can any one explain how .play() or .stop() are access here ?

 //playlist.js
function Playlist() {
   var songs = [];
   var currentIndex = 0;
}

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

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

Playlist.prototype.stop = function(){
     var current = this.songs[currentIndex];
     current.stop();
};

Playlist.prototype.next = function() {
   this.stop();
   this.currentIndex++;
   if(this.currentIndex===this.songs.length){
     this.currentIndex=0;
   }
   this.play();
};

Playlist.prototype.renderIn = function() {

};

  //song.js
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() {

};