https://www.google.com/search?q=nodejs+private+functions&oq=nodejs+private+functions&aqs=chrome..69i57j0l5.2534j0j1&sourceid=chrome&espv=210&es_sm=91&ie=UTF-8

mainly:
http://elegantcode.com/2011/02/04/taking-baby-steps-with-node-js-commonjs-and-creating-custom-modules/