AssemblyScript in WebAssembly

Inspired by TypeScript, AssemblyScript is a strongly typed language. It was written specifically with WebAssembly in mind, and the entire toolchain is oriented around WebAssembly.

Since it has a WASI implementation, AssemblyScript can be used on the Fermyon Platform for writing Wagi or Spin apps.

It is also well-suited for browser-based applications. And it can run inside of Wasmtime and other CLIs.

Available Implementations

AssemblyScript has an official implementation.


To get started with AssemblyScript, you will need to have a Node.js environment, including NPM.

From there, you can get started by creating a new Node project.

Pros and Cons

Things we like about AssemblyScript:

  • Familiar to TypeScript and JavaScript developers
  • Because of that, we can use our normal tools for dev
  • Good integration with NPM and the Node ecosystem
  • Support for common JS idioms (like Console.log instead of println)

We’re neutral about:

  • File sizes, which are larger than we expected, but not enough to be a problem
  • The automatic generation of WAT files, unoptimized binaries, and things we don’t normally use

Things we’re not big fans of:

  • It’s just different enough from TypeScript to be frustrating at times.
  • AssemblyScript WTF-16 instead of the more common UTF-8/UTF-16. This is a hotly debated issue, but our preference is for UTF-8.


This section provides a basic example of building AssemblyScript from source.

All of our examples follow a documented pattern using common tools.

Set up the project like this:

$ mkdir AssemblyScript-example
$ cd AssemblyScript-example
$ npm init
# Answer questions

Install the AssemblyScript compiler:

$ npm install --save-dev assemblyscript
$ npm install --save @assemblyscript/loader

Install the WASI shim

$ npm install --save-dev @assemblyscript/wasi-shim

Now use npx to scaffold out your new project:

$ npx asinit .
Version: 0.27.24

This command will make sure that the following files exist in the project
directory 'AssemblyScript-example':

  Directory holding the AssemblyScript sources being compiled to WebAssembly.

  TypeScript configuration inheriting recommended AssemblyScript settings.

  Example entry file being compiled to WebAssembly to get you started.

  Build artifact directory where compiled WebAssembly files are stored.

  Git configuration that excludes compiled binaries from source control.

  Configuration file defining both a 'debug' and a 'release' target.

  Package info containing the necessary commands to compile to WebAssembly.

  Stater test to check that the module is functioning.

  Starter HTML file that loads the module in a browser.

The command will try to update existing files to match the correct settings
for this instance of the compiler in 'AssemblyScript-example/node_modules/assemblyscript'.

Do you want to proceed? [Y/n] Y

- Making sure that the project directory exists...
  Exists: AssemblyScript-example

- Making sure that the 'assembly' directory exists...
  Created: AssemblyScript-example/assembly

- Making sure that 'assembly/tsconfig.json' is set up...
  Created: AssemblyScript-example/assembly/tsconfig.json

- Making sure that 'assembly/index.ts' exists...
  Created: AssemblyScript-example/assembly/index.ts

- Making sure that the 'build' directory exists...
  Created: AssemblyScript-example/build

- Making sure that 'build/.gitignore' is set up...
  Created: AssemblyScript-example/build/.gitignore

- Making sure that 'package.json' contains the build commands...
  Updated: AssemblyScript-example/package.json

- Making sure that 'asconfig.json' is set up...
  Created: AssemblyScript-example/asconfig.json

- Making sure that the 'tests' directory exists...
  Created: AssemblyScript-example/tests

- Making sure that 'tests/index.js' exists...
  Created: AssemblyScript-example/tests/index.js

- Making sure that 'index.html' exists...
  Created: AssemblyScript-example/index.html


Don't forget to install dependencies before you start:

  npm install

To edit the entry file, open 'assembly/index.ts' in your editor of choice.
Create as many additional files as necessary and use them as imports.

To build the entry file to WebAssembly when you are ready, run:

  npm run asbuild

Running the command above creates the following binaries incl. their respective
text format representations and source maps:


  ^ The debuggable WebAssembly module as generated by the compiler.
    This one matches your sources exactly, without any optimizations.


  ^ The optimized WebAssembly module using default optimization settings.
    You can change the optimization settings in 'package.json'.

To run the tests, do:

  npm test

The AssemblyScript documentation covers all the details:

Have a nice day!

At this point you should have a directory structure that looks like this:

$ tree -L 2        
├── asconfig.json
├── assembly
│   ├── index.ts
│   └── tsconfig.json
├── build
├── index.html
├── node_modules
│   ├── @assemblyscript
│   ├── assemblyscript
│   ├── binaryen
│   └── long
├── package-lock.json
├── package.json
└── tests
    └── index.js

9 directories, 7 files

The assembly/ directory is where the code lives.

Now, we can write a simple AssemblyScript module in the assembly/index.ts file, as shown below:

console.log("content-type: text/plain");
console.log("Hello, World");

While based on JavaScript and TypeScript, AssemblyScript is not a typical scripting language. It must be compiled before it can be executed. The AssemblyScript tools configure NPM to run the compiler for us:

$ npx asc assembly/index.ts -o build/optimized.wasm --optimize --config ./node_modules/@assemblyscript/wasi-shim/asconfig.json

Running in wasmtime

$ wasmtime build/optimized.wasm 
content-type: text/plain
Hello, World

The module emits content-type information and an empty line, so it can be executed in any Wagi runtime such as Wagi, Spin, or Wagi.NET.

Running using spin up

Here’s an example with Spin.

Create a simple spin.toml and edit it as follows::

spin_manifest_version = 2

name = "spin-hello-ts"
version = "0.1.0"
authors = ["Fermyon Engineering <>"]
description = "spin-hello-ts"

route = "/..."
executor = { type = "wagi" } # Note: We are running this using the Wagi spec
component = "spin-hello-ts"

source = "build/optimized.wasm"

Then run Spin to serve our new module at http://localhost:3000/:

$ spin up
Logging component stdio to ".spin/logs/"

Available Routes:
  spin-hello-ts: (wildcard)

At this point you can use a web browser or curl to check the results:

$ curl -i localhost:3000   
HTTP/1.1 200 OK
content-type: text/plain
content-length: 13
date: Mon, 04 Mar 2024 05:09:15 GMT

Hello, World

For more on running AssemblyScript in the browser, read the AssemblyScript documentation

Learn More

Here are some great resources: