So with that in mind - I started working on a DefinitelyTyped file for MobileFirst. I had to guess a bit at exactly how to do it, and I probably did a few things wrong, but you can get the work in progress here: https://github.com/cfjedimaster/MobileFirst-Typings. As you will see in the ReadMe, I've covered a few of the main classes in the WL namespace (this is the core namespace for the API). I'm looking for feedback on how I built it as well as volunteers to help complete the library with a pull request.
In case your curious as to how well this works, check out the video below:
As a side note - you can get definition files for Apache Cordova and Ionic as well!