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

Nick Tolbert
Nick Tolbert
12,003 Points
Posted by Nick Tolbert
Nick Tolbert
12,003 Points

[RESOLVED] am getting an error of Uncaught ReferenceError: song is not defined

can someone tell me why am getting this and how to correct it?

//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() {
   var htmlString = '<li';
   if(this.isplaying){
   htmlString+=' class="current"';
   }
   htmlString+='>';
   htmlString+=this.title; 
   htmlString+=' - ';
   htmlString+=this.artist;
   htmlString+='<span class="duration">';
   htmlString+= this.duration; 
   htmlString+='</span></li>';
  return htmlString;
};


//playlist.js

function Playlist() {
  this.songs = [];
  this.nowPlayingInex = 0;
}

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

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

Playlist.prototype.stop = function(){
  var currentSong = this.songs[this.nowPlayingInex];
  currentSong.stop();  

};

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

};

Playlist.prototype.renderInElement = function(list) {
  list.innerHTML="";
  for(var i=0;i<Songs.length;i++){
    list.innerHTML+= this.songs[i].tohtml();

  }

};

//app.js

var Playlist = new Playlist();

var hereComesTheSun = new song("Here Comes The Sun","The beatles", "2:54");
var iGotAFeeling = new song("I Got a Feeling","The Beatles", "4:30");

Playlist.add(hereComesTheSun);
Playlist.add(iGotAFeeling);
 var playlistElement = document.getElementById("playlist");
  Playlist.renderInElement(playlistElement);

4 Answers

Chad Donohue
Chad Donohue
5,657 Points 
//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() {
   var htmlString = '<li';
   if(this.isplaying){
   htmlString+=' class="current"';
   }
   htmlString+='>';
   htmlString+=this.title; 
   htmlString+=' - ';
   htmlString+=this.artist;
   htmlString+='<span class="duration">';
   htmlString+= this.duration; 
   htmlString+='</span></li>';
  return htmlString;
};


//playlist.js

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

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

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

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

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

Playlist.prototype.renderInElement = function(list) {
  list.innerHTML="";
  for ( var i = 0; i < this.songs.length; i++ ) {
    list.innerHTML+= this.songs[i].toHTML();
  }
};

//app.js

var playlist = new Playlist();

var hereComesTheSun = new Song("Here Comes The Sun","The beatles", "2:54");
var iGotAFeeling = new Song("I Got a Feeling","The Beatles", "4:30");

playlist.add(hereComesTheSun);
playlist.add(iGotAFeeling);
var playlistElement = document.getElementById("playlist");
playlist.renderInElement(playlistElement);
Chad Donohue
Chad Donohue
5,657 Points

I have changed some capitalizations and changed your call in the for loop to toHTML().

A good rule of thumb is to keep your class definitions capitalized and your variable declarations camel-cased.

This way, you know if you are dealing with a class definition or a variable.

Nick Tolbert
Nick Tolbert
12,003 Points
Nick Tolbert
Nick Tolbert
12,003 Points

can you give me an example of class definition ?

Chad Donohue
Chad Donohue
5,657 Points

Class definition

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

Song.prototype.play = function() {
  this.isplaying= true;
};

Using the class with a new instance

var hereComesTheSun = new Song("Here Comes The Sun","The beatles", "2:54");
Nick Tolbert
Nick Tolbert
12,003 Points
Nick Tolbert
Nick Tolbert
12,003 Points

OK, I understand it now, this course is kinda fast pace ; I understand the concepts but there is a lot function grabbing from other function and I don't feel to confident in my ability to layout all the necessary functions. is there any resources that can help with to planing stage or dose this come with practice?

Chad Donohue
Chad Donohue
5,657 Points

It's really a bit of both. Keep practicing and don't be afraid to ask questions like you already have.

As far as resources go, you should check out Eloquent JavaScript

Nick Tolbert
Nick Tolbert
12,003 Points
Nick Tolbert
Nick Tolbert
12,003 Points

ok cool, and thanks again for all the help!

Chad Donohue
Chad Donohue
5,657 Points
Chad Donohue
Chad Donohue
5,657 Points

You need to capitalize 'Song' when you are assigning the object to a variable.

var hereComesTheSun = new Song(...);
Nick Tolbert
Nick Tolbert
12,003 Points
Nick Tolbert
Nick Tolbert
12,003 Points

I went through my code and change both song and songs to Song/Songs but now I have the same error in my for loop on line 35 of playlist.js

Playlist.prototype.renderInElement = function(list) {
  list.innerHTML="";
  for(var i=0; i< Songs.length; i++){
    list.innerHTML+= this.Songs[i].tohtml();

  }

};
Chad Donohue
Chad Donohue
5,657 Points
Chad Donohue
Chad Donohue
5,657 Points

Where is Songs declared in playlist.js?

Nick Tolbert
Nick Tolbert
12,003 Points
Nick Tolbert
Nick Tolbert
12,003 Points

at the top of playlist.js inside the first function

function Playlist() {
  this.Songs = [];
  this.nowPlayingInex = 0;
}
Chad Donohue
Chad Donohue
5,657 Points
Chad Donohue
Chad Donohue
5,657 Points

I think you want

for(var i=0;i<this.songs.length;i++){
    list.innerHTML+= this.songs[i].tohtml();
}
Nick Tolbert
Nick Tolbert
12,003 Points
Nick Tolbert
Nick Tolbert
12,003 Points

thank you for the help chad, after I add the missing this.Songs in the for loop I also needed to change tohtml() to toHML()

Chad Donohue
Chad Donohue
5,657 Points

Glad you figured it out :)