Introduction

AssemblyScript compiles a strict subset of TypeScript (a typed superset of JavaScript) to WebAssembly using Binaryen, looking like:

{% tabs %} {% tab title="fib.ts" %}

export function fib(n: i32): i32 {
  var a = 0, b = 1
  for (let i = 0; i < n; i++) {
    let t = a + b; a = b; b = t
  }
  return b
}

{% endtab %} {% endtabs %}

$> asc fib.ts -b fib.wasm -O3

Its architecture differs from a JavaScript VM in that it compiles a program ahead of time, quite similar to other static compilers. One can think of it as a mix of TypeScript's syntax and C's capabilities.

In a nutshell

On top of WebAssembly types, it not only provides low-level built-ins to access WebAssembly features directly, making it suitable as a thin layer on top of Wasm, but also comes with its own JavaScript-like standard library, making it suitable for non-browser use cases, along a relatively small managed runtime (with memory management and garbage collection) enabling the creation of programs that look and feel much like TypeScript.

For example, on the lowest level, memory can be accessed using the load<T>(offset) and store<T>(offset, value) built-ins that compile to WebAssembly instructions directly

store<i32>(8, load<i32>(0) + load<i32>(4))

while the standard library not only provides optimized native Math implementations (both double and single precision) but also implementations of ArrayBuffer, Uint8Array, String, Map etc. on a higher level:

var view = new Int32Array(12)
view[2] = view[0] + view[1]

In turn it also comes with a bunch of features JavaScript doesn't have out of necessity, like the ability to declare operator overloads that arrays for example use as an implementation helper, yet is really more a subset with bonuses than a superset.

As of today, the compiler still has its limitations and we are waiting for WebAssembly features that are currently undergoing specification (marked as 🦄 throughout the documentation, especially Reference Types 🦄, Interface Types 🦄 and Wasm GC 🦄) to unleash its full potential. But it is open source, built upon an open specification and everyone can contribute, so we are getting there.

Sounds appealing to you? Read on!