SharePoint JavaScript Library
This library extends the Office Fabric-UI JS library, to help create Office 365 components in SharePoint 2013/Online (Classic) environments. Reference the JavaScript library for information on this library, or SharePoint REST Framework for a full overview of the framework.
Important Bootstrap Library Replacement
There have been user voices related to the Office Fabric-UI JavaScript framework, and newer ones that target web components. At this point, it's safe to say that Microsoft is definitely not going to update the Office Fabric-UI JavaScript framework. I will continue to support any issues reported for this library, but do not plan to add new features.
My recommendation is to convert to use the gd-sprest-bs library, which extends the Bootstrap framework.