JavaScript Subclass - how to inherit parent values defined in parameter?

Question

Using JavaScript how do I create a subclass that inherits values from the parent class? These values I need to inherit are defined by the parameters of the parent class. I want to have a one-to-many relationship between the parent class and the child class. Here is my example code where a Song consists of one or more Tracks:

function Song(x){
  this.x = x;
}

function Track(y){
  Song.call(this);
  this.y = y;
}

Track.prototype = new Song;


var mySong = new Song(1);
mySong.guitar = new Track(2);
mySong.bass = new Track(3);
// goal is to output "1 1 2 3" but currently outputs "undefined undefined 2 3"
console.log(mySong.guitar.x + " " + mySong.bass.x + " " + mySong.guitar.y + " " + mySong.bass.y );

This question is similar to this sub-class question (http://stackoverflow.com/questions/1204359/javascript-object-sub-class) but uses variables defined by parameters. Right now when I try to call parentObject.childObject.parentVariable it returns undefined. What do I have to change in the code to make this work or is there a better way to code this one-to-many parent/child relationship?

The whole setup does not make much sense to me. Why should Track be assigned to a Song instance and somehow inherit properties from that particular instance? And even if Track is a subclass of Song, then mySong and mySong.guitar a two totally different instances. — Felix Kling
– Felix Kling, Commented Jan 15, 2011 at 21:10
I agree...something's odd here. If he wants the Song() to have many Track() objects, Song() should have "this.aTracks = new Array();" and then he could "mySong.aTracks.push(new Track("whatever") )". But it's hard to say what the goal here is. — Krumelur
– Krumelur, Commented Jan 15, 2011 at 21:14

Hemlock · Accepted Answer · 2011-01-15 21:36:09Z

0

So, it looks like you don't want inheritance, you want composition.

~~I'll try to find a good link.~~

I didn't find any good links and as I looked at your code more, I got more confused. Here is something that accomplishes what you want, but I'm not sure it's really useful especially since it does a few things I would not normally consider.

function Song(x){
  this.x = x;
}

Song.prototype.addTrack = function(name, track) {
  this[name] = track;
  track.x = this.x;
}

function Track(y){
  this.y = y;
}


var mySong = new Song(1);
mySong.addTrack('guitar', new Track(2));
mySong.addTrack('bass', new Track(3));

console.log(mySong.guitar.x + " " + mySong.bass.x + " " + mySong.guitar.y + " " + mySong.bass.y );

Seems to me that a better solution would be:

function Song(x){
  this.x = x;
  this.tracks = [];
}

Song.prototype.addTrack = function(track) {
  this.tracks.push(track);
}

Song.prototype.output = function() {
   // loop on this.tracks and output what you need using info from 'this' and 
   // each track
}


function Track(y){
  this.y = y;
}

edited Jan 15, 2011 at 21:36

answered Jan 15, 2011 at 21:11

Hemlock

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1 Comment

Greg Over a year ago

I agree that your second solution is best. Having an array to hold all the Tracks for each Song object is what I need. And you are correct that I meant composition and not inheritance. I still have a lot to learn about computer science.

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