Gjs is a JavaScript engine based on Spidermonkey, which contains bindings to Gtk and other libraries that are used to create graphical user interfaces under GNOME. The method by which Gjs communicates with these libraries is through APIs exposed by the GObject Introspection framework.

GNOME Shell is implemented in JavaScript and run by Gjs, and Gjs therefore comes with any GNOME 3 installation. Those who have GNOME 3 installed will not need to install any additional packages. Everyone else can get Gjs through their package managers.

On Debian and derived distributions like Ubuntu:

apt-get install gjs libgjs-dev

On Fedora:

dnf install gjs gjs-devel

On Arch:

pacman -S gjs