Version 1.5 of ArcGIS Experience Builder developer edition is now available on the developers website. In addition to the enhancements from the June update of the online edition, it provides guidelines and samples on how to use the assets like images and videos, write unit testing, and use extension points to initialize the third-party libraries when the app starts or to modify the app config at runtime. You can also search the API reference. See the blog for details.
Cheers,