Music library using object prototypes

music-library.js

var uid = function() {
  return Math.floor((1 + Math.random()) * 0x10000).toString(16).substring(1);
}

function Library (name, creator) {
  this.name = name;
  this.creator = creator;
  this.playlists = [];
  this.tracks = [];
}

function Playlist (name) {
  this.name = name;
  this.id = uid();
  this.tracks = [];
}

function Track (name, rating, length) {
  this.name = name;
  this.rating = rating;
  this.length = length;
  this.id = uid();
}

Library.prototype.createPlaylist = function (name) {
  this.playlists.push(new Playlist(name));
}

Playlist.prototype.addTrack = function (title, rating, length) {
  this.tracks.push(new Track(title, rating, length));
}

Object.defineProperty(Playlist.prototype, 'overallRating', {
  get: function() {
    const sum = this.tracks.reduce((acc, track) => {
      return acc + track.rating;
    }, 0);
    return Math.round((sum / this.tracks.length) * 10) / 10;
  }
})

Object.defineProperty(Playlist.prototype, 'totalLength', {
  get: function() {
    return this.tracks.reduce((acc, track) => {
      return acc + track.length;
    }, 0);
  }
})

const lib = new Library ('Jeffs Library', 'Jeff');
lib.createPlaylist('a playlist');

lib.playlists[0].addTrack('a track', 5, 100);
lib.playlists[0].addTrack('b track', 4, 900);
lib.playlists[0].addTrack('c track', 4, 900);
lib.playlists[0].addTrack('d track', 4, 900);
lib.playlists[0].addTrack('d track', 3, 550);
lib.playlists[0].addTrack('e track', 2, 550);
lib.playlists[0].addTrack('f track', 2, 275);

console.log('Library:', lib);

console.log('Overall rating:', lib.playlists[0].overallRating);
console.log('Total length:', lib.playlists[0].totalLength);