Prototypal Inheritance Multiple Instances of Objects

Question

I'm trying to get the grips of using prototypal inheritance. Here is the code I have below. It is based on the book "Object -Oriented Javascript" by Stoyan Stefanov, with just a few modifications.

Basically I have an Athlete object that extends a Person object. I have created 3 objects. Bob is a Person, while Billy Jean and Steve are Athletes. I added Bob, Billy Jean and Steve in that particular order. I invoked say() and run() functions and getSpeed() and jump(), for the Athlete objects, in all 3 objects in this particular order: Bob, Billy Jean and Steve.

Here is the code below.

<script type="text/javascript">

    function clone(o) {
    var n;
    function F(){};
    F.prototype = o;
    n = new F();

    return n;

   }

    /* Uber is equivalent to the extends method */
   function uber(parent, child) {
     var n = clone(parent);
     n.uber = parent;

   for (var i in child) {
      n[i] = child[i];
   }
   return n;
}

var Person = {
   initialise: function(name)
   {   
      this.name = name;
         },
   say: function()
   {
    console.log('My name is ' + this.name + '. I am a person'); 
   },
   run: function(){
    console.log('I have run 5km');
   },
   jump: function() {
      console.log('I have jumped for joy!');
   }
 };


 var Athlete = {
     initialise: function(name,speed) {
         this.speed = speed; 
         //uber is the parent
    this.uber.initialise(name);
},
    say: function() { console.log('My name is ' + this.name + '. I am an athlete');},
    run: function() { console.log('I have run 20km'); this.jump()},
    getSpeed: function() {console.log('My Speed is: ' + this.speed + 'km Hour');}
  }


  var Athlete = uber(Person, Athlete);

  console.log("Hello, Starting Test...");
  var bob = clone(Person); 
  bob.initialise('Bob');
  bob.say();
  bob.run();

  console.log("Adding Billy Jean...");
  var billyJean = clone(Athlete);
  billyJean.initialise('Billy Jean', 15);

 console.log("Adding Steve...");
 var steve = clone(Athlete);
  steve.initialise('Steve', 25);

 console.log("Asking Billy Jean...");
 billyJean.say();
 billyJean.run();
 billyJean.getSpeed();

 console.log("Asking Steve...");
 steve.say();
 steve.run();
 steve.getSpeed();

</script>

However, when I run the code, although I invoke the functions for Billy Jean first, Steve's properties pop up twice, meaning Steve replaced Billy Jean. Output shown below.

Hello, Starting Test...
My name is Bob. I am a person
I have run 5km
Adding Billy Jean...
Adding Steve...
Asking Billy Jean...
My name is Steve. I am an athlete
I have run 20km
I have jumped for joy!
My Speed is: 15km Hour
Asking Steve Tran...
My name is Steve. I am an athlete
I have run 20km
I have jumped for joy!
My Speed is: 25km Hour

I was just wondering if there is a way to separate Billy Jean and Steve So that I get both their details instead of Steve's details twice?

If it's impossible then what alternative can I use instead to solve this problem? Any solution or help would be a great help.

MBO · Accepted Answer · 2011-10-20 07:58:06Z

1

This particular line is problematic in your example in Athlete.initialize:

//uber is the parent
this.uber.initialise(name);

With this call, you call initialize on object represented by uber, which is shared among your athlets. Change this to:

this.uber.initialize.call(this, name);

to call initialize from uber on your actual object (this) passing additional paremeters.

answered Oct 20, 2011 at 7:58

MBO

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1 Comment

koramaiku Over a year ago

~ That did the trick! Thanks so much for the quick reply, just made the change today and it worked. This is my first time posting on Stack Overflow, had 2 posts with the same solution but i don't know how to vote both of them as correct. Anyway I've given this one a vote up. Thanks again!

Stephen · Accepted Answer · 2011-10-20 08:10:39Z

0

Here's your issue. "this.uber" is referencing an object that is shared between steve and billyJean. Objects are passed around by reference, so that's why this is happening. Try replacing

this.uber.initialise(name);

with

this.uber.initialise.call(this, name);

(In case you aren't aware, 'call' and 'apply' make 'methods' run in different scopes). I don't have any real background on your experience, so hopefully this next ramble isn't insulting.. but, a few thoughts. I've rarely seen javascript done like this. Most of the time it has been something more along the lines of...

var bob = new Person(); 
bob.initialise('Bob');
bob.say();
bob.run();

Why the 'clone'? Did you want a purist approach? If so, the 'clone' isn't any better - you're still calling 'new' and doing a little magic. A purist approach would be more of.. (pardon the jQuery.. being lazy)

Object.prototype.clone = function () {
  return $.extend(true, {}, this);
};

Object.prototype.improve = function (extras) {
  var spawn = $.extend(true, {}, this, extras);
  spawn.uber = this;
  return spawn;
};

var Person = {
  initialise: function(name) {   
    this.name = name;
  },
  say: function() {
    console.log('My name is ' + this.name + '. I am a person'); 
  },
  run: function(){
    console.log('I have run 5km');
  },
  jump: function() {
console.log('I have jumped for joy!');
  }
};

var Athlete = Person.improve({
  initialise: function(name,speed) {
    this.speed = speed; 
    //uber is the parent
    this.uber.initialise.call(this, name);
  },
  say: function() { console.log('My name is ' + this.name + '. I am an athlete');},
  run: function() { console.log('I have run 20km'); this.jump()},
  getSpeed: function() {console.log('My Speed is: ' + this.speed + 'km Hour');}
});

var bob = Person.clone(); 
bob.initialise('Bob');
bob.say();
bob.run();

console.log("Adding Billy Jean...");
var billyJean = Athlete.clone();
billyJean.initialise('Billy Jean', 15);

console.log("Adding Steve...");
var steve = Athlete.clone();
steve.initialise('Steve', 25);

console.log("Asking Billy Jean...");
billyJean.say();
billyJean.run();
billyJean.getSpeed();

console.log("Asking Steve...");
steve.say();
steve.run();
steve.getSpeed();

This works just as well. Note that I'm still using 'call' here - it's because the original is passed by reference. You could undo that.. make it pass a clone.. but really, it's a waste of memory and cycles 90% of the time.

So, there's my best 'purist' approach! I personally don't like this way, as I see 'steve' and 'billyJean' as 'new' Athletes - not clones of an Athletes object - so I'd be more inclined to use a pattern that lets me do 'steve = new Athlete('Steve');'

My two cents, hope it helps.

answered Oct 20, 2011 at 8:10

Stephen

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2 Comments

koramaiku Over a year ago

Thanks so much Stephen for the more detailed explanation and even offering me a recoded version and your opinion and not at all I'm not offended. It was really helpful. I'm just trying to get my head around Javascript and this code that I wrote was based on a book that I found. This was my first post on Stack overflow and like I said in the above response, I have no idea on how to accept both answers. Anyway I've given your answer a tick, but I would've loved to give a vote up on your solution. Anyway thanks again.

Stephen Over a year ago

No problem. It's a lot of fun to get questions like these and try and explain the idea. Javascript is a very interesting language - try and leave whatever other languages you know behind, as JS let's (and sometimes requires) you to do and know things that seem counter intuitive. Good luck and enjoy.

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