ECMAssembly

Spec'd and/or common JavaScript APIs brought to AssemblyScript.

The name is a play on words: ECMAScript -> AssemblyScript -> ECMAssembly

Namely, this provides APIs that require task scheduling from the host environment's (JavaScript's) event loop; APIs such as setTimeout, Promise, etc. AssemblyScript's stdlib currently only provides APIs that can be implemented entirely on their own in Wasm without scheduling (Wasm does not yet provide any APIs for scheduling) therefore without the need for bindings.

Note For DOM APIs, see asdom.

Usage

First:

npm install ecmassembly @assemblyscript/loader

Note > @assemblyscript/loader is needed, and your program will need to be manually loaded with the loader API, and not with AssemblyScript's new auto-bindings in 0.20+, for now.

On the JavaScript side pass required glue code to the Wasm module via imports:

import {ECMAssembly} from 'ecmassembly/index.js'
import ASLoader from '@assemblyscript/loader'

const es = new ECMAssembly()

const imports = {
	...es.wasmImports,
	/*...All your own imports...*/
}

ASLoader.instantiateStreaming(fetch('path/to/module.wasm'), imports).then(wasmModule => {
	// After the Wasm module is created, you need to pass the exports back to the lib:
	es.wasmExports = wasmModule.exports

	// Then finally, run anything from the module that depends on setTimeout, Promise, etc:
	wasmModule.exports.runMyApp()
})

For example, see the example/'s Wasm entrypoint.

In your AssemblyScript project's asconfig.json, make sure the globals are included in entries, along with your own entry point:

{
	"entries": [
		"./node_modules/ecmassembly/assembly/PromiseActions",
		"./node_modules/ecmassembly/assembly/globals.ts",
		"./path/to/your-entry-point.ts"
	]
}

For example, see the example/'s asconfig.json.

In your code you can now use the available APIs similar to in regular JavaScript, for example here is what Promise currently looks like:

let actions: PromiseActions<boolean> | null = null

export function runMyApp() {
	const promise = new Promise<boolean>(_actions => {
		// Temporary hack while AS does not yet support closures (no closing
		// over variable except those that are at the top-level of the module).
		actions = _actions

		// resolve after 1 second
		setTimeout(() => {
			actions!.resolve(true)
		}, 1000)
	})

	promise.then(result => {
		// this runs one second later, and `result` will be `true` here
	})
}

Note AssemblyScript does not support closures for non-top-level variables yet, so a Promise constructor's executor function receives an object with resolve and reject methods instead of two separate resolve and reject args, for the time being, but this will be updated in the future once closures are supported.

Here's what requestAnimationFrame looks like:

// This is out here because there is no closure support for non-top-level variables yet.
let loop: (time: number) => void = (t: number) => {}

export function runMyApp() {
	// Make an infinite game loop:

	loop = (time: number) => {
		// ... render something based on the current elapsed time ...

		requestAnimationFrame(loop)
	}

	requestAnimationFrame(loop)
}

For example, see example/'s index-wasm.ts where it exercises all the APIs.

Finally, make sure when you compile your AS code you pass --exportTable --exportRuntime to the asc CLI. For example:

asc --target release --exportTable --exportRuntime

Portable mode

If you plan to make your code compile to Wasm using AssemblyScript (with asc) and plain JS using TypeScript (f.e. with tsc), then you should make a JS entry point that imports your AS entry point, like so:

// First import JS helpers
import 'assemblyscript/std/portable/index'
import type {} from 'ecmassembly/assembly/PromiseActions-js' // placeholder type for JS

// Then import AS code so that it works in the JS target
export * from './path/to/your-entry-point.js'

Then you will need to write some conditional branching in order to handle the difference between Wasm and JS runtimes when it comes to using Promise. The above Promise example would need to be updated like so:

let actions: PromiseActions<boolean> | null = null

export function runMyApp() {
	const promise = new Promise<boolean>((resolve, reject) => {
		// Temporary hack while AS does not yet support closures (no closing
		// over variable except those that are at the top-level of the module).
		// @ts-expect-error action object is for Wasm only
		actions = resolve

		// resolve after 1 second
		setTimeout(() => {
			if (ASC_TARGET == 0) resolve(time)
			else actions!.resolve(true)
		}, 1000)
	})

	promise.then(result => {
		// this runs one second later, and `result` will be `true` here
	})
}

Where ASC_TARGET == 0 means we're in a JS environment, otherwise the code will use the actions object when compiled to Wasm (ASC_TARGET != 0).

Note When closures for non-top-level variables arrive in AssemblyScript, this conditional checking will not be needed, and the Promise API will work exactly the same way in either environment.

APIs so far

• requestAnimationFrame/cancelAnimationFrame
• setTimeout/clearTimeout
• setInterval/clearInterval
• Promise (rudimentary initial implementation, still lacking things like proper promise chaining and static methods, etc)
  • Complete the API, make it more to spec.
  • Remove PromiseActions after closure support.
• queueMicrotask