It's still a scripting language, and you still can't write an OS in it. There's no low-level access to the hardware. You could of course write all the low level parts in C, but then you haven't written an OS in JavaScript.

C also doesn't have low level access to the hardware. The interesting bits of the OS are written in assembly.

I think the only bit that really needs to be in assembly is the context switch, but even that can be embedded in the C (which you might consider cheating, but isn't even an option in JavaScript).