They are effectively sugar on JS promises which are effectively sugar around async event callbacks. This is why node rose so quickly in popularity as a server.

They are effectively sugar on JS promises which are effectively sugar around async event callbacks. This is why node rose so quickly in popularity as a server. The async IO handling could be leveraged with callbacks to scale low number of processes to handle large number of requests. Plus easy to learn and lots of people knew some JS. JS always had an event loop (in browser so async events work) and in nodeJS it can be used for async IO. So JS has basically always handled async of sorts (although that has improved significantly).