Items are not showing 2

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

}

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




//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 +='>' + this.title + ' - ' + this.artist + '<span class="duration">' + this.duration + '</span></li>';
    return htmlString;
};

var playlist = new Playlist();


var herecomeourson= new Song("Here come our son","The Beatles", "2:54");
var song2 = new Song("Song2","Blur", "3:04");

playlist.add(herecomeourson);
playlist.add(song2);  

var playListElement= document.getElementById("playlist");
playlist.renderInElement(playListElement);