The best isolation is inside a Service Worker, where the script is served with Content-Security-Policy: sandbox header.[1]

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/...

Or just an iframe?

I think that this is a plugin library for teams that want to offer a platform for the public (or an LLM-AI) to submit code to. If your team writes some code, you don't generally sandbox it from yourself, you just amend your program: you don't need a sandbox. But, if you want to run code that you don't trust, you should run it in a way that prevents it from causing problems if it is actually dangerous (like a virus or accidentally overwrites your files with blank files). That's what a sandbox like kyushu promises to do.

So, with a sandbox library like this, you could - say - write a website that hosts games (like itch.io or newgrounds) that hosts games on the world wide web. The sandbox part can give you confidence that, if a villain's programmer henchmen uploads a virus instead of a game, it can't infect your platform or other games on the website. Or, if a LLM-AI written game is accidentally tries to take up all the memory of the computer, it can't ask the operating system for more than is in the sandbox.

Haha, glad someone noticed those testimonials ;)

Others mentioned better use cases than I could probably come with. Not sure it's a strong use case but, one thing I could maybe mention too is the fact that it ships as a standalone artifact. It's portable and, if reproducible, can provide some sort of guarantee on what's effectively running for those who care.

it is also perfect for running untrusted user code safely, if you want to buld a plugin system or your own edge functions this can be really helpful

ELI5: Imagine you want to run a heavy, powerful 3D video game engine inside a standard web browser or a lightweight desktop app, without making it slow or unsafe.

JavaScript alone can't handle that kind of heavy lifting efficiently. That’s where Wasm comes in. It lets you run high-performance native code (like C++) at near-native speed safely in the sandbox.

For example, I'm currently using Wasm to run a complex 3D geometry engine (Manifold) inside a lightweight CAD app (Nasscad). It gives you web flexibility with desktop power.

I had previous experience with QuickJS - respectively using the rquickjs crate (awesome project) - so my approach was first asserting whether it was possible to run a Wasmtime binary that both executes the JS code and handles HTTP requests and responses.

Then, the second part which was really important to me, was figuring out if I could find a way to embed the developer's JS code within the worker without requiring them to install Cargo. (thanks to Wizer it's possible, love it).

Once I had those two, the rest was basically execution (not saying it was straightforward though ;)

I was also a bit lucky: at the same time as I was developing it, Rolldown announced the version 1 of their standalone crate. So it was the perfect timing to use it as well.

As for StackBlitz WebContainers, I actually don't know much about it. They run in the browser as I understand, so fundamentally different but, feature wise I'm sure this project is way more mature and therefore offers way more features.

I haven't benchmarked it yet but, guts feeling, would expect it to be (a bit?) slower. Cloudflare has likely spent quite some effort optimizing it and also runs fast machine. Kyushu uses QuickJS which adds a layer that needs to be loaded but, it's pre-initialization in memory which should mitigate the cost.