This is not as general as a monad though, it's just feeding the output of one function into the next.

I'm sure you know this, but the thing that makes monads a more general solution is that (because bind depends on a value produced at runtime) one can dynamically alter control flow, i.e:

    fetch = do
      user <- fetchUser "url"
      if userEmpty user
        then do
          fetchUserDetails "details"
          fetch -- recurse
        else
          fetchAddress user

For that to work, your functions inside the `composeAsync` array would need to able to return nested promises and at that point you've just reimplemented the continuation monad :)