async is pretty terrible though, you cannot pass the results from one execution to another, i.e. passing results from one function in async.series to another, which results in developers tending to pollute variable scope by defining above and filling it in inside each callback. This prevents the ability to write clean isolated testable functions.

So promises are way more composable than callback based solutions.

Which was: throwing away the entire language's compositional features, including the concept of input arguments and return values, results with... poor compositionality, of course.