.add();

Hi troy the .add() function simply adds a song to the playlist. However to do this there are a number of steps involved.

First off when you create the Song constructor function, you are giving it a list of properties that can be filled with values when creating an instance of the object:

function Song(title, artist, duration) {
  this.title = title;
  this.artist = artist;
  this.duration = duration;
  this.isPlaying = false;
}

When you create the Playlist constructor function, you add an empty array of songs that can be added to later on:

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

By adding the add method via using .prototype you are asking the program to add or push a song into the songs array:

Playlist.prototype.add = function(song) {
  this.songs.push(song); //add the song to the songs array
};

Then you are creating a new Playlist and Song object:

var playlist = new Playlist(); //create a new playlist

var hereComesTheSun = new Song ("Here Comes the Sun", "The Beatles", "2:54"); //title, artist, duration

The song and its details is then added to the playlist object via .add():

playlist.add(hereComesTheSun); //add the song "Here Comes The Sun" to the playlist

hope this helps!!