I don't even think the worst part is the download (which can be cached). The worst part is the fact that you're running your script, interpreted by a weirdly generated unoptimized script, interpreted by a scripting engine which only then runs on metal. I'm shooting in the dark, but I'd bet the performance and memory usage would be a disaster for anything but Hello, world.

Actually, right now, even Hello world is a disaster. My guess is that this will drive some interesting improvements to emscripten and LLVM so there may be some benefits down the line.

There's also going to be an enormous startup time as it has to parse and compile all that code, even if it's already cached.

65MB is our current unoptimized, uncompressed version. I expect the final version to be around 5-10MB. Still large, but not quite as bad.

I'm not super familiar with Narcissus in practice, but on paper your projects sound very similar. Were there any interesting reasons why you opted to make this project instead?

We wanted the full power of the SpiderMonkey JS API and maximum browser compatibility.

i can see some possible for this where you could add support to your application for plugin development or even macro-type scripting within a sandbox.