Actually, it's slightly more complex because if the result of the function passed to then were somehow a Promise<Promise<something>>, it would be flattened recursively so that the ultimate result is just Promise<something>. (Of course, due to .then's flattening semantics it's highly unlikely you'd have a Promise<Promise<...>> in the first places).

The distinction between map and flatMap makes sense for, say, lists, because you often do want to keep the non-flattened structure around. From promises, I can't think of a reason why that would be desired. You just want to represent the final result after all necessary waiting is completed. I suppose a library could provide stricter map and flatMap for people who deeply care about this.