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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"><
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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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</a>. Features documented here may not be available in released versions of
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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="#polyfill-for-js-objects-in-wasm" id="polyfill-for-js-objects-in-wasm">Polyfill for "JS objects in wasm"</a></h1>
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<p>One of the main goals of <code>wasm-bindgen</code> is to allow working with and passing
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around JS objects in wasm, but that's not allowed today! While indeed true,
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that's where the polyfill comes in.</p>
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<p>The question here is how we shoehorn JS objects into a <code>u32</code> for wasm to use.
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The current strategy for this approach is to maintain a module-local variable
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in the generated <code>foo.js</code> file: a <code>heap</code>.</p>
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<h3><a class="header" href="#temporary-js-objects-on-the-stack" id="temporary-js-objects-on-the-stack">Temporary JS objects on the "stack"</a></h3>
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<p>The first slots in the <code>heap</code> in <code>foo.js</code> are considered a stack. This stack,
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like typical program execution stacks, grows down. JS objects are pushed on the
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bottom of the stack, and their index in the stack is the identifier that's passed
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to wasm. A stack pointer is maintained to figure out where the next item is
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pushed.</p>
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<p>JS objects are then only removed from the bottom of the stack as well. Removal
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is simply storing null then incrementing a counter. Because of the "stack-y"
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nature of this scheme it only works for when wasm doesn't hold onto a JS object
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(aka it only gets a "reference" in Rust parlance).</p>
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<p>Let's take a look at an 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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// foo.rs
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#[wasm_bindgen]
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pub fn foo(a: &JsValue) {
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// ...
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}
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#}</code></pre></pre>
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<p>Here we're using the special <code>JsValue</code> type from the <code>wasm-bindgen</code> library
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itself. Our exported function, <code>foo</code>, takes a <em>reference</em> to an object. This
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notably means that it can't persist the object past the lifetime of this
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function call.</p>
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<p>Now what we actually want to generate is a JS module that looks like (in
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TypeScript parlance)</p>
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<pre><code class="language-ts">// foo.d.ts
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export function foo(a: any);
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</code></pre>
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<p>and what we actually generate looks something like:</p>
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<pre><code class="language-js">// foo.js
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import * as wasm from './foo_bg';
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const heap = new Array(32);
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heap.push(undefined, null, true, false);
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let stack_pointer = 32;
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function addBorrowedObject(obj) {
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stack_pointer -= 1;
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heap[stack_pointer] = obj;
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return stack_pointer;
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}
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export function foo(arg0) {
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const idx0 = addBorrowedObject(arg0);
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try {
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wasm.foo(idx0);
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} finally {
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heap[stack_pointer++] = undefined;
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}
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}
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</code></pre>
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<p>Here we can see a few notable points of action:</p>
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<ul>
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<li>The wasm file was renamed to <code>foo_bg.wasm</code>, and we can see how the JS module
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generated here is importing from the wasm file.</li>
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<li>Next we can see our <code>heap</code> module variable which is to store all JS values
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reference-able from wasm.</li>
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<li>Our exported function <code>foo</code>, takes an arbitrary argument, <code>arg0</code>, which is
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converted to an index with the <code>addBorrowedObject</code> object function. The index
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is then passed to wasm so wasm can operate with it.</li>
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<li>Finally, we have a <code>finally</code> which frees the stack slot as it's no longer
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used, popping the value that was pushed at the start of the function.</li>
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</ul>
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<p>It's also helpful to dig into the Rust side of things to see what's going on
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there! Let's take a look at the code that <code>#[wasm_bindgen]</code> generates in Rust:</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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// what the user wrote
|
||
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pub fn foo(a: &JsValue) {
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||
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// ...
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||
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}
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||
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#[export_name = "foo"]
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|
|
pub extern "C" fn __wasm_bindgen_generated_foo(arg0: u32) {
|
||
|
|
let arg0 = unsafe {
|
||
|
|
ManuallyDrop::new(JsValue::__from_idx(arg0))
|
||
|
|
};
|
||
|
|
let arg0 = &*arg0;
|
||
|
|
foo(arg0);
|
||
|
|
}
|
||
|
|
#}</code></pre></pre>
|
||
|
|
<p>And as with the JS, the notable points here are:</p>
|
||
|
|
<ul>
|
||
|
|
<li>The original function, <code>foo</code>, is unmodified in the output</li>
|
||
|
|
<li>A generated function here (with a unique name) is the one that's actually
|
||
|
|
exported from the wasm module</li>
|
||
|
|
<li>Our generated function takes an integer argument (our index) and then wraps it
|
||
|
|
in a <code>JsValue</code>. There's some trickery here that's not worth going into just
|
||
|
|
yet, but we'll see in a bit what's happening under the hood.</li>
|
||
|
|
</ul>
|
||
|
|
<h3><a class="header" href="#long-lived-js-objects" id="long-lived-js-objects">Long-lived JS objects</a></h3>
|
||
|
|
<p>The above strategy is useful when JS objects are only temporarily used in Rust,
|
||
|
|
for example only during one function call. Sometimes, though, objects may have a
|
||
|
|
dynamic lifetime or otherwise need to be stored on Rust's heap. To cope with
|
||
|
|
this there's a second half of management of JS objects, naturally corresponding
|
||
|
|
to the other side of the JS <code>heap</code> array.</p>
|
||
|
|
<p>JS Objects passed to wasm that are not references are assumed to have a dynamic
|
||
|
|
lifetime inside of the wasm module. As a result the strict push/pop of the stack
|
||
|
|
won't work and we need more permanent storage for the JS objects. To cope with
|
||
|
|
this we build our own "slab allocator" of sorts.</p>
|
||
|
|
<p>A picture (or code) is worth a thousand words so let's show what happens with an
|
||
|
|
example.</p>
|
||
|
|
<pre><pre class="playpen"><code class="language-rust">
|
||
|
|
# #![allow(unused_variables)]
|
||
|
|
#fn main() {
|
||
|
|
// foo.rs
|
||
|
|
#[wasm_bindgen]
|
||
|
|
pub fn foo(a: JsValue) {
|
||
|
|
// ...
|
||
|
|
}
|
||
|
|
#}</code></pre></pre>
|
||
|
|
<p>Note that the <code>&</code> is missing in front of the <code>JsValue</code> we had before, and in
|
||
|
|
Rust parlance this means it's taking ownership of the JS value. The exported ES
|
||
|
|
module interface is the same as before, but the ownership mechanics are slightly
|
||
|
|
different. Let's see the generated JS's slab in action:</p>
|
||
|
|
<pre><code class="language-js">import * as wasm from './foo_bg'; // imports from wasm file
|
||
|
|
|
||
|
|
const heap = new Array(32);
|
||
|
|
heap.push(undefined, null, true, false);
|
||
|
|
let heap_next = 36;
|
||
|
|
|
||
|
|
function addHeapObject(obj) {
|
||
|
|
if (heap_next === heap.length)
|
||
|
|
heap.push(heap.length + 1);
|
||
|
|
const idx = heap_next;
|
||
|
|
heap_next = heap[idx];
|
||
|
|
heap[idx] = obj;
|
||
|
|
return idx;
|
||
|
|
}
|
||
|
|
|
||
|
|
export function foo(arg0) {
|
||
|
|
const idx0 = addHeapObject(arg0);
|
||
|
|
wasm.foo(idx0);
|
||
|
|
}
|
||
|
|
|
||
|
|
export function __wbindgen_object_drop_ref(idx) {
|
||
|
|
heap[idx ] = heap_next;
|
||
|
|
heap_next = idx;
|
||
|
|
}
|
||
|
|
</code></pre>
|
||
|
|
<p>Unlike before we're now calling <code>addHeapObject</code> on the argument to <code>foo</code> rather
|
||
|
|
than <code>addBorrowedObject</code>. This function will use <code>heap</code> and <code>heap_next</code> as a
|
||
|
|
slab allocator to acquire a slot to store the object, placing a structure there
|
||
|
|
once it's found. Note that this is going on the right-half of the array, unlike
|
||
|
|
the stack which resides on the left half. This discipline mirrors the stack/heap
|
||
|
|
in normal programs, roughly.</p>
|
||
|
|
<p>Another curious aspect of this generated module is the
|
||
|
|
<code>__wbindgen_object_drop_ref</code> function. This is one that's actually imported to
|
||
|
|
wasm rather than used in this module! This function is used to signal the end of
|
||
|
|
the lifetime of a <code>JsValue</code> in Rust, or in other words when it goes out of
|
||
|
|
scope. Otherwise though this function is largely just a general "slab free"
|
||
|
|
implementation.</p>
|
||
|
|
<p>And finally, let's take a look at the Rust generated again too:</p>
|
||
|
|
<pre><pre class="playpen"><code class="language-rust">
|
||
|
|
# #![allow(unused_variables)]
|
||
|
|
#fn main() {
|
||
|
|
// what the user wrote
|
||
|
|
pub fn foo(a: JsValue) {
|
||
|
|
// ...
|
||
|
|
}
|
||
|
|
|
||
|
|
#[export_name = "foo"]
|
||
|
|
pub extern "C" fn __wasm_bindgen_generated_foo(arg0: u32) {
|
||
|
|
let arg0 = unsafe {
|
||
|
|
JsValue::__from_idx(arg0)
|
||
|
|
};
|
||
|
|
foo(arg0);
|
||
|
|
}
|
||
|
|
#}</code></pre></pre>
|
||
|
|
<p>Ah that looks much more familiar! Not much interesting is happening here, so
|
||
|
|
let's move on to...</p>
|
||
|
|
<h3><a class="header" href="#anatomy-of-jsvalue" id="anatomy-of-jsvalue">Anatomy of <code>JsValue</code></a></h3>
|
||
|
|
<p>Currently the <code>JsValue</code> struct is actually quite simple in Rust, it's:</p>
|
||
|
|
<pre><pre class="playpen"><code class="language-rust">
|
||
|
|
# #![allow(unused_variables)]
|
||
|
|
#fn main() {
|
||
|
|
pub struct JsValue {
|
||
|
|
idx: u32,
|
||
|
|
}
|
||
|
|
|
||
|
|
// "private" constructors
|
||
|
|
|
||
|
|
impl Drop for JsValue {
|
||
|
|
fn drop(&mut self) {
|
||
|
|
unsafe {
|
||
|
|
__wbindgen_object_drop_ref(self.idx);
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
#}</code></pre></pre>
|
||
|
|
<p>Or in other words it's a newtype wrapper around a <code>u32</code>, the index that we're
|
||
|
|
passed from wasm. The destructor here is where the <code>__wbindgen_object_drop_ref</code>
|
||
|
|
function is called to relinquish our reference count of the JS object, freeing
|
||
|
|
up our slot in the <code>slab</code> that we saw above.</p>
|
||
|
|
<p>If you'll recall as well, when we took <code>&JsValue</code> above we generated a wrapper
|
||
|
|
of <code>ManuallyDrop</code> around the local binding, and that's because we wanted to
|
||
|
|
avoid invoking this destructor when the object comes from the stack.</p>
|
||
|
|
<h3><a class="header" href="#working-with-heap-in-reality" id="working-with-heap-in-reality">Working with <code>heap</code> in reality</a></h3>
|
||
|
|
<p>The above explanations are pretty close to what happens today, but in reality
|
||
|
|
there's a few differences especially around handling constant values like
|
||
|
|
<code>undefined</code>, <code>null</code>, etc. Be sure to check out the actual generated JS and the
|
||
|
|
generation code for the full details!</p>
|
||
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