Use node functions in my html

Create a route to your getSongById() function in node and then, from your html file make an ajax request to that url.

Let's say, in your app.js, have something like:

app.get('/random-video', function(req, res) {    console.log('random video method called');    var numberOfSongs = db.getNextFreeId()-1;    idToGet=Math.floor(Math.random() * numberOfSongs);    db.getSongById(idToGet, function(err, song){       console.log('found random song: '+ song);       res.send(JSON.stringify(song));    });    });

You'll also have to modify your getSongById() function to be async, like:

database.prototype.getSongById=function(id2, cb){    songModel.findOne({id : id2}, function(err,obj){        if (err) {            console.log('Not Found, error: '+err);            cb(err);        } else if (obj) {            console.log('Found:', obj);            cb(null, obj);        }    });};

Then, in your html page, once you have jQuery loaded, do something like:

<script>$(document).ready(function(){   $.ajax({        url: '/random-video',        contentType: 'application/json; charset=utf-8',    }).done(function(song){       //do stuff with the song    });})</script>