[JaRail]

As a general answer, doing something simple in pure JS can be more efficient than the overhead associated with making a native call from JS. The costs are going to vary depending on how the JIT handles things. That's probably the best general reason though as to how it's possible.

[scrabble]

As I understand it, native promises don't make native system calls. They are still pure js. They are just implemented differently from Bluebird which optimizes for the most common use cases.