Songs aren't rendering properly, but the data is there.

function Song(title, artist, duration) {
  this.title = title;
  this.artist = artist;
  this.duration = duration;
  this.isPlaying = false;
}

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;
};

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];
  }
};

var playlist = new Playlist();

var heyEverybody = new Song("Hey Everybody", "Gargamel!", "1:15");
var murmaider = new Song("Murmaider", "Dethklock", "4:25");

playlist.add(heyEverybody);
playlist.add(murmaider);

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

playlist.renderInElement(playlistElement);