This is an example of how to set up a basic local developing and testing environment for building a high-level webGIS API. Note that it will NOT work for you, since the dummy spatial files (geojson) are too large for GitHub storage.
1) Download and install Node.js from https://nodejs.org/en/download!
2) Open any terminal, move to your working directory where all html and js files are stored, for example: cd D:\DEV.
If you see the following line appear in the terminal: Server running with CORS at http://localhost:8080, you have successfully created a local webserver!
3) Go to any browser and run any html files: http://localhost:8080/dummy_MAIN.html.
The webGIS API appears only if the geojson files exist and their urls are set correctly (change the urls)!
Some screenshots: