quickjs-emscripten VS quickjs

Based on your comment below I think you figured out the difference - but if you're looking to execute JS, you can pick between ShadowRealm (where available, or using a polyfill) or my library quickjs-emscripten.

Pros of quickjs-emscripten over ShadowRealm:

- You can use quickjs today in any browser with WASM. ShadowRealm isn't available yet, and polyfills have had security issues in the past. See https://www.figma.com/blog/an-update-on-plugin-security/

- In ShadowRealm eval, untrusted code can consume arbitrary CPU cycles. With QuickJS, you can control the CPU time used during an `eval` using an [interrupt handler] that's called periodically during the eval.

- In ShadowRealm eval, untrusted code can allocate arbitrary amounts of memory. With QuickJS, you can control both the [stack size] and the [heap size] available inside the runtime.

- quickjs-emscripten can do interesting things with custom module loaders and facades that allow synchronous code inside the runtime to call async code on the host.

Pros of ShadowRealm over QuickJS:

- ShadowRealm will (presumably?) execute code using your native runtime, probably v8, JavaScriptCore, or SpiderMonkey. Quickjs is orders of magnitude slower than JIT'd javascript performance of v8 etc. It's also slower than v8/JSC's interpreters, although not by a huge amount. See [benchmarks] from 2019.

- You can easily call and pass values to ShadowRealm imported functions. Talking to quickjs-emscripten guest code requires a lot of fiddly and manual object building.

- Overall the quickjs(-emscripten) API is verbose, and requires manual memory management of references to values inside the quickjs runtime.

[interrupt handler]: https://github.com/justjake/quickjs-emscripten/blob/main/doc...

[stack size]: https://github.com/justjake/quickjs-emscripten/blob/main/doc...

[heap size]: https://github.com/justjake/quickjs-emscripten/blob/main/doc...

[benchmarks]: https://bellard.org/quickjs/bench.html

The thing I want to achieve with WebAssembly is still proving a lot harder than I had anticipated.

I want to be able to take strings of untrusted code provided by users and execute them in a safe sandbox.

I have all sorts of things I want this for - think custom templates for a web application, custom workflow automation scripts (Zapier-style), running transformations against JSON data.

When you're dealing with untrusted code you need a really robust sandbox. WebAssembly really should be that sandbox.

I'd like to support Python, JavaScript and maybe other languages too. I want to take a user-provided string of code in one of those languages and execute that in a sandbox with a strict limit on both memory usage and time taken (so I can't be crashed by a "while True" loop). If memory or time limit are exceeded, I want to get an exception which I can catch and return an error message to the user.

I've been exploring options for this for quite a while now. The furthest I've got was running Pyodide inside of Deno: https://til.simonwillison.net/deno/pyodide-sandbox

Surprisingly I've not found a good pattern for running a JavaScript interpreter in a WASM sandbox yet. https://github.com/justjake/quickjs-emscripten looks promising but I've not found the right recipe to call it from server-side Python or Deno yet.

Can Extism help with this? I'm confident I'm not the only person who's looking for a solution here!

"Safely execute untrusted Javascript in your Javascript, and execute synchronous code that uses async functions" quickjs-emscripten, NPM

This maybe, as a start?

https://github.com/justjake/quickjs-emscripten

QuickJS Running in WebAssembly\ (17 comments)

The library was inspired by Figma’s blog posts about their plug-in system: https://github.com/justjake/quickjs-emscripten#background

If you need to call into user-generated Javascript synchronously or have greater control over the sandbox environment, you can use WebAssembly to run a Javascript interpreter: https://github.com/justjake/quickjs-emscripten#quickjs-emscr...

QuickJS in WebAssembly is much slower than your browser's native Javascript runtime, but possibly faster than async calls using postMessage. As an added bonus, it can make async functions in the host appear to be synchronous inside the sandbox using asyncify: https://emscripten.org/docs/porting/asyncify.html.

You can use One Weird Trick with generator functions to make your code "generic" over synchronicity. I use this technique to avoid needing to implement both sync and async versions of some functions in my quickjs-emscripten library.

The great part about this technique as a library author is that unlike choosing to use a Promise return type, this technique is invisible in my public API. I can write a function like `export function coolAlgorithm(getData: (request: I) => O | Promise): R | Promise`, and we get automatic performance improvement if the user's function happens to return synchronously, without mystery generator stuff showing up in the function signature.

Helper to make a function that can be either sync or async: https://github.com/justjake/quickjs-emscripten/blob/ff211447...

Uses: https://cs.github.com/justjake/quickjs-emscripten?q=yield*+l...

This seems like a pretty nice, recently enabled way of getting a sandboxed js environment: QuickJS compiled to WASM: https://github.com/justjake/quickjs-emscripten.

It uses service workers to cache static files, by the time it opens up you already free to be offline, try toggle network switch to verify.

It has download link at bottom of the about page ([accdoo.app/about]) which you could then self host it by dropping into any static hosting services.

btw, the Pi feature was by-product from the original App but I won't expand here, if you'd like to learn more, please checkout its two Show HN post (39115559 and 39138957) previously.

[wiki]: https://en.wikipedia.org/wiki/Chudnovsky_algorithm

[quickjs/pi]: https://bellard.org/quickjs/pi.html

[pi_bigint.js]: https://github.com/bellard/quickjs/blob/master/examples/pi_b...

[accdoo.app/about]: https://accdoo.app/about#releases

[39115559]: https://news.ycombinator.com/item?id=39115559

[39138957]: https://news.ycombinator.com/item?id=39138957

Just go with quickjs, I think this is what you are looking for.

https://bellard.org/quickjs/

QuickJS

I think QuickJS, written in C, is a user-"friendly" starting point for implementing ECMA-262. Documentation QuickJS Javascript Engine.

There is a readme on the project's main page: https://bellard.org/quickjs/

The newsworthy bit here is that the activity seemed to have stalled for year or two and now Fabrice pushed a few fixes and made a new release.

Just to demonstrate GitHub repositories do not necessarily reflect upon a programmers' body of work, Fabrice Bellard has one (1) repository published on GitHub, quickjs. Compare the list of work on Bellard's home page https://bellard.org/.

> I am still confused, it's a JavaScript runtime intended to be deployed to JavaScript/Wasm runtimes?

Seemingly.

> Why does a JavaScript runtime need a JavaScript runtime?

Because if you want to create a Service Worker server for CloudFlare Workers and other JavaScript/Wasm runtimes, that's the only option for doing that AFAIK.

FWIW, this isn't a new idea. For example, Figma uses QuickJS (https://bellard.org/quickjs/) for their plug-in runtime: https://www.figma.com/blog/an-update-on-plugin-security/