Compiling from Rust to WebAssembly

There are two main use cases for Rust and WebAssembly:

• Build an entire application — an entire web app based in Rust.
• Build a part of an application — using Rust in an existing JavaScript frontend.

For now, the Rust team is focusing on the latter case, and so that's what we cover here. For the former case, check out projects like yew.

In this tutorial, we build a package using wasm-pack, a tool for building JavaScript packages in Rust. This package will contain only WebAssembly and JavaScript code, and so the users of the package won't need Rust installed. They may not even notice that it's written in Rust.

Let's go through all the required steps to get our environment set up.

Install Rust by going to the Install Rust page and following the instructions. This installs a tool called "rustup", which lets you manage multiple versions of Rust. By default, it installs the latest stable Rust release, which you can use for general Rust development. Rustup installs rustc, the Rust compiler, as well as cargo, Rust's package manager, rust-std, Rust's standard libraries, and some helpful docs — rust-docs.

To build the package, we need an additional tool, wasm-pack. This helps compile the code to WebAssembly, as well as produce the right packaging for use in the browser. To download and install it, enter the following command into your terminal:

cargo install wasm-pack

Enough setup; let's create a new package in Rust. Navigate to wherever you keep your personal projects, and type this:

$ cargo new --lib hello-wasm
     Created library `hello-wasm` project

This creates a new library in a subdirectory named hello-wasm with everything you need to get going:

├── Cargo.toml
└── src
    └── lib.rs

First, we have Cargo.toml; this is the file that we use to configure our build. If you've used Gemfile from Bundler or package.json from npm, this is likely to be familiar; Cargo works in a similar manner to both of them.

Next, Cargo has generated some Rust code for us in src/lib.rs:

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }
}

We won't use this test code at all, so go ahead and delete it.

Let's put this code into src/lib.rs instead:

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
extern {
    pub fn alert(s: &str);
}

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

This is the contents of our Rust project. It has three main parts; let's talk about each of them in turn. We give a high-level explanation here, and gloss over some details; to learn more about Rust, please check the free online book The Rust Programming Language.

Using wasm-bindgen to communicate between Rust and JavaScript

The first part looks like this:

use wasm_bindgen::prelude::*;

Libraries are called "crates" in Rust.

Get it? Cargo ships crates.

The first line contains a use command, which imports code from a library into your code. In this case, we're importing everything in the wasm_bindgen::prelude module. We use these features in the next section.

Before we move on to the next section, we should talk a bit more about wasm-bindgen.

wasm-pack uses wasm-bindgen, another tool, to provide a bridge between the types of JavaScript and Rust. It allows JavaScript to call a Rust API with a string, or a Rust function to catch a JavaScript exception.

We use wasm-bindgen's functionality in our package. In fact, that's the next section.

Calling external functions in JavaScript from Rust

The next part looks like this:

#[wasm_bindgen]
extern {
    pub fn alert(s: &str);
}

The bit inside the #[ ] is called an "attribute", and it modifies the next statement somehow. In this case, that statement is an extern, which tells Rust that we want to call some externally defined functions. The attribute says "wasm-bindgen knows how to find these functions".

The third line is a function signature, written in Rust. It says "the alert function takes one argument, a string named s."

As you might suspect, this is the alert function provided by JavaScript. We call this function in the next section.

Whenever you want to call JavaScript functions, you can add them to this file, and wasm-bindgen takes care of setting everything up for you. Not everything is supported yet, but we're working on it. Please file bugs if something is missing.

Producing Rust functions that JavaScript can call

The final part is this one:

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

Once again, we see the #[wasm_bindgen] attribute. In this case, it's not modifying an extern block, but a fn; this means that we want this Rust function to be able to be called by JavaScript. It's the opposite of extern: these aren't the functions we need, but rather the functions we're giving out to the world.

This function is named greet, and takes one argument, a string (written &str), name. It then calls the alert function we asked for in the extern block above. It passes a call to the format! macro, which lets us concatenate strings.

The format! macro takes two arguments in this case, a format string, and a variable to put in it. The format string is the "Hello, {}!" bit. It contains {}s, where variables will be interpolated. The variable we're passing is name, the argument to the function, so if we call greet("Steve") we should see "Hello, Steve!".

This is passed to alert(), so when we call this function we will see an alert box with "Hello, Steve!" in it.

Now that our library is written, let's build it.

To compile our code correctly, we first need to configure it with Cargo.toml. Open this file, and change its contents to look like this:

[package]
name = "hello-wasm"
version = "0.1.0"
authors = ["Your Name <you@example.com>"]
description = "A sample project with wasm-pack"
license = "MIT/Apache-2.0"
repository = "https://github.com/yourgithubusername/hello-wasm"
edition = "2018"

[lib]
crate-type = ["cdylib"]

[dependencies]
wasm-bindgen = "0.2"

Fill in your own repository and use the same info that git uses for the authors field.

The big part to add is the [package]. The [lib] part tells Rust to build a cdylib version of our package; we won't get into what that means in this tutorial. For more, consult the Cargo and Rust Linkage documentation.

The last section is the [dependencies] section. Here's where we tell Cargo what version of wasm-bindgen we want to depend on; in this case, that's any 0.2.z version (but not 0.3.0 or above).

Now that we've got everything set up, let's build the package. Type this into your terminal:

wasm-pack build --target web

This does a number of things (and they take a lot of time, especially the first time you run wasm-pack). To learn about them in detail, check out this blog post on Mozilla Hacks. In short, wasm-pack build:

1. Compiles your Rust code to WebAssembly.
2. Runs wasm-bindgen on that WebAssembly, generating a JavaScript file that wraps up that WebAssembly file into a module the browser can understand.
3. Creates a pkg directory and moves that JavaScript file and your WebAssembly code into it.
4. Reads your Cargo.toml and produces an equivalent package.json.
5. Copies your README.md (if you have one) into the package.

The end result? You have a package inside the pkg directory.

A digression about code size

If you check out the generated WebAssembly code size, it may be a few hundred kilobytes. We haven't instructed Rust to optimize for size at all, and doing so cuts down on the size a lot. This is beyond the scope of this tutorial, but if you'd like to learn more, check out the Rust WebAssembly Working Group's documentation on Shrinking .wasm Size.

Now that we've got a compiled Wasm module, let's run it in the browser. Let's start by creating a file named index.html in the root of the project, so we end up with the following project structure:

├── Cargo.lock
├── Cargo.toml
├── index.html
├── pkg
│   ├── hello_wasm.d.ts
│   ├── hello_wasm.js
│   ├── hello_wasm_bg.wasm
│   ├── hello_wasm_bg.wasm.d.ts
│   └── package.json
├── src
│   └── lib.rs
└── target
    ├── CACHEDIR.TAG
    ├── release
    └── wasm32-unknown-unknown

Put the following content in the index.html file:

<!DOCTYPE html>
<html lang="en-US">
  <head>
    <meta charset="utf-8" />
    <title>hello-wasm example</title>
  </head>
  <body>
    <script type="module">
      import init, { greet } from "./pkg/hello_wasm.js";
      init().then(() => {
        greet("WebAssembly");
      });
    </script>
  </body>
</html>

The script in this file will import the js glue code, initialize the Wasm module, and call the greet function we wrote in rust.

Serve the root directory of the project with a local web server, (e.g. python3 -m http.server). If you're not sure how to do that, refer to Running a simple local HTTP server.

Load index.html from the web server (if you used the Python3 example: http://localhost:8000). An alert box appears on the screen, with Hello, WebAssembly! in it. We've successfully called from JavaScript into Rust, and from Rust into JavaScript.

If you want to use the WebAssembly module with npm, we'll need to make a few changes.

Let's start by recompiling our Rust with the target bundler option:

wasm-pack build --target bundler

We are building an npm package, so you need to have Node.js and npm installed.

To get Node.js and npm, go to the Get npm! page and follow the instructions. This tutorial targets node 16, if you need to switch between node versions, you can use nvm.

Next, let's use npm link to make this package available to other JavaScript packages installed

cd pkg
npm link

We now have an npm package, written in Rust, but compiled to WebAssembly. It's ready for use from JavaScript, and doesn't require the user to have Rust installed; the code included was the WebAssembly code, not the Rust source.

Let's build a website that uses our new npm package. Many people use npm packages through various bundler tools, and we'll be using one of them, webpack, in this tutorial. It's only a bit complex, and shows a realistic use-case.

Let's move back out of the pkg directory, and make a new directory, site, to try this out in:

cd ..
mkdir site
cd site
npm link hello-wasm

Create a new file, package.json, and put the following code in it:

{
  "scripts": {
    "serve": "webpack-dev-server"
  },
  "dependencies": {
    "hello-wasm": "^0.1.0"
  },
  "devDependencies": {
    "webpack": "^4.25.1",
    "webpack-cli": "^3.1.2",
    "webpack-dev-server": "^3.1.10"
  }
}

Next, we need to configure Webpack. Create webpack.config.js and put the following in it:

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

Next, create a file named index.js, and give it these contents:

import("./node_modules/hello-wasm/hello_wasm.js").then((js) => {
  js.greet("WebAssembly with npm");
});

This imports the new module from the node_modules folder. This isn't considered a best practice, but this is a demo, so it's OK for now. Once it's loaded, it calls the greet function from that module, passing "WebAssembly" as a string. Note how there's nothing special here, yet we're calling into Rust code. As far as the JavaScript code can tell, this is just a normal module.

Finally, we need to add a HTML file to load the JavaScript. Create an index.html file and add the following:

<!DOCTYPE html>
<html lang="en-US">
  <head>
    <meta charset="utf-8" />
    <title>hello-wasm example</title>
  </head>
  <body>
    <script src="./index.js"></script>
  </body>
</html>

The hello-wasm/site directory should look like this:

├── index.html
├── index.js
├── node_modules
│   └── hello-wasm -> ../../pkg
├── package.json
└── webpack.config.js

We're done making files. Let's give this a shot:

npm install
npm run serve

This starts a web server. Load http://localhost:8080 and an alert box appears on the screen, with Hello, WebAssembly with npm! in it. We've successfully used the Rust module with npm.

This is the end of our tutorial; we hope you've found it useful.

There's lots of exciting work going on in this space; if you'd like to help make it even better, check out the Rust WebAssembly Working Group.