Brian Mckenna criticised current spec. He proposes to use FP approach to achieve much better modularity.

Suggest to read it, really good ideas with just three changes.

https://brianmckenna.org/blog/category_theory_promisesaplus

His proposal is to incorporate into spec three simple apis:

1. Promise.of(a) will turn anything into promise.
2. Promise#then(f) should take one function, not two.
3. Promise#onRejected(f): move onRejected to prototype instead of second arg.

edit: see promises-aplus/constructor-spec#24 for more discussion