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https://github.com/tanishiking/scala-js-javy-playground

Scala --(scala.js)--> JS --(javy)--> WASM
https://github.com/tanishiking/scala-js-javy-playground

scala scalajs webassembly

Last synced: 24 days ago
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Scala --(scala.js)--> JS --(javy)--> WASM

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README 

        
# scala-js-javy-playground



- [bytecodealliance/javy: JS to WebAssembly toolchain](https://github.com/bytecodealliance/javy)

- [scala-js/scala-js: Scala.js, the Scala to JavaScript compiler](https://github.com/scala-js/scala-js)



Scala --(scala.js)--> JS --(javy)--> WASM



## Hello World



```scala

// Hello.scala

import scala.scalajs.js

import java.lang.Throwable



object Hello:

  def main(args: Array[String]): Unit =

    val console = js.Dynamic.global.console

    try

      throw new Error("test")

    catch

      case e: Throwable => console.log(e.getMessage)

```



```sh

$ scala-cli package --js Hello.scala -o build/hello.js --force



$ javy compile build/hello.js -o destination/hello.wasm



$ wasmtime destination/hello.wasm

java.lang.Error: test

```



## Event loop support is experimental

[The event loop - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Event_loop)



> Similarly, we haven’t enabled the event loop in the QuickJS instance that Javy uses. That means that async/await, Promises, and functions like setTimeout are syntactically available but never trigger their callbacks. We want to enable this functionality in Javy, but have to clear up a couple of open questions, like if and how to integrate the event loop with the host system or how to implement setTimeout() from inside a WASI environment.



> With all of this in place, we’ve built a JavaScript runtime that behaves very similarly to other runtimes out there. We can generate WASI-compatible Wasm modules from JavaScript and even split them into a static engine part and a dynamic user code part. This forms the foundation of how we brought JavaScript to Shopify Functions. As I said, we want Javy to be a general-purpose tool for JavaScript-in-Wasm, so everything that is specific to Shopify Functions needs to be built on top of Javy. 

[Bringing Javascript to WebAssembly for Shopify Functions (2023)](https://shopify.engineering/javascript-in-webassembly-for-shopify-functions)



```js

// index.js

function fetchData() {

  return Promise.resolve()

    .then(_ => "some data!")

}

const f = fetchData()

f.then(data =>

  console.log(data)

)

```



```sh

$ javy compile index.js -o destination/index.wasm



$ wasmtime destination/index.wasm

Error while running JS: Adding tasks to the event queue is not supported

Error: failed to run main module `destination/index.wasm`



Caused by:

    0: failed to invoke command default

    1: error while executing at wasm backtrace:

           0: 0x5cfa1 - !

           1: 0x6f59c - !

           2: 0xb6630 - !

    2: wasm trap: wasm `unreachable` instruction executed

```



which means, we cannot use `Future` in Scala...



```scala

// Promise.scala

import scala.concurrent._

import scala.util.Success

import concurrent.ExecutionContext.Implicits.global



object Promise:

  def fetchData(): Future[String] = Future { "some data!" }

  def main(args: Array[String]): Unit =

    val f = fetchData()

    f.onComplete:

      case Success(data) => println(data)

```



```sh

$ scala-cli package --js Promise.scala -o build/promise.js --force

$ javy compile build/promise.js -o destination/promise.wasm

$ wasmtime destination/promise.wasm

Error while running JS: Adding tasks to the event queue is not supported

Error: failed to run main module `destination/promise.wasm`



Caused by:

    0: failed to invoke command default

    1: error while executing at wasm backtrace:

           0: 0x5cfa1 - !

           1: 0x6f59c - !

           2: 0xb6630 - !

    2: wasm trap: wasm `unreachable` instruction executed

```



because Scala's `Future` would be translated into `Promise` (or `setTimeout`) in JS.



## experimental_event_loop

[Support for async / await · Issue #387 · bytecodealliance/javy](https://github.com/bytecodealliance/javy/issues/387)



Build `javy` with an `experimental_event_loop` feature flag, and install `javy-cli` globally.



```sh

$ cargo build --features experimental_event_loop -p javy-core --target=wasm32-wasi -r

$ cargo install --path crates/cli



$ javy compile build/promise.js -o destination/promise.wasm

$ wasmtime destination/promise.wasm

some data!

```