Error-'undefined'

Hi I have followed the video in JavaScript object about building playlist application. When I run the program I get the error saying that htmlSring is undefined, even though it has been defined. Please find the code below:

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 === songs.length) { this.nowPlayingIndex = 0; } this.play(); };

function Song(title, duration, artist) { this.title = title; this.duration = duration; this.artist = artist; this.isPlaying = false; } Song.prototype.play = function () { this.isPlaying = true; }; Song.prototype.stop = function () { this.isPlaying = false; }; Playlist.prototype.renderInElement = function (list) { list.innerHTML = ""; for (var i = 0; i < this.songs.length; i++) {

    var string = this.songs[i].toHTML();
    list.innerHTML += string;

}

};

Song.prototype.toHTML = function () {

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

};

var mySong1 = new Song("tum mere ho", "akhtar", "2:32"); var mySong2 = new Song("jab hum jawan hon gay", "aslam", "4:32"); var playlist = new Playlist(); playlist.add(mySong1); playlist.add(mySong2); var playListElement = document.getElementById("playlist"); playlist.renderInElement(playListElement);