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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" class="active"><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"><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 href="../reference/attributes/on-js-imports/getter-and-setter.html"><strong aria-hidden="true">2.15.1.4.</strong> getter and setter</a></li><li><a href="../reference/attributes/on-js-imports/final.html"><strong aria-hidden="true">2.15.1.5.</strong> final</a></li><li><a href="../reference/attributes/on-js-imports/indexing-getter-setter-deleter.html"><strong aria-hidden="true">2.15.1.6.</strong> indexing_getter, indexing_setter, and indexing_deleter</a></li><li><a href="../reference/attributes/on-js-imports/js_class.html"><strong aria-hidden="true">2.15.1.7.</strong> js_class = "Blah"</a></li><li><a href="../reference/attributes/on-js-imports/js_name.html"><strong aria-hidden="true">2.15.1.8.</strong> js_name</a></li><li><a href="../reference/attributes/on-js-imports/js_namespace.html"><strong aria-hidden="true">2.15.1.9.</strong> js_namespace</a></li><li><a href="../reference/attributes/on-js-imports/method.html"><strong aria-hidden="true">2.15.1.10.</strong> method</a></li><li><a href="../reference/attributes/on-js-imports/module.html"><strong aria-hidden="true">2.15.1.11.</strong> module = "blah"</a></li><li><a href="../reference/attributes/on-js-imports/raw_module.html"><strong aria-hidden="true">2.15.1.12.</strong> raw_module = "blah"</a></li><li><a href="../reference/attributes/on-js-imports/static_method_of.html"><strong aria-hidden="true">2.15.1.13.</strong> static_method_of = Blah</a></li><li><a href="../reference/attributes/on-js-imports/structural.html"><strong aria-hidden="true">2.15.1.14.</strong> structural</a></li><li><a href="../reference/attributes/on-js-imports/variadic.html"><strong aria-hidden="true">2.15.1.15.</strong> variadic</a></li><li><a href="../reference/attributes/on-js-imports/vendor_prefix.html"><strong aria-hidden="true">2.15.1.16.</strong> vendor_prefix</a></li></ol></li><li><a href="../reference/attributes/on-rust-exports/index.html"><strong aria-hidden="true">2.15.2.</strong> On Rust Exports</a></li><li><ol 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 href="../reference/attributes/on-rust-exports/inspectable.html"><strong aria-hidden="true">2.15.2.8.</strong> inspectable</a></li><li><a href="../reference/attributes/on-rust-exports/skip_typescript.html"><strong aria-hidden="true">2.15.2.9.</strong> skip_typescript</a></li><li><a href="../reference/attributes/on-rust-exports/typescript_type.html"><strong aria-hidden="true">2.15.2.10.</strong> typescript_type</a></li></ol></li></ol></li></ol></li><li><a href="../web-sys/index.html"><strong aria-hidden="true">3.</strong> web-sys</a></li><li><ol class="section"><li><a href="../web-sys/using-web-sys.html"><strong aria-hidden="true">3.1.</strong> Using web-sys</a></li><li><a href="../web-sys/cargo-features.html"><strong aria-hidden="true">3.2.</strong> Cargo Features</a></li><li><a href="../web-sys/function-overloads.html"><strong aria-hidden="true">3.3.</strong> Function Overloads</a></li><li><a href="../web-sys/type-translations.html"><strong aria-hidden="true">3.4.</strong> Type 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 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<h1><a class="header" href="#passing-rust-closures-to-imported-javascript-functions" id="passing-rust-closures-to-imported-javascript-functions">Passing Rust Closures to Imported JavaScript Functions</a></h1>
<p>The <code>#[wasm_bindgen]</code> attribute supports Rust closures being passed to
JavaScript in two variants:</p>
<ol>
<li>
<p>Stack-lifetime closures that should not be invoked by JavaScript again after
the imported JavaScript function that the closure was passed to returns.</p>
</li>
<li>
<p>Heap-allocated closures that can be invoked any number of times, but must be
explicitly deallocated when finished.</p>
</li>
</ol>
<h2><a class="header" href="#stack-lifetime-closures" id="stack-lifetime-closures">Stack-Lifetime Closures</a></h2>
<p>Closures with a stack lifetime are passed to JavaScript as either <code>&Fn</code> or <code>&mut FnMut</code> trait objects:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
// Import JS functions that take closures
#[wasm_bindgen]
extern "C" {
fn takes_immutable_closure(f: &Fn());
fn takes_mutable_closure(f: &mut FnMut());
}
// Usage
takes_immutable_closure(&|| {
// ...
});
let mut times_called = 0;
takes_mutable_closure(&mut || {
times_called += 1;
});
#}</code></pre></pre>
<p><strong>Once these imported functions return, the closures that were given to them
will become invalidated, and any future attempts to call those closures from
JavaScript will raise an exception.</strong></p>
<p>Closures also support arguments and return values like exports do, for example:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen]
extern "C" {
fn takes_closure_that_takes_int_and_returns_string(x: &Fn(u32) -> String);
}
takes_closure_that_takes_int_and_returns_string(&|x: u32| -> String {
format!("x is {}", x)
});
#}</code></pre></pre>
<h2><a class="header" href="#heap-allocated-closures" id="heap-allocated-closures">Heap-Allocated Closures</a></h2>
<p>Sometimes the discipline of stack-lifetime closures is not desired. For example,
you'd like to schedule a closure to be run on the next turn of the event loop in
JavaScript through <code>setTimeout</code>. For this, you want the imported function to
return but the JavaScript closure still needs to be valid!</p>
<p>For this scenario, you need the <code>Closure</code> type, which is defined in the
<code>wasm_bindgen</code> crate, exported in <code>wasm_bindgen::prelude</code>, and represents a
"long lived" closure.
The <code>Closure</code> type is currently behind a feature which needs to be enabled:</p>
<pre><code class="language-toml">[dependencies]
wasm-bindgen = {version = "^0.2", features = ["nightly"]}
</code></pre>
<p>The validity of the JavaScript closure is tied to the lifetime of the <code>Closure</code>
in Rust. <strong>Once a <code>Closure</code> is dropped, it will deallocate its internal memory
and invalidate the corresponding JavaScript function so that any further
attempts to invoke it raise an exception.</strong></p>
<p>Like stack closures a <code>Closure</code> supports both <code>Fn</code> and <code>FnMut</code> closures, as well
as arguments and returns.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen]
extern "C" {
fn setInterval(closure: &Closure<FnMut()>, millis: u32) -> f64;
fn cancelInterval(token: f64);
#[wasm_bindgen(js_namespace = console)]
fn log(s: &str);
}
#[wasm_bindgen]
pub struct Interval {
closure: Closure<FnMut()>,
token: f64,
}
impl Interval {
pub fn new<F: 'static>(millis: u32, f: F) -> Interval
where
F: FnMut()
{
// Construct a new closure.
let closure = Closure::new(f);
// Pass the closuer to JS, to run every n milliseconds.
let token = setInterval(&closure, millis);
Interval { closure, token }
}
}
// When the Interval is destroyed, cancel its `setInterval` timer.
impl Drop for Interval {
fn drop(&mut self) {
cancelInterval(self.token);
}
}
// Keep logging "hello" every second until the resulting `Interval` is dropped.
#[wasm_bindgen]
pub fn hello() -> Interval {
Interval::new(1_000, || log("hello"))
}
#}</code></pre></pre>
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