This is a library built for GJS to allow defining GTK widgets in a declarative way. It also provides services and other utilities to interact with the system so that these widgets can have functionality. GJS is a JavaScript runtime built on Firefox's SpiderMonkey JavaScript engine and the GNOME platform libraries, the same runtime GNOME Shell runs on.
It was heavily inspired by EWW.
Currently, only Wayland is supported, but it also works on X11, see #19.
To get started read the wiki.
