class="current" does not shift to current song..

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 = [];//song empty array
  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.newPlayingIndex++;
  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();
  };
};

var playlist = new Playlist();

var hereComesTheSun = new Song("Here Comes the Sun","The Beatles","2:54");
var walkingOnSunshine = new Song("Walking on Sunshine","Katrina and the Waves","3:43");

playlist.add(hereComesTheSun);
playlist.add(walkingOnSunshine);

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

playlist.renderInElement(playlistElement);

var playButton = document.getElementById("play");
playButton.onclick = function() {
  playlist.play();
  playlist.renderInElement(playlistElement);
};
var nextButton = document.getElementById("next");
nextButton.onclick = function() {
  playlist.next();
  playlist.renderInElement(playlistElement);
};
var stopButton = document.getElementById("stop");
stopButton.onclick = function() {
  playlist.stop();
  playlist.renderInElement(playlistElement);
};