I'm not sure how it's possible but it's definitely not rare. Lodash' methods are faster than their respective native implementations (Lodash `map` > `Array.prototype.map`).

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.

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.

Bluebird isn't written in C. It doesn't even depend on a compiled node addon.

Oh you're right. https://github.com/petkaantonov/bluebird/blob/master/src/pro...