wasm-bindgen 0.2.8

Easy support for interacting between JS and Rust.
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A project for facilitating high-level interactions between wasm modules and JS.

Introduction blog post

Build Status Build status

This project is sort of half polyfill for features like the host bindings proposal and half features for empowering high-level interactions between JS and wasm-compiled code (currently mostly from Rust). More specifically this project allows JS/wasm to communicate with strings, JS objects, classes, etc, as opposed to purely integers and floats. Using wasm-bindgen for example you can define a JS class in Rust or take a string from JS or return one. The functionality is growing as well!

Currently this tool is Rust-focused but the underlying foundation is language-independent, and it's hoping that over time as this tool stabilizes that it can be used for languages like C/C++!

Notable features of this project includes:

This project is still relatively new but feedback is of course always welcome! If you're curious about the design plus even more information about what this crate can do, check out the design doc.

Basic usage

Let's implement the equivalent of "Hello, world!" for this crate.

Note: Currently this projects uses nightly Rust which you can acquire through rustup and configure with rustup default nightly

If you'd like you dive straight into an online example, but if you'd prefer to follow along in your own console let's install the tools we need:

$ rustup target add wasm32-unknown-unknown
$ cargo install wasm-bindgen-cli

The first command here installs the wasm target so you can compile to it, and the latter will install the wasm-bindgen CLI tool we'll be using later.

Next up let's make our project

$ cargo new js-hello-world --lib

Now let's add a dependency on this project inside Cargo.toml as well as configuring our build output:

crate-type = ["cdylib"]

wasm-bindgen = "0.2"

Next up our actual code! We'll write this in src/lib.rs:

#![feature(proc_macro, wasm_custom_section, wasm_import_module)]

extern crate wasm_bindgen;

use wasm_bindgen::prelude::*;

extern {
    fn alert(s: &str);

pub fn greet(name: &str) {
    alert(&format!("Hello, {}!", name));

And that's it! If we were to write the greet function naively without the #[wasm_bindgen] attribute then JS wouldn't be able to communicate with the types like str, so slapping a #[wasm_bindgen] on the function and the import of alert ensures that the right shims are generated.

Next up let's build our project:

$ cargo build --target wasm32-unknown-unknown

After this you'll have a wasm file at target/wasm32-unknown-unknown/debug/js_hello_world.wasm. Don't be alarmed at the size, this is an unoptimized program!

Now that we've generated the wasm module it's time to run the bindgen tool itself! This tool will postprocess the wasm file rustc generated, generating a new wasm file and a set of JS bindings as well. Let's invoke it!

$ wasm-bindgen target/wasm32-unknown-unknown/debug/js_hello_world.wasm \
  --out-dir .

This is the main point where the magic happens. The js_hello_world.wasm file emitted by rustc contains descriptors of how to communicate via richer types than wasm currently supports. The wasm-bindgen tool will interpret this information, emitting a replacement module for the wasm file.

The previous js_hello_world.wasm file is interpreted as if it were an ES6 module. The js_hello_world.js file emitted by wasm-bindgen should have the intended interface of the wasm file, notably with rich types like strings, classes, etc.

The wasm-bindgen tool also emits a few other files needed to implement this module. For example js_hello_world_bg.wasm is the original wasm file but postprocessed a bit. It's intended that the js_hello_world_bg.wasm file, like before, acts like an ES6 module.

At this point you'll probably plug these files into a larger build system. Files emitted by wasm-bindgen act like normal ES6 modules (one just happens to be wasm). As of the time of this writing there's unfortunately not a lot of tools that natively do this, but Webpack's 4.0 beta release has native wasm support!. Let's take a look at that and see how it works.

First create an index.js file:

const js = import("./js_hello_world");

js.then(js => {

Note that we're using import(..) here because Webpack doesn't support synchronously importing modules from the main chunk just yet.

Next our JS dependencies by creating a package.json:

  "scripts": {
    "serve": "webpack-dev-server"
  "devDependencies": {
    "webpack": "^4.0.1",
    "webpack-cli": "^2.0.10",
    "webpack-dev-server": "^3.1.0"

and our webpack configuration

// webpack.config.js
const path = require('path');

module.exports = {
  entry: "./index.js",
  output: {
    path: path.resolve(__dirname, "dist"),
    filename: "index.js",
  mode: "development"

Our corresponding index.html:

    <meta content="text/html;charset=utf-8" http-equiv="Content-Type"/>
    <script src='./index.js'></script>

And finally:

$ npm run serve

If you open https://localhost:8080 in a browser you should see a Hello, world! dialog pop up! This works in Firefox out of the box but not in Chrome due to a webpack issue. See the hello_world README for a workaround.

If that was all a bit much, no worries! You can execute this code online thanks to WebAssembly Studio or you can follow along on GitHub to see all the files necessary as well as a script to set it all up.

What just happened?

Phew! That was a lot of words and a lot ended up happening along the way. There were two main pieces of magic happening: the #[wasm_bindgen] attribute and the wasm-bindgen CLI tool.

The #[wasm_bindgen] attribute

This attribute, exported from the wasm-bindgen crate, is the entrypoint to exposing Rust functions to JS. This is a procedural macro (hence requiring the nightly Rust toolchain) which will generate the appropriate shims in Rust to translate from your type signature to one that JS can interface with. Finally the attribute also serializes some information to the output artifact which wasm-bindgen-the-tool will discard after it parses.

There's a more thorough explanation below of the various bits and pieces of the attribute, but it suffices for now to say that you can attach it to free functions, structs, impl blocks for those structs and extern { ... } blocks. Some Rust features like generics, lifetime parameters, etc, aren't supported on functions tagged with #[wasm_bindgen] right now.

The wasm-bindgen CLI tool

The next half of what happened here was all in the wasm-bindgen tool. This tool opened up the wasm module that rustc generated and found an encoded description of what was passed to the #[wasm_bindgen] attribute. You can think of this as the #[wasm_bindgen] attribute created a special section of the output module which wasm-bindgen strips and processes.

This information gave wasm-bindgen all it needed to know to generate the JS file that we then imported. The JS file wraps instantiating the underlying wasm module (aka calling WebAssembly.instantiate) and then provides wrappers for classes/functions within.

What else can we do?

Much more! Here's a taste of various features you can use in this project. You can also explore this code online:

// src/lib.rs
#![feature(proc_macro, wasm_custom_section, wasm_import_module)]

extern crate wasm_bindgen;

use wasm_bindgen::prelude::*;

// Strings can both be passed in and received
pub fn concat(a: &str, b: &str) -> String {
    let mut a = a.to_string();
    return a

// A struct will show up as a class on the JS side of things
pub struct Foo {
    contents: u32,

impl Foo {
    pub fn new() -> Foo {
        Foo { contents: 0 }

    // Methods can be defined with `&mut self` or `&self`, and arguments you
    // can pass to a normal free function also all work in methods.
    pub fn add(&mut self, amt: u32) -> u32 {
        self.contents += amt;
        return self.contents

    // You can also take a limited set of references to other types as well.
    pub fn add_other(&mut self, bar: &Bar) {
        self.contents += bar.contents;

    // Ownership can work too!
    pub fn consume_other(&mut self, bar: Bar) {
        self.contents += bar.contents;

pub struct Bar {
    contents: u32,
    opaque: JsValue, // defined in `wasm_bindgen`, imported via prelude

#[wasm_bindgen(module = "./index")] // what ES6 module to import from
extern {
    fn bar_on_reset(to: &str, opaque: &JsValue);

    // We can import classes and annotate functionality on those classes as well
    type Awesome;
    fn new() -> Awesome;
    fn get_internal(this: &Awesome) -> u32;

impl Bar {
    pub fn from_str(s: &str, opaque: JsValue) -> Bar {
        let contents = s.parse().unwrap_or_else(|_| {
        Bar { contents, opaque }

    pub fn reset(&mut self, s: &str) {
        if let Ok(n) = s.parse() {
            bar_on_reset(s, &self.opaque);
            self.contents = n;

The generated JS bindings for this invocation of the macro look like this. You can view them in action like so:

and our corresponding index.js:

import { Foo, Bar, concat } from "./js_hello_world";
import { booted } from "./js_hello_world_wasm";

export function bar_on_reset(s, token) {
  console.log(`this instance of bar was reset to ${s}`);

function assertEq(a, b) {
  if (a !== b)
    throw new Error(`${a} != ${b}`);
  console.log(`found ${a} === ${b}`);

function main() {
  assertEq(concat('a', 'b'), 'ab');

  // Note the `new Foo()` syntax cannot be used, static function
  // constructors must be used instead. Additionally objects allocated
  // corresponding to Rust structs will need to be deallocated on the
  // Rust side of things with an explicit call to `free`.
  let foo = Foo.new();
  assertEq(foo.add(10), 10);

  // Pass objects to one another
  let foo1 = Foo.new();
  let bar = Bar.from_str("22", { opaque: 'object' });

  // We also don't have to `free` the `bar` variable as this function is
  // transferring ownership to `foo1`

  assertEq(foo1.add(2), 22 + 34 + 2);

  alert('all passed!')

export class Awesome {
  constructor() {
    this.internal = 32;

  get_internal() {
    return this.internal;



The #[wasm_bindgen] attribute supports some Rust closures being passed to JS. Examples of what you can do are:

extern {
    fn foo(a: &Fn()); // could also be `&mut FnMut()`

Here a function foo is imported from JS where the first argument is a stack closure. You can call this function with a &Fn() argument and JS will receive a JS function. When the foo function returns, however, the JS function will be invalidated and any future usage of it will raise an exception.

Closures also support arguments and return values like exports do, for example:

extern {
    type Foo;

    fn bar(a: &Fn(u32, String) -> Foo);

Sometimes the stack behavior of these 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 JS through setTimeout. For this you want the imported function to return but the JS closure still needs to be valid!

To support this use case you can do:

use wasm_bindgen::prelude::*;

extern {
    fn baz(a: &Closure<Fn()>);

The Closure type is defined in the wasm_bindgen crate and represents a "long lived" closure. The JS closure passed to baz is still valid after baz returns, and the validity of the JS closure is tied to the lifetime of the Closure in Rust. Once Closure is dropped it will deallocate its internal memory and invalidate the corresponding JS function.

Like stack closures a Closure also supports FnMut:

use wasm_bindgen::prelude::*;

extern {
    fn another(a: &Closure<FnMut() -> u32>);

At this time you cannot pass a JS closure to Rust, you can only pass a Rust closure to JS in limited circumstances.

Feature reference

Here this section will attempt to be a reference for the various features implemented in this project. This is likely not exhaustive but the tests should also be a great place to look for examples.

The #[wasm_bindgen] attribute can be attached to functions, structs, impls, and foreign modules. Impls can only contain functions, and the attribute cannot be attached to functions in an impl block or functions in a foreign module. No lifetime parameters or type parameters are allowed on any of these types. Foreign modules must have the "C" abi (or none listed). Free functions with #[wasm_bindgen] might no have the "C" abi or none listed and also not needed to annotate with the #[no_mangle] attribute.

All structs referenced through arguments to functions should be defined in the macro itself. Arguments allowed implement the WasmBoundary trait, and examples are:

  • Integers (not u64/i64)
  • Floats
  • Borrowed strings (&str)
  • Owned strings (String)
  • Exported structs (Foo, annotated with #[wasm_bindgen])
  • Exported C-like enums (Foo, annotated with #[wasm_bindgen])
  • Imported types in a foreign module annotated with #[wasm_bindgen]
  • Borrowed exported structs (&Foo or &mut Bar)
  • The JsValue type and &JsValue (not mutable references)
  • Vectors and slices of supported integer types and of the JsValue type.

All of the above can also be returned except borrowed references. Passing Vec<JsValue> as an argument to a function is not currently supported. Strings are implemented with shim functions to copy data in/out of the Rust heap. That is, a string passed to Rust from JS is copied to the Rust heap (using a generated shim to malloc some space) and then will be freed appropriately.

Owned values are implemented through boxes. When you return a Foo it's actually turned into Box<RefCell<Foo>> under the hood and returned to JS as a pointer. The pointer is to have a defined ABI, and the RefCell is to ensure safety with reentrancy and aliasing in JS. In general you shouldn't see RefCell panics with normal usage.

JS-values-in-Rust are implemented through indexes that index a table generated as part of the JS bindings. This table is managed via the ownership specified in Rust and through the bindings that we're returning. More information about this can be found in the design doc.

All of these constructs currently create relatively straightforward code on the JS side of things, mostly having a 1:1 match in Rust with JS.

CLI Reference

The wasm-bindgen tool has a number of options available to it to tweak the JS that is generated. By default the generated JS uses ES modules and is compatible with both Node and browsers (but will likely require a bundler for both use cases).

Supported flags of the CLI tool can be learned via wasm-bindgen --help, but some notable options are:

  • --nodejs - this flag will tailor output for Node instead of browsers, allowing for native usage of require of the generated JS and internally using require instead of ES modules. When using this flag no further postprocessing (aka a bundler) should be necessary to work with the wasm.

  • --browser - this flag will tailor the output specifically for browsers, making it incompatible with Node. This will basically make the generated JS a tiny bit smaller as runtime checks for Node won't be necessary.

  • --no-modules - the default output of wasm-bindgen uses ES modules but this option indicates that ES modules should not be used and output should be tailored for a web browser. In this mode window.wasm_bindgen will be a function that takes a path to the wasm file to fetch and instantiate. Afterwards exported functions from the wasm are available through window.wasm_bindgen.foo. Note that the name wasm_bindgen can be configured with the --no-modules-global FOO flag.

  • --no-typescript - by default a *.d.ts file is generated for the generated JS file, but this flag will disable generating this TypeScript file.

  • --debug - generates a bit more JS and wasm in "debug mode" to help catch programmer errors, but this output isn't intended to be shipped to production


This project is licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


In order to run the tests you will need node.js version 8.9.4 or above. Running the tests is done by running cargo test.