> And, if so, does that mean that once the API has been bootstrapped, one could actually write an OS in js?
I bet somebody has done that.
https://www.google.com/search?q=os+kernel+in+javascript
Seems like a small number of hobbyists have attempted.
I've heard of people doing this with other high level languages. Basically you need enough low level code to bootstrap a VM. Once you have that, you can make the high level language decide some logic that traditionally would be in C code, like manipulating page tables or whatever.
You'd need to write an entire hardware abstraction layer to do anything useful. There's projects that do this for microcontrollers - eg MicroPython and Espruino.
Automatic Garbage Collection in a kernel probably won't work:
I vaguely remember hearing about someone trying to use .Net in the Windows kernel.
The big problem is garbage collection: If I remember correctly, the fact that "any" operation can fail with an out of memory exception was a huge problem. Another problem was that random pauses for garbage collections in the kernel had major stability issues.
In short, I hope that the js kernel is for amusement and education; otherwise it would need a much more advanced garbage collector then earl 2000's .Net.