The stuff I do (numerical modelling) is still done in native code, because performance matters a lot. I've seen something similar done in 640KB, but it was not worth having; this starts to get useful at the single-figure GB level. It sits behind various cloud-based applications, because nobody has yet made an operating system that's all Javascript.

Oh, gawd. Now I've said it, someone will think of writing it. Edit: NodeOS (https://github.com/NodeOS) uses a Linux kernel, and, presumably, device drivers. Its userland is all Node.js, but I was thinking of a kernel in Javascript.