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<ol class="chapter"><li class="affix"><a href="../introduction.html">Introduction</a></li><li class="spacer"></li><li><a href="../examples/index.html"><strong aria-hidden="true">1.</strong> Examples</a></li><li><ol class="section"><li><a href="../examples/hello-world.html"><strong aria-hidden="true">1.1.</strong> Hello, World!</a></li><li><a href="../examples/console-log.html"><strong aria-hidden="true">1.2.</strong> Using console.log</a></li><li><a href="../examples/add.html"><strong aria-hidden="true">1.3.</strong> Small wasm files</a></li><li><a href="../examples/without-a-bundler.html"><strong aria-hidden="true">1.4.</strong> Without a Bundler</a></li><li><a href="../examples/wasm2js.html"><strong aria-hidden="true">1.5.</strong> Converting WebAssembly to JS</a></li><li><a href="../examples/import-js.html"><strong aria-hidden="true">1.6.</strong> Importing functions from JS</a></li><li><a href="../examples/char.html"><strong aria-hidden="true">1.7.</strong> Working with char</a></li><li><a href="../examples/wasm-in-wasm.html"><strong aria-hidden="true">1.8.</strong> js-sys: WebAssembly in WebAssembly</a></li><li><a href="../examples/dom.html"><strong aria-hidden="true">1.9.</strong> web-sys: DOM hello world</a></li><li><a href="../examples/closures.html"><strong aria-hidden="true">1.10.</strong> web-sys: Closures</a></li><li><a href="../examples/performance.html"><strong aria-hidden="true">1.11.</strong> web-sys: performance.now</a></li><li><a href="../examples/fetch.html"><strong aria-hidden="true">1.12.</strong> web-sys: using fetch</a></li><li><a href="../examples/2d-canvas.html"><strong aria-hidden="true">1.13.</strong> web-sys: canvas hello world</a></li><li><a href="../examples/julia.html"><strong aria-hidden="true">1.14.</strong> web-sys: canvas Julia set</a></li><li><a href="../examples/web-audio.html"><strong aria-hidden="true">1.15.</strong> web-sys: WebAudio</a></li><li><a href="../examples/webgl.html"><strong aria-hidden="true">1.16.</strong> web-sys: WebGL</a></li><li><a href="../examples/websockets.html"><strong aria-hidden="true">1.17.</strong> web-sys: WebSockets</a></li><li><a href="../examples/webrtc_datachannel.html"><strong aria-hidden="true">1.18.</strong> web-sys: WebRTC DataChannel</a></li><li><a href="../examples/request-animation-frame.html"><strong aria-hidden="true">1.19.</strong> web-sys: requestAnimationFrame</a></li><li><a href="../examples/paint.html"><strong aria-hidden="true">1.20.</strong> web-sys: A Simple Paint Program</a></li><li><a href="../examples/raytrace.html"><strong aria-hidden="true">1.21.</strong> Parallel Raytracing</a></li><li><a href="../examples/todomvc.html"><strong aria-hidden="true">1.22.</strong> web-sys: A TODO MVC App</a></li></ol></li><li><a href="../reference/index.html"><strong aria-hidden="true">2.</strong> Reference</a></li><li><ol class="section"><li><a href="../reference/deployment.html"><strong aria-hidden="true">2.1.</strong> Deployment</a></li><li><a href="../reference/js-snippets.html"><strong aria-hidden="true">2.2.</strong> JS snippets</a></li><li><a href="../reference/passing-rust-closures-to-js.html"><strong aria-hidden="true">2.3.</strong> Passing Rust Closures to JS</a></li><li><a href="../reference/receiving-js-closures-in-rust.html"><strong aria-hidden="true">2.4.</strong> Receiving JS Closures in Rust</a></li><li><a href="../reference/js-promises-and-rust-futures.html" class="active"><strong aria-hidden="true">2.5.</strong> Promises and Futures</a></li><li><a href="../reference/iterating-over-js-values.html"><strong aria-hidden="true">2.6.</strong> Iterating over JS Values</a></li><li><a href="../reference/arbitrary-data-with-serde.html"><strong aria-hidden="true">2.7.</strong> Arbitrary Data with Serde</a></li><li><a href="../reference/accessing-properties-of-untyped-js-values.html"><strong aria-hidden="true">2.8.</strong> Accessing Properties of Untyped JS Values</a></li><li><a href="../reference/working-with-duck-typed-interfaces.html"><strong aria-hidden="true">2.9.</strong> Working with Duck-Typed Interfaces</a></li><li>
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This is the <strong>unpublished</strong> documentation of
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<code>wasm-bindgen</code>, the published documentation is available
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<a href="https://rustwasm.github.io/docs/wasm-bindgen/">
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on the main Rust and WebAssembly documentation site
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<code>wasm-bindgen</code>.
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<h1 class="menu-title">The `wasm-bindgen` Guide</h1>
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<main>
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<h1><a class="header" href="#working-with-a-js-promise-and-a-rust-future" id="working-with-a-js-promise-and-a-rust-future">Working with a JS <code>Promise</code> and a Rust <code>Future</code></a></h1>
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<p>Many APIs on the web work with a <code>Promise</code>, such as an <code>async</code> function in JS.
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Naturally you'll probably want to interoperate with them from Rust! To do that
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you can use the <code>wasm-bindgen-futures</code> crate as well as Rust <code>async</code>
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functions.</p>
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<p>The first thing you might encounter is the need for working with a <code>Promise</code>.
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For this you'll want to use <a href="https://docs.rs/js-sys/*/js_sys/struct.Promise.html"><code>js_sys::Promise</code></a>. Once you've got one of those
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values you can convert that value to <code>wasm_bindgen_futures::JsFuture</code>. This type
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implements the <code>std::future::Future</code> trait which allows naturally using it in an
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<code>async</code> function. For example:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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async fn get_from_js() -> Result<JsValue, JsValue> {
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let promise = js_sys::Promise::resolve(&42.into());
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let result = wasm_bindgen_futures::JsFuture::from(promise).await?;
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Ok(result)
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}
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#}</code></pre></pre>
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<p>Here we can see how converting a <code>Promise</code> to Rust creates a <code>impl Future<Output = Result<JsValue, JsValue>></code>. This corresponds to <code>then</code> and <code>catch</code> in JS where
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a successful promise becomes <code>Ok</code> and an erroneous promise becomes <code>Err</code>.</p>
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<p>Next up you'll probably want to export a Rust function to JS that returns a
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promise. To do this you can use an <code>async</code> function and <code>#[wasm_bindgen]</code>:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[wasm_bindgen]
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pub async fn foo() {
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// ...
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}
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#}</code></pre></pre>
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<p>When invoked from JS the <code>foo</code> function here will return a <code>Promise</code>, so you can
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import this as:</p>
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<pre><code class="language-js">import { foo } from "my-module";
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async function shim() {
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const result = await foo();
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// ...
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}
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</code></pre>
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<h2><a class="header" href="#return-values-of-async-fn" id="return-values-of-async-fn">Return values of <code>async fn</code></a></h2>
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<p>When using an <code>async fn</code> in Rust and exporting it to JS there's some
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restrictions on the return type. The return value of an exported Rust function
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will eventually become <code>Result<JsValue, JsValue></code> where <code>Ok</code> turns into a
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successfully resolved promise and <code>Err</code> is equivalent to throwing an exception.</p>
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<p>The following types are supported as return types from an <code>async fn</code>:</p>
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<ul>
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<li><code>()</code> - turns into a successful <code>undefined</code> in JS</li>
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<li><code>T: Into<JsValue></code> - turns into a successful JS value</li>
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<li><code>Result<(), E: Into<JsValue>></code> - if <code>Ok(())</code> turns into a successful
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<code>undefined</code> and otherwise turns into a failed promise with <code>E</code> converted to a
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JS value</li>
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<li><code>Result<T: Into<JsValue>, E: Into<JsValue>></code> - like the previous case except
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both data payloads are converted into a <code>JsValue</code>.</li>
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</ul>
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<p>Note that many types implement being converted into a <code>JsValue</code>, such as all
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imported types via <code>#[wasm_bindgen]</code> (aka those in <code>js-sys</code> or <code>web-sys</code>),
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primitives like <code>u32</code>, and all exported <code>#[wasm_bindgen]</code> types. In general,
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you should be able to write code without having too many explicit conversions,
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and the macro should take care of the rest!</p>
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<h2><a class="header" href="#using-wasm-bindgen-futures" id="using-wasm-bindgen-futures">Using <code>wasm-bindgen-futures</code></a></h2>
|
||
|
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<p>The <code>wasm-bindgen-futures</code> crate bridges the gap between JavaScript <code>Promise</code>s
|
||
|
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and Rust <code>Future</code>s. Its <code>JsFuture</code> type provides conversion from a JavaScript
|
||
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<code>Promise</code> into a Rust <code>Future</code>, and its <code>future_to_promise</code> function converts a
|
||
|
|
Rust <code>Future</code> into a JavaScript <code>Promise</code> and schedules it to be driven to
|
||
|
|
completion.</p>
|
||
|
|
<p>Learn more:</p>
|
||
|
|
<ul>
|
||
|
|
<li><a href="https://crates.io/crates/wasm-bindgen-futures"><code>wasm_bindgen_futures</code> on crates.io</a></li>
|
||
|
|
<li><a href="https://rustwasm.github.io/wasm-bindgen/api/wasm_bindgen_futures/"><code>wasm-bindgen-futures</code> API documentation and example usage</a></li>
|
||
|
|
</ul>
|
||
|
|
<h2><a class="header" href="#compatibility-with-versions-of-future" id="compatibility-with-versions-of-future">Compatibility with versions of <code>Future</code></a></h2>
|
||
|
|
<p>The current crate on crates.io, <code>wasm-bindgen-futures 0.4.*</code>, supports
|
||
|
|
<code>std::future::Future</code> and <code>async</code>/<code>await</code> in Rust. This typically requires Rust
|
||
|
|
1.39.0+ (as of this writing on 2019-09-05 it's the nightly channel of Rust).</p>
|
||
|
|
<p>If you're using the <code>Future</code> trait from the <code>futures</code> <code>0.1.*</code> crate then you'll
|
||
|
|
want to use the <code>0.3.*</code> track of <code>wasm-bindgen-futures</code> on crates.io.</p>
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