ben.js

var Track = (function() {
  function Track(title, artist) {
    this.title  = title  || 'Untitled';
    this.artist = artist || 'Unknown';
  }

  Track.prototype.toString = function() {
    return this.title + ',\t\t' + this.artist;
  };

  return Track;
}());

var MusicLibrary = (function() {
  function MusicLibrary() {
    var tracks = [];
    this.getTracks = function() {
      return tracks;
    };
  }

  MusicLibrary.prototype.addTrack = function(track) {
    if (track instanceof Track) {
      return this.getTracks().push(track);
    }
  };

  MusicLibrary.prototype.iterate = function(callback) {
    if (!callback || typeof callback !== 'function') {
      return;
    }
    return this.getTracks().map(callback);
  };

  MusicLibrary.prototype.print = function() {
    this.iterate(function(track) {
      console.log(track.toString());
    });
  };

  return MusicLibrary;
}());

/* Testing internal iterator */
var lib = new MusicLibrary();

lib.addTrack(new Track('Ocean', 'John Butler'));
lib.addTrack(new Track('MS', 'Alt-J'));
lib.addTrack(new Track('Nuvole Bianche', 'Einaudi'));
lib.addTrack(new Track('Grown Woman', 'Beyonce'));

console.log('Printing...');
lib.print();