JavaScript in WebAssembly

Compiling JavaScript to WebAssembly is different than using JavaScript to talk to a WebAssembly module. This article is focused on how to take JavaScript code and build it into a WebAssembly module.


JavaScript in WebAssembly is a recent development. Shopify’s platform can generate and run Wasm modules. If Shopify’s javy tool is built with --feature standalone-wasi, then it can create wasm32-wasi modules that can be run on the Fermyon platform. While SpiderMonkey can be compiled into JavaScript, only proof-of-concept code exists for building and running WASM this way. However, we expect the SpiderMonkey implementation to mature rapidly.

Available JavaScript Implementations

There are three popular ways of building JavaScript into WebAssembly.

  1. Use the Mozilla SpiderMonkey engine
  2. Use the QuickJS implementation and compile the runtime and script into a Wasm module
  3. Use the Ducktape implementation of a JavaScript runtime, usually to “safe eval” JS inside of JS using Wasm as an indirection layer

Recently, Suborbital has introduced a version of Javy that supports some of their extensions.

You can compile JavaScript and TypeScript to Wasm for the Spin runtime using the Spin JavaScript SDK. The Spin SDK borrows heavily from Javy, using the same approach of providing a CLI utility to convert a JS file into a Wasm file.

JCO is a fully native tool for working with WebAssembly Components in JavaScript.

Example (Using Spin)

The Spin SDK makes it very easy to build Javascript/TypeScript Wasm applications simply by using a Spin template that handles all of the heavy lifting. If you would like to try out the Spin SDK for Javascript please follow along with the example below.


If you have not done so already, please install Spin. Having Spin installed will allow us to easily use js2wasm and Spin application templates.


$ spin plugin update
$ spin plugin install js2wasm

The Spin JS/TS SDK Template

The Spin JS/TS SDK provides a couple of Spin templates for quickly starting a new JS or TS application. These templates can be installed using the following command:

$ spin templates install --git

You will now see http-ts and http-js available when listing installed templates:

$ spin templates list


Create a new Spin application, using the http-js template:

$ spin new -t http-js javascript-example --accept-defaults

Take a look at the scaffolded program in javascript-example/src/index.js:

export async function handleRequest(request) {

    return {
        status: 200,
        headers: { "content-type": "text/plain" },
        body: "Hello from JS-SDK"

Compile a Wasm binary and then start up a local server:

$ cd javascript-example
$ npm install
$ spin build --up
Building component javascript-example with `npm run build`
// --snip--
Available Routes:
  javascript-example: (wildcard)

Test it with curl:

$ curl localhost:3000/
Hello from JS-SDK

The Wasm binary can be found at target/javascript-example.wasm.

Learn More

Here are some great resources:

  • Shopify has created an easy-to-use builder that uses QuickJS. It is called Javy
  • A short article on Wasm.Builders covers JavaScript, MessagePack, and Javy
  • Bytecode Alliance’s builder for Spidermonkey provides binary releases
  • QuickJS is a tiny JavaScript runtime that can be compiled to Wasm.
  • Making JavaScript Run Fast on WebAssembly from Bytecode Alliance explains how SpiderMonkey and Wasm work together.
  • wasm-jseval uses Ducktape compiled to Wasm to eval() JavaScript inside of JavaScript. Think of it as sandboxed JS
  • QuickJS-Emscripten does something similar to Wasm-JSEval, but with QuickJS instead of Ducktape
  • While much conversation is dominated by JS devs who use Wasm in-browser, there is good discussion on the WebAssembly Discord server’s #javascript channel.