Producing WebAssembly better for Rust & for all languages

One big 2018 goal for that Rust community is to become an internet language. By targeting WebAssembly, Corrosion can run on the web just like JavaScript. But what does this mean? Can it mean that Rust is trying to replace JavaScript?

The answer to that query is no. We don’ t anticipate Rust WebAssembly apps to be created completely in Rust. In fact , all of us expect the bulk of application code will be JS, even in most Rust WebAssembly applications.

This is because JS is a good choice for most things. It’ s quick and easy to get up and running along with JavaScript. On top of that, there’ s a delightful ecosystem full of JavaScript developers that have created incredibly innovative approaches to various problems on the web.

Rust logo and JS logo with a heart in between

But sometimes for specific areas of an application, Rust+WebAssembly is the right device for the job… like when you’ re parsing source maps , or determining what changes to make to the DEM, like Ember .

So just for Rust+WebAssembly, the path forward doesn’ to stop at compiling Rust to WebAssembly. We need to make sure that WebAssembly fits in to the JavaScript ecosystem. Web developers need to be capable to use WebAssembly as if it had been JavaScript.

But WebAssembly isn’ t there yet. To help make this happen, we need to build equipment to make WebAssembly easier to load, plus easier to interact with from JS. This particular work will help Rust. But it may also help all other languages that focus on WebAssembly.

Pipeline from compiling, to producing bindings, to packaging, to bundling

What WebAssembly usability challenges are usually we tackling? Here are a few:

  1. How do you make it easy to complete objects between WebAssembly and JS?
  2. How do you package everything up for npm ?
  3. How do programmers easily combine JS and WASM packages, whether in bundlers or even browsers?

However, what are we making possible within Rust?

Rust can call JavaScript functions. JavaScript can call Rust functions. Rust can call functions from the host system, like alert . Rust crates will be able to have dependencies on npm packages. And all through all of this, Rust and JavaScript is going to be passing objects around in a way that is sensible to both of them.

Rust crate graph

So that’ s what we make possible in Rust. Now let’ s look at the WebAssembly usability issues that we need to tackle.

Q. How do you make it easy to complete objects between WebAssembly and  JS?

A. wasm-bindgen

One of the hardest areas of working with WebAssembly is getting different kinds of beliefs into and out of functions. That’ s because WebAssembly currently just has two types: integers plus floating point numbers.

This means you can’ t simply pass a string into a WebAssembly function. Instead, you have to go through lots of steps:

  1. Over the JS side, encode the thread into numbers (using something like the particular TextEncoder API)
    Encoder ring encoding Hello straight into number equivalent
  2. Put those amounts into WebAssembly’ s memory, that is basically an array of numbers
    JS putting numbers straight into WebAssembly's memory
  3. Pass the range index for the first letter from the string to the WebAssembly function
  4. On the WebAssembly side, use that integer like a pointer to pull out the amounts

And that’ s only what’ s required for strings. If you have more complicated types, then you’ re likely to have a more convoluted process to find the data back and forth.

In case you’ re using a lot of WebAssembly code, you’ ll probably subjective this kind of glue code out right into a library. Wouldn’ t it end up being nice if you didn’ t need to write all that glue code, although? If you could just pass complicated values across the language boundary and also have them magically work?

That’ s what wasm-bindgen does. In case you add a few annotations to your Corrosion code, it will automatically create the particular code that’ s needed (on both sides) to make more complex varieties work.

JS passing the string Hello there to wasm-bindgen, which does all the other work

This means calling JS functions from Rust using whichever types those functions expect:

    type console;

    # [wasm_bindgen(static = console)] 
    fn log(s: &str);

bar fn foo() 


… Or using structs within Rust and having them work as lessons in JS:

// Rust
pub struct Foo 
    contents: u32,

impl Foo 
    pub fn new() -> Foo 
        Foo  contents: 0 
    pub fn add(&mut self, amt: u32) -> u32 
        self.contents += amt;
        return self.contents

// JS
import  Foo  from inch. /js_hello_world";
let foo = Foo. new();
assertEq(foo. add(10), 10);
foo. free();

… Or many other niceties.

Under the hood, wasm-bindgen is designed to become language-independent. This means that as the tool balances it should be possible to expand assistance for constructs in other languages, such as C/C++.

Alex Crichton will be writing more about wasm-bindgen in a little while, so watch for that post.

Q. How do you package everything up for  npm?

A. wasm-pack

Once we put it all together, we have lots of files. There’ s the put together WebAssembly file. Then there’ ersus all of the JavaScript  —   each dependencies and the JS generated simply by wasm-bindgen . We require a way to package them all up. In addition, if we’ ve added any kind of npm dependencies, we need to put all those into the package. json manifest file.

multiple data files being packaged up and released to npm

Again, it would be fine if this could be done for us. Plus that’ s what wasm-pack does. This is an one-stop shop for going from a put together WebAsssembly file to an npm deal.

It will run wasm-bindgen to suit your needs. Then, it will take all of the files plus package them up. It will take a package. json on top, filling in all the npm dependencies from your Rust program code. Then, all a person need to do is npm publish .

Again, the foundations of the tool are language-independent, so we anticipate it to support multiple language environments.

Ashley Williams is going to be writing more about wasm-pack next month, so that’ s another post to watch designed for.

Q. How do programmers easily combine JS and WASM, whether in bundlers, browsers, or  Node?

A. SERA modules

Now that we’ ve published our WebAssembly in order to npm, how do we make it user friendly that WebAssembly in a JS software?

Make it easy to include the WebAssembly package as a dependency… to include it in JS component dependency graphs.

module graph with JS and WASM modules

Currently, WebAssembly has an imperative JS API for producing modules. You have to write code to try and do every step, from fetching the particular file to preparing the dependencies. It’ s hard work.

But now that native module assistance is in browsers, we can add a declarative API. Specifically, we can use the HA SIDO module API. With this, working with WebAssembly modules should be as easy as importing all of them.

import myFunction from "myModule. wasm"

We’ re working with TC39 and the WebAssembly community group to standardize this particular.

But we don’ t just need to standardize ES component support. Even once browsers plus Node support ES modules, designers will still likely use bundlers. That’ s because bundlers slow up the number of requests that you have to make for component files, which means it takes less time in order to download your code.

Bundlers do this by combining a variety of modules from different files into one file, and then adding a little bit of the runtime to the top to load all of them.

a module graph being combined into one file

Bundlers will still have to use the JS API to create the particular modules, at least in the short term. But customers will be authoring with ES component syntax. Those users will anticipate their modules to act as if these were ES modules. We’ ll have to add some features to WebAssembly to be able to easier for bundlers to copy ES modules.

We are writing more about the effort to add HA SIDO module integration to the WebAssembly specification. I’ ll also be diving straight into bundlers and their support with regard to WebAssembly over the coming months.


To become an useful as a web language, Corrosion needs to work well with the JavaScript environment. We have some work to do to obtain there, and fortunately that work can help other languages, too. Do you want to help to make WebAssembly better for every language? Sign up for us! We’ re happy to help you get started  : )

Lin is definitely an engineer on the Mozilla Developer Relationships team. She tinkers with JavaScript, WebAssembly, Rust, and Servo, as well as draws code cartoons.

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