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JavaScript Object-Oriented JavaScript (2015) Constructor Functions and Prototypes Making the UI Work

Posted by ianedwards4
ianedwards4
1,576 Points

Adding a "back" button to the "treetunes" player

Does anyone know how you would add a "back" button to the interface of the treetunes fake media player? I understand how to add a new button in the html and change app.js, but what code would you use in playlist.js?

Here is the 'next' code from playlist.js, which is what needs to be modified:

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

I think this part for a back button would be something like...

Playlist.prototype.back = function() { 
  this.stop(); 
this.nowPlayingIndex = ???; 
  if(this.nowPlayingIndex === 0) { 
    this.nowPlayingIndex = ???;
   }  
  this.play(); 
};

Any ideas? Thanks.

2 Answers

James Anwyl
.a{fill-rule:evenodd;}techdegree seal-36
James Anwyl
Full Stack JavaScript Techdegree Graduate 49,960 Points

Here what I came up with:

Playlist.prototype.back = function() {
   this.stop();
   this.nowPlayingIndex--;
   if(this.nowPlayingIndex <= -1) {
         this.nowPlayingIndex += this.songs.length;
     }

  this.play();

};

Some explanation:

this.nowPlayingIndex--;

Subtract 1 from the index.

this.nowPlayingIndex += this.songs.length;

Says: when the index equals -1 (i.e when you press back on the first song in the list), set nowPlayingIndex equal to 0 + length of songs array (which will take us to the last song in the list)

Hope this helps :)

EDIT: also fixed the formatting of your code, see here for a guide to using markdown on the forums

ianedwards4
ianedwards4
1,576 Points

Thanks for this answer, that totally works. Thanks a lot !

James Anwyl
.a{fill-rule:evenodd;}techdegree seal-36
James Anwyl
Full Stack JavaScript Techdegree Graduate 49,960 Points

In what way does it break the interface?

Are you including this line as well:

if(this.nowPlayingIndex <= -1) {
         this.nowPlayingIndex += this.songs.length;
     }

I tested it in Chrome, Firefox and Edge and it works fine for me.

Hope this helps :)

EDIT: Ah awesome, glad you got it working :)

Chris Dyer
Chris Dyer
18,518 Points
Chris Dyer
Chris Dyer
18,518 Points

This is a bit more verbose for new learners. Syntactically James is correct. This just reads a bit easier for me.

Playlist.prototype.back = function() {
  // Stop the current track.
  this.stop();
  // If the index is equal to zero, move index to the element at the end of the list.
  if (this.nowPlayingIndex === 0) {
    this.nowPlayingIndex = (this.songs.length - 1);
  // Otherwise, we are safe to decrement.
  } else {
    this.nowPlayingIndex--;
  }
  // Kick back on the tunes.
  this.play();
}