server.js

const http = require('http'),
      fs   = require('fs'),
      port = 3000

const server = http.createServer( function( request,response ) {
  switch( request.url ) {
    case '/':
      sendFile( response, 'index.html' )
      break
    case '/index.html':
      sendFile( response, 'index.html' )
      break
      case 'styles.css':
      sendFile( response, 'styles.css' )
      break
    default:
      response.end( '404 Error: File Not Found' )
  }
})

server.listen( process.env.PORT || port )

const sendFile = function( response, filename ) {
   fs.readFile( filename, function( err, content ) {
     response.end( content, 'utf-8' )
   })
}

$(document).on("scroll", function() {
  var pageTop = $(document).scrollTop();
  var pageBottom = pageTop + $(window).height();
  var tags = $(".tag");

  for (var i = 0; i < tags.length; i++) {
    var tag = tags[i];

    if ($(tag).position().top < pageBottom) {
      $(tag).addClass("visible");
    } else {
      $(tag).removeClass("visible");
    }
  }
});