Several revisions of the ECMAScript standard were released to improve the language and address its limitations. Furthermore new and more expressive languages (yes TypeScript, we hear you) were released to add missing features while maintaining the complete compatibility with existing runtime engines. All this has led to the fragmented ecosystem we are all fighting everyday, making the development of large projects very challenging.
- Leave the cave you are currently living in;
- Download the ECMAScript specification and count how many steps are required to perform just an OrdinaryGetOwnProperty;
Therefore, when in need of native performance, developers had to turn to native code (plug-ins, NPAPI, etc.) with frequently disastrous consequences in terms of security.
So why WebAssembly?
Here is where WebAssembly comes to the rescue.
If you have never heard about WebAssembly, don’t miss the talk Diwanshi Pandey, software engineer at Red Hat, gave at Codemotion Milan 2019!
WebAssembly is a compilation target for C, C++, Go, Rust and others. Developers can code in any of those languages and then compile their source into wasm binaries which will then run alongside JS code in their applications.
However, these are not the reasons why WebAssembly is so special. After all, on a purely technical level binary wasm is not very different from Java’s bytecode or .NET Common Intermediate Language. But WebAssembly is an open standard and it is already implemented in all the major browsers. That means you don’t need a native plug-in to run wasm code in your browser, no matter what platform, OS or device you are using. And this is pretty unique.
… and why Rust?
Go would be a better alternative in this case, as it is concise and easy to read. However, there is always a trade-off between performance and readability. Go’s runtime offers a lot of features but these come at the cost of the binary size: the smallest wasm file compiled from Go is around 2 MB, while 10 MB is the most typical size. That is huge compared to the files compiled from C++ and Rust, which are typically just a few KB.
Getting started: setup
To begin working with Rust and WebAssembly, one must obviously have the Rust toolchain installed. Nowadays the best (and recommended) way to manage the Rust toolchain is via the rustup tool.
Installing rustup is as easy as launching a Bash script (the rustup-init.sh script to be precise) with this one-line command:
# curl https://sh.rustup.rs -sSf | sh
If you are running Windows, luckily we leave in Nadella’s era and thus we have plenty of options to get Bash up and running: the Windows Subsystem for Linux, Git for Windows own Bash, Cygwin, just to name a few. If you don’t want to use Bash at all, there is a .exe version of rustup-init as well as standalone installers available for download from Rust language project website.
The script will download and compile all the required tools including rustc (the Rust compiler) and cargo (Rust’s package manager).
By default, the toolchain will be installed in $HOME/.cargo so be sure to add the $HOME/.cargo/bin to your PATH environment variable, or simply run the command:
# source $HOME/.cargo/env
when opening a new shell.
Once the Rust toolchain is installed, we can proceed installing wasm-pack. Wasm-pack is an all in one tool to build, test and publish Rust-generated wasm modules. To install it, simply type this command in your Bash prompt:
# curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh
or again, download the .exe file if you are on Windows from the official website.
Another useful tool to install is
cargo-generate, which allows to create new projects based on existing templates. Since the Rust toolchain is already installed, we can use cargo itself to download, compile and install cargo-generate, with this command:
# cargo install cargo-generate
In case you do not have those installed, we recommend to install git, node and npm as these may come in handy pretty soon.
Our first WebAssembly module
Let’s now dive in and build our first Hello World project in Rust/WebAssembly: a web page that shows an alert!
First of all, we will create a new project using cargo generate, using a git repository as a template:
# cargo generate --git https://github.com/rustwasm/wasm-pack-template
When prompted for the project’s name, just type the name “hello”. The script will create the directory “hello” and populate it with the files shipped with the template:
- Cargo.toml – which specifies dependencies and metadata for cargo (a sort of Rust’s equivalent of package.json);
- src/lib.rs – which contains the code of our wasm module;
- src/utils.rs – which provides common utilities to debug wasm code;
Let’s have a look at the lib.rs code in detail:
In this simple example, wasm_bindgen is used to import the browser’s alert() function, so our Rust code can call it and display the “Hello, hello!” message. That’s because the string created by cargo generate is “Hello, <project_name>!” and our project is named “hello”. Feel free to change this message according to your preference.
When targeting wasm, the wee_alloc allocator is the preferred choice, as it was explicitly designed for smaller code size, although it can be a bit slower than other allocators.
# wasm-pack build
Once finished, the compiled artifacts are available in the pkg/ directory. This directory should contain these files:
- hello_bg.wasm – our WebAssembly binary, which exports the greet() function;
- package.json – the metadata about this generated WebAssembly package;
Calling WebAssembly from a web page
The Rust project offers an npm init template to generate a web application that will call our WebAssembly module. Simply type the command:
# npm init wasm-app www
To use our WebAssembly package here, we simply must add it as a dependency in the package.json file, by adding the following line under the “devDependencies” node:
Then, install the dependencies with the well known command:
# npm install
Once the installation is finished, modify the index.js file to call our WebAssembly greet() function, with this code:
import * as wasm from "hello"; wasm.greet();
Since we have Webpack and its local server installed, we can run the application with the command:
# npm run start
Now point your browser to http://localhost:8080/ to be greeted with the message from our Rust code.
Congratulations, you have created your first Rust/WebAssembly project!
Again, if you want to learn more about Rust and WebAssembly, don’t miss the talk Diwanshi Pandey, software engineer at Red Hat, gave at Codemotion Milan 2019!
Diwanshi addressed more advanced topics and showed some interesting demos.