Each released versions of GeoJS can be found on Github.
GeoJS is hosted on cdnjs. To use GeoJS, include the following in your HTML head:
<script src="https://cdnjs.cloudflare.com/ajax/libs/geojs/1.4.2/geo.min.js" integrity="sha256-t9oLE+BFJc7iWaTIqMFk+8dYJn5kmfg/FIdKNuAxMII=" crossorigin="anonymous"></script>
This will include minified version of GeoJS and its dependencies on your page. The integrity hash will make sure the integrity of the file.
npm install geojs
This will install source code and built version GeoJS and it's optional dependencies to node_modules.
Once GeoJS is installed with npm, you could include GeoJS using as an ES6 module.
import geo from 'geojs';
Then you could use GeoJS with the variable
geo in your script.