Cannot read property 'add' of undefined

I keep getting an error in regards to the playlist.add but I am not sure what I am doing wrong? var playlist = Playlist ();

var giveItToYou = new Song ("Give it to you", "Robin Thicke", "3:50"); var fancy= new Song("Fancy", "Iggy Azalea", "3:20"); var wiggle= new Song("Wiggle", "Jason Derulo", "3:20");
var upDown= new Song("Up Down", "T-pain", "3:52"); var bringTheNoise= new Song("Bring the Noise", "M.I.A", "3:24"); var goGirl= new Song("Go Girl", "Ciara", "3:56");

playlist.add(giveItToYou); playlist.add(fancy); playlist.add(wiggle); playlist.add(upDown); playlist.add(bringTheNoise); playlist.add(goGirl);

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); }