NodeOS
Lightweight operating system using Node.js as userspace
...There could be some differences to adjust better to each target platform, but the general structure is barebones custom Linux kernel with an initramfs that boots to a Node.js REPL, initramfs Initram environment to mount the users partition & boot the system, and usersfs multi-user environment with the same behaviour of traditional OSes. All the layers are bootable, leading barebones to a raw naked Node.js REPL prompt as PID 1, while initramfs exec actual NodeOS code to isolate user code from the core system.