Introduction

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

{% code-tabs %} {% code-tabs-item 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;
}

{% endcode-tabs-item %} {% endcode-tabs %}

$> 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. On top of WebAssembly types, it provides both low-level built-ins to access WebAssembly features directly as well as a JavaScript-like standard library on top of a relatively small managed runtime 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 provides implementations of ArrayBuffer , Uint8Array , String , Map etc. on a higher level:

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

The compiler still has its limitations and we are still waiting for WebAssembly features that are currently undergoing specification (marked as 🦄 throughout the documentation) to unleash its full potential. But it is open source and everyone can contribute, so we are getting there.

Sounds appealing to you? Read on!