabs/comlink/tree/master/dist
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[delivrjs]: https://cdn.jsdelivr.net/
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[es6 proxy]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy
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[proxy-polyfill]: https://github.com/GoogleChrome/proxy-polyfill
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[endpoint]: src/protocol.ts
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[structured cloning]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm
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[structured clone table]: structured-clone-table.md
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[event]: https://developer.mozilla.org/en-US/docs/Web/API/Event
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[worker_threads]: https://nodejs.org/api/worker_threads.html
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[weakref proposal]: https://github.com/tc39/proposal-weakrefs
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## Additional Resources
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- [Simplify Web Worker code with Comlink](https://davidea.st/articles/comlink-simple-web-worker)
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---
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License Apache-2.0
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