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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><a href="../reference/cli.html"><strong aria-hidden="true">2.10.</strong> Command Line Interface</a></li><li><a href="../reference/optimize-size.html"><strong aria-hidden="true">2.11.</strong> Optimizing for Size</a></li><li><a href="../reference/rust-targets.html"><strong aria-hidden="true">2.12.</strong> Supported Rust Targets</a></li><li><a href="../reference/browser-support.html"><strong aria-hidden="true">2.13.</strong> Supported Browsers</a></li><li><a href="../reference/types.html"><strong aria-hidden="true">2.14.</strong> Supported Types</a></li><li><ol class="section"><li><a href="../reference/types/imported-js-types.html"><strong aria-hidden="true">2.14.1.</strong> Imported JavaScript Types</a></li><li><a href="../reference/types/exported-rust-types.html"><strong aria-hidden="true">2.14.2.</strong> Exported Rust Types</a></li><li><a href="../reference/types/jsvalue.html"><strong aria-hidden="true">2.14.3.</strong> JsValue</a></li><li><a href="../reference/types/boxed-jsvalue-slice.html"><strong aria-hidden="true">2.14.4.</strong> Box<[JsValue]></a></li><li><a href="../reference/types/pointers.html"><strong aria-hidden="true">2.14.5.</strong> *const T and *mut T</a></li><li><a href="../reference/types/numbers.html"><strong aria-hidden="true">2.14.6.</strong> Numbers</a></li><li><a href="../reference/types/bool.html"><strong aria-hidden="true">2.14.7.</strong> bool</a></li><li><a href="../reference/types/char.html"><strong aria-hidden="true">2.14.8.</strong> char</a></li><li><a href="../reference/types/str.html"><strong aria-hidden="true">2.14.9.</strong> str</a></li><li><a href="../reference/types/string.html"><strong aria-hidden="true">2.14.10.</strong> String</a></li><li><a href="../reference/types/number-slices.html"><strong aria-hidden="true">2.14.11.</strong> Number Slices</a></li><li><a href="../reference/types/boxed-number-slices.html"><strong aria-hidden="true">2.14.12.</strong> Boxed Number Slices</a></li><li><a href="../reference/types/result.html"><strong aria-hidden="true">2.14.13.</strong> Result<T, JsValue></a></li></ol></li><li><a href="../reference/attributes/index.html"><strong aria-hidden="true">2.15.</strong> #[wasm_bindgen] Attributes</a></li><li><ol class="section"><li><a href="../reference/attributes/on-js-imports/index.html"><strong aria-hidden="true">2.15.1.</strong> On JavaScript Imports</a></li><li><ol class="section"><li><a href="../reference/attributes/on-js-imports/catch.html"><strong aria-hidden="true">2.15.1.1.</strong> catch</a></li><li><a href="../reference/attributes/on-js-imports/constructor.html"><strong aria-hidden="true">2.15.1.2.</strong> constructor</a></li><li><a href="../reference/attributes/on-js-imports/extends.html"><strong aria-hidden="true">2.15.1.3.</strong> extends</a></li><li><a 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class="section"><li><a href="../reference/attributes/on-rust-exports/constructor.html"><strong aria-hidden="true">2.15.2.1.</strong> constructor</a></li><li><a href="../reference/attributes/on-rust-exports/js_name.html"><strong aria-hidden="true">2.15.2.2.</strong> js_name = Blah</a></li><li><a href="../reference/attributes/on-rust-exports/readonly.html"><strong aria-hidden="true">2.15.2.3.</strong> readonly</a></li><li><a href="../reference/attributes/on-rust-exports/skip.html"><strong aria-hidden="true">2.15.2.4.</strong> skip</a></li><li><a href="../reference/attributes/on-rust-exports/start.html"><strong aria-hidden="true">2.15.2.5.</strong> start</a></li><li><a href="../reference/attributes/on-rust-exports/typescript_custom_section.html"><strong aria-hidden="true">2.15.2.6.</strong> typescript_custom_section</a></li><li><a href="../reference/attributes/on-rust-exports/getter-and-setter.html"><strong aria-hidden="true">2.15.2.7.</strong> getter and setter</a></li><li><a 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Translations</a></li><li><a href="../web-sys/inheritance.html"><strong aria-hidden="true">3.5.</strong> Inheritance</a></li><li><a href="../web-sys/unstable-apis.html"><strong aria-hidden="true">3.6.</strong> Unstable APIs</a></li></ol></li><li><a href="../wasm-bindgen-test/index.html"><strong aria-hidden="true">4.</strong> Testing with wasm-bindgen-test</a></li><li><ol class="section"><li><a href="../wasm-bindgen-test/usage.html"><strong aria-hidden="true">4.1.</strong> Usage</a></li><li><a href="../wasm-bindgen-test/asynchronous-tests.html"><strong aria-hidden="true">4.2.</strong> Writing Asynchronous Tests</a></li><li><a href="../wasm-bindgen-test/browsers.html"><strong aria-hidden="true">4.3.</strong> Testing in Headless Browsers</a></li><li><a href="../wasm-bindgen-test/continuous-integration.html"><strong aria-hidden="true">4.4.</strong> Continuous Integration</a></li></ol></li><li><a href="../contributing/index.html"><strong aria-hidden="true">5.</strong> Contributing to wasm-bindgen</a></li><li><ol class="section"><li><a href="../contributing/testing.html"><strong aria-hidden="true">5.1.</strong> Testing</a></li><li><a href="../contributing/design/index.html"><strong aria-hidden="true">5.2.</strong> Internal Design</a></li><li><ol class="section"><li><a href="../contributing/design/js-objects-in-rust.html"><strong aria-hidden="true">5.2.1.</strong> JS Objects in Rust</a></li><li><a href="../contributing/design/exporting-rust.html"><strong aria-hidden="true">5.2.2.</strong> Exporting a function to JS</a></li><li><a href="../contributing/design/exporting-rust-struct.html"><strong aria-hidden="true">5.2.3.</strong> Exporting a struct to JS</a></li><li><a href="../contributing/design/importing-js.html"><strong aria-hidden="true">5.2.4.</strong> Importing a function from JS</a></li><li><a href="../contributing/design/importing-js-struct.html"><strong aria-hidden="true">5.2.5.</strong> Importing a class from JS</a></li><li><a href="../contributing/design/rust-type-conversions.html"><strong aria-hidden="true">5.2.6.</strong> Rust Type conversions</a></li><li><a href="../contributing/design/describe.html"><strong aria-hidden="true">5.2.7.</strong> Types in wasm-bindgen</a></li></ol></li><li><a href="../contributing/js-sys/index.html"><strong aria-hidden="true">5.3.</strong> js-sys</a></li><li><ol class="section"><li><a href="../contributing/js-sys/testing.html"><strong aria-hidden="true">5.3.1.</strong> Testing</a></li><li><a href="../contributing/js-sys/adding-more-apis.html"><strong aria-hidden="true">5.3.2.</strong> Adding More APIs</a></li></ol></li><li><a href="../contributing/web-sys/index.html"><strong aria-hidden="true">5.4.</strong> web-sys</a></li><li><ol class="section"><li><a href="../contributing/web-sys/overview.html"><strong aria-hidden="true">5.4.1.</strong> Overview</a></li><li><a href="../contributing/web-sys/testing.html"><strong aria-hidden="true">5.4.2.</strong> Testing</a></li><li><a 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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>
|
|
<p>Many APIs on the web work with a <code>Promise</code>, such as an <code>async</code> function in JS.
|
|
Naturally you'll probably want to interoperate with them from Rust! To do that
|
|
you can use the <code>wasm-bindgen-futures</code> crate as well as Rust <code>async</code>
|
|
functions.</p>
|
|
<p>The first thing you might encounter is the need for working with a <code>Promise</code>.
|
|
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
|
|
values you can convert that value to <code>wasm_bindgen_futures::JsFuture</code>. This type
|
|
implements the <code>std::future::Future</code> trait which allows naturally using it in an
|
|
<code>async</code> function. For example:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
async fn get_from_js() -> Result<JsValue, JsValue> {
|
|
let promise = js_sys::Promise::resolve(&42.into());
|
|
let result = wasm_bindgen_futures::JsFuture::from(promise).await?;
|
|
Ok(result)
|
|
}
|
|
#}</code></pre></pre>
|
|
<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
|
|
a successful promise becomes <code>Ok</code> and an erroneous promise becomes <code>Err</code>.</p>
|
|
<p>Next up you'll probably want to export a Rust function to JS that returns a
|
|
promise. To do this you can use an <code>async</code> function and <code>#[wasm_bindgen]</code>:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
#[wasm_bindgen]
|
|
pub async fn foo() {
|
|
// ...
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>When invoked from JS the <code>foo</code> function here will return a <code>Promise</code>, so you can
|
|
import this as:</p>
|
|
<pre><code class="language-js">import { foo } from "my-module";
|
|
|
|
async function shim() {
|
|
const result = await foo();
|
|
// ...
|
|
}
|
|
</code></pre>
|
|
<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>
|
|
<p>When using an <code>async fn</code> in Rust and exporting it to JS there's some
|
|
restrictions on the return type. The return value of an exported Rust function
|
|
will eventually become <code>Result<JsValue, JsValue></code> where <code>Ok</code> turns into a
|
|
successfully resolved promise and <code>Err</code> is equivalent to throwing an exception.</p>
|
|
<p>The following types are supported as return types from an <code>async fn</code>:</p>
|
|
<ul>
|
|
<li><code>()</code> - turns into a successful <code>undefined</code> in JS</li>
|
|
<li><code>T: Into<JsValue></code> - turns into a successful JS value</li>
|
|
<li><code>Result<(), E: Into<JsValue>></code> - if <code>Ok(())</code> turns into a successful
|
|
<code>undefined</code> and otherwise turns into a failed promise with <code>E</code> converted to a
|
|
JS value</li>
|
|
<li><code>Result<T: Into<JsValue>, E: Into<JsValue>></code> - like the previous case except
|
|
both data payloads are converted into a <code>JsValue</code>.</li>
|
|
</ul>
|
|
<p>Note that many types implement being converted into a <code>JsValue</code>, such as all
|
|
imported types via <code>#[wasm_bindgen]</code> (aka those in <code>js-sys</code> or <code>web-sys</code>),
|
|
primitives like <code>u32</code>, and all exported <code>#[wasm_bindgen]</code> types. In general,
|
|
you should be able to write code without having too many explicit conversions,
|
|
and the macro should take care of the rest!</p>
|
|
<h2><a class="header" href="#using-wasm-bindgen-futures" id="using-wasm-bindgen-futures">Using <code>wasm-bindgen-futures</code></a></h2>
|
|
<p>The <code>wasm-bindgen-futures</code> crate bridges the gap between JavaScript <code>Promise</code>s
|
|
and Rust <code>Future</code>s. Its <code>JsFuture</code> type provides conversion from a JavaScript
|
|
<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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