wasm-bindgen

A CLI and Rust dependency for acting as a polyfill for Wasm host bindings, allowing you to interoperate with JS from wasm with types like strings, JS objects, etc.

This project is a "temporary" polyfill for the host bindings proposal which is intended to empower wasm modules to interact with host objects such as strings, JS objects, etc. This project enables defining JS classes in wasm, taking strings from JS in wasm, and calling into JS with JS objects previously provided.

Currently this tool is Rust-focused but the underlying foundation is language-independent, and it's hoping that over time as this tool stabilizes that it can be used for languages like C/C++!

Notable features of this project includes:

• Exposing Rust structs to JS as classes
• Exposing Rust functions to JS
• Managing arguments between JS/Rust (strings, numbers, classes, objects, etc)
• Importing JS functions with richer types (strings, objects)
• Receiving arbitrary JS objects in Rust, passing them through to JS
• Generates Typescript for now instead of JS (although that may come later)

Planned features include:

• Field setters/getters in JS through Rust functions
• ... and more coming soon!

This project is still very "early days" but feedback is of course always welcome!

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

First up, let's add the wasm target and generate a Rust project:

$ rustup target add wasm32-unknown-unknown
$ cargo new js-hello-world

Now let's add a dependency on this project inside Cargo.toml as well as configuring our build output:

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

[dependencies]
wasm-bindgen = { git = 'https://github.com/alexcrichton/wasm-bindgen' }

Next up our actual code! We'll write this in src/lib.rs:

#![feature(proc_macro)]

extern crate wasm_bindgen;

use wasm_bindgen::prelude::*;

wasm_bindgen! {
    pub fn greet(name: &str) -> String {
        format!("Hello, {}!", name)
    }
}

Here we're wrapping the code we'd like to export to JS in the wasm_bindgen! macro. We'll see more features later, but it suffices to say that most Rust syntax fits inside here, it's not too special beyond what it generates!

Next up let's build our project:

$ cargo build --release --target wasm32-unknown-unknown

Note that we're using --release here because unfortunately the current LLVM backend for wasm has a few bugs in non-optimized mode. Those bugs will hopefully get smoothed out over time!

After this you'll have a wasm file at target/wasm32-unknown-unknown/release/js_hello_world.wasm. If you'd like you can use wasm-gc to make this file a little smaller

Now that we've generated the wasm module it's time to run the bindgen tool itself! Let's install it:

$ cargo install --git https://github.com/alexcrichton/wasm-bindgen

This'll install a wasm-bindgen binary next to your cargo binary. 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/release/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 secondary file, js_hello_world_wasm.wasm. This is the original wasm file but postprocessed a bit. It's intended that the js_hello_world_wasm.wasm file, like before, acts like an ES6 module. The js_hello_world.wasm file, for example, uses import to import functionality from the wasm.

Note that you can also pass a --nodejs argument to wasm-bindgen for emitting Node-compatible JS as well as a --typescript argument to emit a *.d.ts file describing the exported contents.

At this point you'll typically plug these files into a larger build system. Both 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 they're coming!). In the meantime we can use the wasm2es6js utility (aka "hack") from the wasm-bindgen tool we previously installed along with the parcel-bundler packager. Note that these steps will differ depending on your build system.

Alright first create an index.js file:

import { greet } from "./js_hello_world";
import { booted } from "./js_hello_world_wasm";

booted.then(() => {
  alert(greet("World!"))
});

Then a corresponding index.html:

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

And run a local server with these files:

# Convert `*.wasm` to `*.js` where the JS internally instantiates the wasm
$ wasm2es6js js_hello_world_wasm.wasm -o js_hello_world_wasm.js --base64

# Install parcel and run it against the index files we use below.
$ npm install -g parcel-bundler
$ parcel index.html -p 8000

If you open that in a browser you should see a Hello, world! dialog pop up!

What just happened?

Phew! That was a lot of words and a lot ended up happening along the way. There were two main pieces of magic happening: the wasm_bindgen! macro and the wasm-bindgen CLI tool.

The wasm_bindgen! macro

This macro, exported from the wasm-bindgen crate, is the entrypoint to exposing Rust functions to JS. This is a procedural macro (hence requiring the nightly Rust toolchain) which will transform the definitions inside and prepare appropriate wrappers to receive JS-compatible types and convert them to Rust-compatible types.

There's a more thorough explanation below of the various bits and pieces of the macro, but it suffices for now to say that you can have free functions, structs, and impl blocks for those structs in the macro right now. Many Rust features aren't supported in these blocks like generics, lifetime parameters, etc. Additionally not all types can be taken or returned from the functions. In general though simple-ish types should work just fine!

The wasm-bindgen CLI tool

The next half of what happened here was all in the wasm-bindgen tool. This tool opened up the wasm module that rustc generated and found an encoded description of what was passed to the wasm_bindgen! macro. You can think of this as the wasm_bindgen! macro created a special section of the output module which wasm-bindgen strips and processes.

This information gave wasm-bindgen all it needed to know to generate the JS file that we then imported. The JS file wraps instantiating the underlying wasm module (aka calling WebAssembly.instantiate) and then provides wrappers for classes/functions within.

What else can we do?

Much more! Here's a taste of various features you can use in this project:

// src/lib.rs
#![feature(proc_macro)]

extern crate wasm_bindgen;

use wasm_bindgen::prelude::*;

wasm_bindgen! {
    // Strings can both be passed in and received
    pub fn concat(a: &str, b: &str) -> String {
        let mut a = a.to_string();
        a.push_str(b);
        return a
    }

    // A struct will show up as a class on the JS side of things
    pub struct Foo {
        contents: u32,
    }

    impl Foo {
        pub fn new() -> Foo {
            Foo { contents: 0 }
        }

        // Methods can be defined with `&mut self` or `&self`, and arguments you
        // can pass to a normal free function also all work in methods.
        pub fn add(&mut self, amt: u32) -> u32 {
            self.contents += amt;
            return self.contents
        }

        // You can also take a limited set of references to other types as well.
        pub fn add_other(&mut self, bar: &Bar) {
            self.contents += bar.contents;
        }

        // Ownership can work too!
        pub fn consume_other(&mut self, bar: Bar) {
            self.contents += bar.contents;
        }
    }

    pub struct Bar {
        contents: u32,
        opaque: JsObject, // defined in `wasm_bindgen`, imported via prelude
    }

    #[wasm_module = "./index"] // what ES6 module to import this functionality from
    extern "JS" {
        fn bar_on_reset(to: &str, opaque: &JsObject);
    }

    impl Bar {
        pub fn from_str(s: &str, opaque: JsObject) -> Bar {
            Bar { contents: s.parse().unwrap_or(0), opaque }
        }

        pub fn reset(&mut self, s: &str) {
            if let Ok(n) = s.parse() {
                bar_on_reset(s, &self.opaque);
                self.contents = n;
            }
        }
    }
}

The generated JS bindings for this invocation of the macro look like this. You can view them in action like so:

and our corresponding index.js:

import { Foo, Bar, concat } from "./js_hello_world";
import { booted } from "./js_hello_world_wasm";

export function bar_on_reset(s, token) {
  console.log(token);
  console.log(`this instance of bar was reset to ${s}`);
}

function assertEq(a, b) {
  if (a !== b)
    throw new Error(`${a} != ${b}`);
  console.log(`found ${a} === ${b}`);
}

function main() {
  assertEq(concat('a', 'b'), 'ab');

  // Note the `new Foo()` syntax cannot be used, static function
  // constructors must be used instead. Additionally objects allocated
  // corresponding to Rust structs will need to be deallocated on the
  // Rust side of things with an explicit call to `free`.
  let foo = Foo.new();
  assertEq(foo.add(10), 10);
  foo.free();

  // Pass objects to one another
  let foo1 = Foo.new();
  let bar = Bar.from_str("22", { opaque: 'object' });
  foo1.add_other(bar);

  // We also don't have to `free` the `bar` variable as this function is
  // transferring ownership to `foo1`
  bar.reset('34');
  foo1.consume_other(bar);

  assertEq(foo1.add(2), 22 + 34 + 2);
  foo1.free();

  alert('all passed!')
}

booted.then(main);

Feature reference

Here this section will attempt to be a reference for the various features implemented in this project. This is likely not exhaustive but the tests should also be a great place to look for examples.

In the wasm_bindgen! macro you can have four items: functions, structs, impls, and foreign modules. Impls can only contain functions. No lifetime parameters or type parameters are allowed on any of these types. Foreign modules must have the "JS" abi and currently only allow integer/string arguments and integer return values.

All structs referenced through arguments to functions should be defined in the macro itself. Arguments allowed are:

• Integers (not u64/i64)
• Floats
• Borrowed strings (&str)
• Owned strings (String)
• Owned structs (Foo) defined in the same bindgen macro
• Borrowed structs (&Foo or &mut Bar) defined in the same bindgen macro
• The JsObject type and &JsObject (not mutable references)

All of the above can also be returned except borrowed references. Strings are implemented with shim functions to copy data in/out of the Rust heap. That is, a string passed to Rust from JS is copied to the Rust heap (using a generated shim to malloc some space) and then will be freed appropriately.

Owned values are implemented through boxes. When you return a Foo it's actually turned into Box<RefCell<Foo>> under the hood and returned to JS as a pointer. The pointer is to have a defined ABI, and the RefCell is to ensure safety with reentrancy and aliasing in JS. In general you shouldn't see RefCell panics with normal usage.

JS-values-in-Rust are implemented through indexes that index a table generated as part of the JS bindings. This table is managed via the ownership specified in Rust and through the bindings that we're returning.

All of these constructs currently create relatively straightforward code on the JS side of things, mostly having a 1:1 match in Rust with JS.

License

This project is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.