Comment by MobiusHorizons 19 hours ago
I'm curious what practical purpose you could have for running a js execution engine in an environment that already contains a (substantially faster) js execution engine? Is it just for the joy of doing it (if so good for you, absolutely nothing wrong with that).
WebAssembly also runs in places other than the web, where there isn't a JavaScript interpreter at hand. It'd be nice to have a fast JavaScript engine that integrates inside the WebAssembly sandbox, and can call and be called by other languages targeting WebAssembly.
That way, programs that embed WebAssembly in order to be scriptable can let people use their choice of languages, including JavaScript.
It allows, for example, to create bindings as I did for raylib graphics library. exaequOS can run any program that can be built to WebAssembly It will soon support WASI p1 and p2. So many programming languages will be possible for creating programs targeting exaequOS
Is there not a way to use the browser native js execution environment for that? You lose a non-trivial amount of performance running js inside quickjs inside of wasm vs the browser native js engine. I wouldn't be surprised if that's 10 or even 20 times slower, and of course requires loading more code into the browser (slower startup, more ram usage). Maybe you don't care about that, but all of that is pretty orthogonal to the environments I an embedded engine like this is intended for.
This is because the execution is very predictable, so the JIT in the runtime can emit optimized code with the knowledge of how the code is going to run. Embedding unpredictable code (like a javascript interpreter) typically has substantially worse performance when executing under a JIT. This is in addition to the fact that Quickjs (despite being pretty good) can't match the performance of sophisticated JIT implementations like V8 or JavaScriptCore
Sandboxing.
Figma for example used QuickJS, the prior version of the library this post is about, to sandbox user authored Javascript plugins: https://www.figma.com/blog/an-update-on-plugin-security/
It's pretty handy for things like untrusted user authored JS scripts that run on a user's client.