Where in this code is class ="current" being removed from the currently playing song before it's added to the next song?

Whether or not a song should have the 'current' class attached is dealt with in the Song.prototype.toHTML function with an if structure.

// It should look something like this...
  if(this.isPlaying){
    htmlString += ' class="current"';  // If the song is playing, add the class current.
  }  

// When a song is stopped, isPlaying should be set to false.
Song.prototype.stop = function() {
  this.isPlaying = false;
};

Playlist.prototype.next = function() {
  this.stop(); // The first thing the next function does is stop the current song.
  // .  .  .
};

If you're still seeing this behavior after looking into these code snippets, you could post your solution and we can see if we can't figure out what's going wrong. Happy coding!

Did you remember to run the playlist.renderInElement(playlistElement); code in each of the onclick handlers? He mentioned that you need to render the HTML again...

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);
}

Also make sure that you're resetting the innerHTML property before changing it:

list.innerHTML = "";

I had the same issue. The solution for me was that in my Playlist.next function I had written this.stop instead of this.stop(). Maybe you did the same? Check your parenthesis.