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 can 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 --toolchain nightly
$ cargo +nightly 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 +nightly 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/

extern crate wasm_bindgen;
use wasm_bindgen::prelude::*;

extern "C" {
    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 +nightly 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, define our JS dependencies by creating a package.json:

  "scripts": {
    "serve": "webpack-dev-server"
  "devDependencies": {
    "html-webpack-plugin": "^3.2.0",
    "webpack": "^4.0.1",
    "webpack-cli": "^3.1.1",
    "webpack-dev-server": "^3.1.0"

and our webpack configuration:

// webpack.config.js
const path = require('path');
const HtmlWebpackPlugin = require('html-webpack-plugin');

module.exports = {
  entry: "./index.js",
  output: {
    path: path.resolve(__dirname, "dist"),
    filename: "index.js",
  plugins: [
    new HtmlWebpackPlugin({
      title: "Getting started with WASM"
  mode: "development"

And finally:

$ npm install
$ npm run serve

If you open http://localhost:8080 in a browser you should see a Hello, world! dialog pop up!

Notice that html-webpack-plugin has generated an HTML page which includes index.js.

Finally, you may want to deploy your application to a web server like Apache or NGINX. For that, simply run:

$ npx webpack

The output will be in the dist directory. You can now copy it to the root of your web server.

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.