[kevingoslar]

There is a way to support both styles at the same time, and most libraries (that I'm aware of) use it. If a callback is given, call it with the result. If not, return a promise for the result. This doesn't require a rewrite of existing libraries, but merely a new minor version.

I don't see how compile-time type checking would help here. When checking out a new library, I read the documentation, or maybe it's test suite, and then use it accordingly. My tests ensure that I use it correctly, i.e. give it types it understands, give it the right data (the id of the correct user account for example), and handle the outcome correctly (i.e. display it in the right format).

JavaScript has run-time type checking, which could be used for automatic promisification, but I agree that's a slippery road, for many reasons.