While - in general - you are quite right, most of the bigger libraries that I use in production offer interfaces for both styles. Mongoose for instance. You can

While - in general - you are quite right, most of the bigger libraries that I use in production offer interfaces for both styles. Mongoose for instance. You can easily use it with Promises or with the old-school callback style. Same for unit test libraries. And this trend will only continue. There is a lot of cool research being done in combining JS with hardcore FP (fantasy-land, ramda) and this will of course influence future developments. Not to forget the FRP community around React/Bacon etc... I predict that in a year or two, the majority of devs will use Promises as if they always have been there.... And hey. Promises are Monads [1], so maybe we will also soon see a lot of Eithers and Maybes in Production code. I taught today a lecture about this at my company, and people with no prior exposure to FP immediately loved it. Thats also one cool thing of the js/node community. We just adopt stuff and try it out. With microservices, this also poses no problem. If it doesn't work, do it differently in the next service. [1] I know, I know. A+ Promises are violating some laws BUT the general idea holds. And there are full monadic promise libraries like Data.Task or Fluture available.