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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"><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" class="active"><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 href="../../contributing/web-sys/logging.html"><strong aria-hidden="true">5.4.3.</strong> Logging</a></li><li><a href="../../contributing/web-sys/supporting-more-web-apis.html"><strong aria-hidden="true">5.4.4.</strong> Supporting More Web APIs</a></li></ol></li><li><a href="../../contributing/publishing.html"><strong aria-hidden="true">5.5.</strong> Publishing</a></li><li><a href="../../contributing/team.html"><strong aria-hidden="true">5.6.</strong> Team</a></li></ol></li></ol>
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<div id="content" class="content">
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<main>
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<h1><a class="header" href="#exporting-a-function-to-js" id="exporting-a-function-to-js">Exporting a function to JS</a></h1>
|
|
<p>Alright now that we've got a good grasp on JS objects and how they're working,
|
|
let's take a look at another feature of <code>wasm-bindgen</code>: exporting functionality
|
|
with types that are richer than just numbers.</p>
|
|
<p>The basic idea around exporting functionality with more flavorful types is that
|
|
the wasm exports won't actually be called directly. Instead the generated
|
|
<code>foo.js</code> module will have shims for all exported functions in the wasm module.</p>
|
|
<p>The most interesting conversion here happens with strings so let's take a look
|
|
at that.</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
#[wasm_bindgen]
|
|
pub fn greet(a: &str) -> String {
|
|
format!("Hello, {}!", a)
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>Here we'd like to define an ES module that looks like</p>
|
|
<pre><code class="language-ts">// foo.d.ts
|
|
export function greet(a: string): string;
|
|
</code></pre>
|
|
<p>To see what's going on, let's take a look at the generated shim</p>
|
|
<pre><code class="language-js">import * as wasm from './foo_bg';
|
|
|
|
function passStringToWasm(arg) {
|
|
const buf = new TextEncoder('utf-8').encode(arg);
|
|
const len = buf.length;
|
|
const ptr = wasm.__wbindgen_malloc(len);
|
|
let array = new Uint8Array(wasm.memory.buffer);
|
|
array.set(buf, ptr);
|
|
return [ptr, len];
|
|
}
|
|
|
|
function getStringFromWasm(ptr, len) {
|
|
const mem = new Uint8Array(wasm.memory.buffer);
|
|
const slice = mem.slice(ptr, ptr + len);
|
|
const ret = new TextDecoder('utf-8').decode(slice);
|
|
return ret;
|
|
}
|
|
|
|
export function greet(arg0) {
|
|
const [ptr0, len0] = passStringToWasm(arg0);
|
|
try {
|
|
const ret = wasm.greet(ptr0, len0);
|
|
const ptr = wasm.__wbindgen_boxed_str_ptr(ret);
|
|
const len = wasm.__wbindgen_boxed_str_len(ret);
|
|
const realRet = getStringFromWasm(ptr, len);
|
|
wasm.__wbindgen_boxed_str_free(ret);
|
|
return realRet;
|
|
} finally {
|
|
wasm.__wbindgen_free(ptr0, len0);
|
|
}
|
|
}
|
|
</code></pre>
|
|
<p>Phew, that's quite a lot! We can sort of see though if we look closely what's
|
|
happening:</p>
|
|
<ul>
|
|
<li>
|
|
<p>Strings are passed to wasm via two arguments, a pointer and a length. Right
|
|
now we have to copy the string onto the wasm heap which means we'll be using
|
|
<code>TextEncoder</code> to actually do the encoding. Once this is done we use an
|
|
internal function in <code>wasm-bindgen</code> to allocate space for the string to go,
|
|
and then we'll pass that ptr/length to wasm later on.</p>
|
|
</li>
|
|
<li>
|
|
<p>Returning strings from wasm is a little tricky as we need to return a ptr/len
|
|
pair, but wasm currently only supports one return value (multiple return values
|
|
<a href="https://github.com/WebAssembly/design/issues/1146">is being standardized</a>).
|
|
To work around this in the meantime, we're actually returning a pointer to a
|
|
ptr/len pair, and then using functions to access the various fields.</p>
|
|
</li>
|
|
<li>
|
|
<p>Some cleanup ends up happening in wasm. The <code>__wbindgen_boxed_str_free</code>
|
|
function is used to free the return value of <code>greet</code> after it's been decoded
|
|
onto the JS heap (using <code>TextDecoder</code>). The <code>__wbindgen_free</code> is then used to
|
|
free the space we allocated to pass the string argument once the function call
|
|
is done.</p>
|
|
</li>
|
|
</ul>
|
|
<p>Next let's take a look at the Rust side of things as well. Here we'll be looking
|
|
at a mostly abbreviated and/or "simplified" in the sense of this is what it
|
|
compiles down to:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
pub extern "C" fn greet(a: &str) -> String {
|
|
format!("Hello, {}!", a)
|
|
}
|
|
|
|
#[export_name = "greet"]
|
|
pub extern "C" fn __wasm_bindgen_generated_greet(
|
|
arg0_ptr: *const u8,
|
|
arg0_len: usize,
|
|
) -> *mut String {
|
|
let arg0 = unsafe {
|
|
let slice = ::std::slice::from_raw_parts(arg0_ptr, arg0_len);
|
|
::std::str::from_utf8_unchecked(slice)
|
|
};
|
|
let _ret = greet(arg0);
|
|
Box::into_raw(Box::new(_ret))
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>Here we can see again that our <code>greet</code> function is unmodified and has a wrapper
|
|
to call it. This wrapper will take the ptr/len argument and convert it to a
|
|
string slice, while the return value is boxed up into just a pointer and is
|
|
then returned up to was for reading via the <code>__wbindgen_boxed_str_*</code> functions.</p>
|
|
<p>So in general exporting a function involves a shim both in JS and in Rust with
|
|
each side translating to or from wasm arguments to the native types of each
|
|
language. The <code>wasm-bindgen</code> tool manages hooking up all these shims while the
|
|
<code>#[wasm_bindgen]</code> macro takes care of the Rust shim as well.</p>
|
|
<p>Most arguments have a relatively clear way to convert them, bit if you've got
|
|
any questions just let me know!</p>
|
|
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