What does the command "node ." do?
By default Node.js is trying to load a module located in the folder that you are passing to it as an argument (. - just bash variant of the current folder). Then it runs whatever is written in the "main" section of the package.json file found in this folder.
In your case, it'll try to run node ./index.js
Doc: https://docs.npmjs.com/files/package.json#main
Good point from @djechlin: if no package.json found in the folder or if no "main" section is present, then Node.js will try to run the index.js file in this particular folder you are passing.