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!"

Clone the Project Template

The project template contains a "hello world" program. It comes pre-configured with sane defaults, so you can quickly build, integrate, and package your code for the Web.

Clone this tutorials code repository, enter its directory, and checkout the chapter-zero branch:

git clone https://github.com/rustwasm/wasm_game_of_life.git
cd ./wasm_game_of_life
git checkout -b chapter-zero origin/chapter-zero

What's Inside

Let's take a look at the contents of our project:

├── bootstrap.js
├── Cargo.lock
├── Cargo.toml
├── index.html
├── index.js
├── package.json
├── package-lock.json
├── src
│   └── lib.rs
└── webpack.config.js

Most of these are configuration files, but there are a few files we should highlight.


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

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


The index.js is the main entry point for our Web page's JavaScript. It imports the project's WebAssembly module, and calls the module's greet function.

import { greet } from "./wasm_game_of_life";

greet("Rust and WebAssembly");


The src/lib.rs 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 takes a name parameter and alerts a greeting message.

# #![allow(unused_variables)]

#fn main() {
extern crate wasm_bindgen;

use wasm_bindgen::prelude::*;

extern {
    fn alert(s: &str);

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

Building and Serving

First, ensure that the JavaScript build dependencies are installed for this project:

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.

To build the Rust crate as WebAssembly and generate the wasm_bindgen glue, run this command:

npm run build-debug

The first build may take a little while, since dependencies need to be compiled. But don't worry: subsequent builds, when dependencies don't need to be recompiled, will be much faster.

This creates a "debug" build of the Rust crates: a build that does not have optimizations applied, and has symbols included for better debugging in your browser's developer tools. You can also create a "release" build that has optimization passes applied with this command:

npm run build-release

This is the command we want to use to create .wasm binaries for profiling and deploying to production.

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:

npm run serve

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

Screenshot of the "Hello, Rust and WebAssembly!" Web page alert

Anytime you make changes and want them reflected on http://localhost:8080/, just re-run the npm run build-debug command.


  • Modify index.js to greet you by your name instead of by "Rust and WebAssembly".