Song list not being rendered Help!

var playlist = new Playlist();

var yellowSubmarine = new Song("Yellow Submarine", "The Beatles", "3:56");
var everlong = new Song ("Everlong", "Foo Fighters", "3:12");

playlist.add(yellowSubmarine);
playlist.add(everlong);

var playlistElement = document.getElementById("playlist");

function Playlist() {
  this.songs = [];
  this.nowPlayingIndex = 0;
}

Playlist.prototype.add = function(song) {
  this.songs.push(song);
};

Playlist.prototype.play = function() {
  var currentSong = this.songs[this.nowPlayingIndex];
  currentSong.play();
};

Playlist.prototype.stop = function(){
  var currentSong = this.songs[this.nowPlayingIndex];
  currentSong.stop();
};

Playlist.prototype.next = function() {
  this.stop();
  this.nowPlayingIndex++;
  if(this.nowPlayingIndex === this.songs.length){
    this.nowPlayingIndex = 0;
  }
  this.play();
};

Playlist.prototype.renderInElement = function(list) {
  list.innerHTML = "";
  for(var i = 0; i < this.songs.length; i++){
    list.innerHTML += this.songs[i].toHTML();
  }
};

function Song(title, artist, duration) {
  this.title = title;
  this.artist = artist;
  this.duration = duration;
  this.isPlaying = false; 
}
console.log(Song.title);
Song.prototype.play = function() {
  this.isPlaying = true;
};

Song.prototype.stop = function() {
  this.isPlaying = false;
};

Song.prototype.toHTML = function() {
 var htmlString = '<li ';
 if(this.isPlaying){
  htmlString += ' class="current"';
  }
  htmlString += '>';
  htmlString += this.title;
  htmlString += ' - ';
  htmlString += this.artist;
  htmlString += '<span class="duration">';
  htmlString += this.duration;
  htmlString += '</span></li>';
  return htmlString;
};