This is unpublished documentation of working with Rust and WebAssembly, the published documentation is available on the main Rust and WebAssembly documentation site . Features documented here may not be available in released versions of tooling for Rust and WebAssembly.

Hello, World!

This section will show you how to build and run your first Rust and WebAssembly program: a Web page that alerts "Hello, World!"

Make sure you have followed the setup instructions before beginning.

Clone the Project Template

The project template comes pre-configured with sane defaults, so you can quickly build, integrate, and package your code for the Web.

Clone the project template with this command:

cargo generate --git

This should prompt you for the new project's name. We will use "wasm-game-of-life".


What's Inside

Enter the new wasm-game-of-life project

cd wasm-game-of-life

and let's take a look at its contents:

├── Cargo.toml
└── src

Let's take a look at a couple of these files in detail.


The Cargo.toml file specifies dependencies and metadata for cargo, Rust's package manager and build tool. This one comes pre-configured with a wasm-bindgen dependency, a few optional dependencies we will dig into later, and the crate-type properly initialized for generating .wasm libraries.


The src/ file is the root of the Rust crate that we are compiling to WebAssembly. It uses wasm-bindgen to interface with JavaScript. It imports the window.alert JavaScript function, and exports the greet Rust function, which alerts a greeting message.

# #![allow(unused_variables)]
#fn main() {
extern crate cfg_if;
extern crate wasm_bindgen;

mod utils;

use cfg_if::cfg_if;
use wasm_bindgen::prelude::*;

cfg_if! {
    // When the `wee_alloc` feature is enabled, use `wee_alloc` as the global
    // allocator.
    if #[cfg(feature = "wee_alloc")] {
        extern crate wee_alloc;
        static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;

extern {
    fn alert(s: &str);

pub fn greet() {
    alert("Hello, wasm-game-of-life!");



The src/ module provides common utilities to make working with Rust compiled to WebAssembly easier. We will take a look at some of these utilities in more detail later in the tutorial, such as when we look at debugging our wasm code, but we can ignore this file for now.

Build the Project

We use wasm-pack to orchestrate the following build steps:

  • Ensure that we have Rust 1.30 or newer and the wasm32-unknown-unknown target installed via rustup,
  • Compile our Rust sources into a WebAssembly .wasm binary via cargo,
  • Use wasm-bindgen to generate the JavaScript API for using our Rust-generated WebAssembly.

To do all of that, run this command inside the project directory:

wasm-pack build

When the build has completed, we can find its artifacts in the pkg directory, and it should have these contents:

├── package.json
├── wasm_game_of_life_bg.wasm
├── wasm_game_of_life.d.ts
└── wasm_game_of_life.js

The file is copied from the main project, but the others are completely new.


The .wasm file is the WebAssembly binary that is generated by the Rust compiler from our Rust sources. It contains the compiled-to-wasm versions of all of our Rust functions and data. For example, it has an exported "greet" function.


The .js file is generated by wasm-bindgen and contains JavaScript glue for importing DOM and JavaScript functions into Rust and exposing a nice API to the WebAssembly functions to JavaScript. For example, there is a JavaScript greet function that wraps the greet function exported from the WebAssembly module. Right now, this glue isn't doing much, but when we start passing more interesting values back and forth between wasm and JavaScript, it will help shepherd those values across the boundary.

import * as wasm from './wasm_game_of_life_bg';

// ...

export function greet() {
    return wasm.greet();


The .d.ts file contains TypeScript type declarations for the JavaScript glue. If you are using TypeScript, you can have your calls into WebAssembly functions type checked, and your IDE can provide autocompletions and suggestions! If you aren't using TypeScript, you can safely ignore this file.

export function greet(): void;


The package.json file contains metadata about the generated JavaScript and WebAssembly package. This is used by npm and JavaScript bundlers to determine dependencies across packages, package names, versions, and a bunch of other stuff. It helps us integrate with JavaScript tooling and allows us to publish our package to npm.

  "name": "wasm-game-of-life",
  "collaborators": [
    "Your Name <>"
  "description": null,
  "version": "0.1.0",
  "license": null,
  "repository": null,
  "files": [
  "main": "wasm_game_of_life.js",
  "types": "wasm_game_of_life.d.ts"

Putting it into a Web Page

To take our wasm-game-of-life package and use it in a Web page, we use the create-wasm-app JavaScript project template.

Run this command within the wasm-game-of-life directory:

npm init wasm-app www

Here's what our new wasm-game-of-life/www subdirectory contains:

├── bootstrap.js
├── index.html
├── index.js
├── package.json
└── webpack.config.js

Once again, let's take a closer look at some of these files.


This package.json comes pre-configured with webpack and webpack-dev-server dependencies, as well as a dependency on hello-wasm-pack, which is a version of the initial wasm-pack-template package that has been published to npm.


This file configures webpack and its local development server. It comes pre-configured, and you shouldn't have to tweak this at all to get webpack and its local development server working.


This is the root HTML file for the Web page. It doesn't do much other than load bootstrap.js, which is a very thin wrapper around index.js.

<!DOCTYPE html>
    <meta charset="utf-8">
    <title>Hello wasm-pack!</title>
    <script src="./bootstrap.js"></script>


The index.js is the main entry point for our Web page's JavaScript. It imports the hello-wasm-pack npm package, which contains the default wasm-pack-template's compiled WebAssembly and JavaScript glue, then it calls hello-wasm-pack's greet function.

import * as wasm from "hello-wasm-pack";


Install the dependencies

First, ensure that the local development server and its dependencies are installed by running npm install within the wasm-game-of-life/www subdirectory:

npm install

This command only needs to be run once, and will install the webpack JavaScript bundler and its development server.

Note that webpack is not required for working with Rust and WebAssembly, it is just the bundler and development server we've chosen for convenience here. Parcel and Rollup should also support importing WebAssembly as ECMAScript modules. You can also use Rust and WebAssembly without a bundler if you prefer!

Using our Local wasm-game-of-life Package in www

Rather than use the hello-wasm-pack package from npm, we want to use our local wasm-game-of-life package instead. This will allow us to incrementally develop our Game of Life program.

Open up wasm-game-of-life/www/package.json and edit the "dependencies" to include a "wasm-game-of-life": "file:../pkg" entry:

  // ...
  "dependencies": {
    "wasm-game-of-life": "file:../pkg", // Add this line!
    // ...

Next, modify wasm-game-of-life/www/index.js to import wasm-game-of-life instead of the hello-wasm-pack package:

import * as wasm from "wasm-game-of-life";


Since we declared a new dependency, we need to install it:

npm install

Our Web page is now ready to be served locally!

Serving Locally

Next, open a new terminal for the development server. Running the server in a new terminal lets us leave it running in the background, and doesn't block us from running other commands in the meantime. In the new terminal, run this command from within the wasm-game-of-life/www directory:

npm run start

Navigate your Web browser to http://localhost:8080/ and you should be greeted with an alert message:

Screenshot of the "Hello, wasm-game-of-life!" Web page alert

Anytime you make changes and want them reflected on http://localhost:8080/, just re-run the wasm-pack build command within the wasm-game-of-life directory.


  • Modify the greet function in wasm-game-of-life/src/ to take a name: &str parameter that customizes the alerted message, and pass your name to the greet function from inside wasm-game-of-life/www/index.js. Rebuild the .wasm binary with wasm-pack build, then refresh http://localhost:8080/ in your Web browser and you should see a customized greeting!


    New version of the greet function in wasm-game-of-life/src/

    # #![allow(unused_variables)]
    #fn main() {
    pub fn greet(name: &str) {
        alert(&format!("Hello, {}!", name));

    New invocation of greet in wasm-game-of-life/www/index.js:

    wasm.greet("Your Name");