The console display "this.songs[i].toHTML is not a functio"???

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

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

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

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

var playlist= new Playlist();

var hereComesTheSun= new Song('Here Comes The Sun', '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);

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+= '<span calss="duration">'
    htmlString+= this.duration;
    htmlString+= '</span></li>';

    return htmlString;
};