Each .then replaces the promise before, instead of wrapping the source promise around it. At each step you have a resolved value, with a monad you'd have to .fl

Each .then replaces the promise before, instead of wrapping the source promise around it. At each step you have a resolved value, with a monad you'd have to .flatMap because mappining a monad to a monad nests it, which you have to later flatMap to extract the boxed values. Compare rxjs streams which are monads to promises which are functors. monads are functors but not vice versa.