How to use composition with JavaScript

Question

I have something like this:

dog = function () {
  this.age;
  this.name;
  this.dog1 = new dog.shepherd();
}

dog.prototype = {
  bark : function () {
    alert( (this.getName()) + " is barking." );
  },

  [3]**getName** : function () {
    return this.name;
  }
}

dog.shepherd = function () {
  this.something;
}

dog.shepherd.prototype = function () {
  guard : function () {
        alert( ([2]**parent.getName()**) + " is barking." );
  }
}

window.onload = function () {
  var n = new dog();
  [1]**n.guard()**;
}

How can I use a function which is prototyped to the dog from dog.speherd prototype?

In other words, when [1] is executed, then [2] is written in pseudo-code, to call [3].

I know this code shouldn't work; it's just to help me explain what I want.

It's not really clear what it is that you want/expect to happen. What effect are you trying to achieve? — Pointy
– Pointy, Commented Nov 26, 2011 at 16:51
Christoph: Are you aware that dog.shepherd = function () { this.something; } has no impact on inheritance? You're just referencing one function on a property of another function. — RightSaidFred
– RightSaidFred, Commented Nov 26, 2011 at 16:54
Are you just trying to make a Shepherd subclass for dog or are you trying to have dogs have the shepherd methods? — hugomg
– hugomg, Commented Nov 26, 2011 at 16:58
I've been thinking about it. I wanted to reference one function to another, but I should use inheritance here as I think more deeply. — Chris
– Chris, Commented Nov 26, 2011 at 19:16

Custodio · Accepted Answer · 2014-02-07 17:09:50Z

Your title says you want to use composition, but your question body implies inheritance. Here's how you would do this with composition, which is probably the better option. A shepherd "is not a" dog after all (nor vice versa) so inheritance probably isn't right here.

Also, you usually don't want to set the entire prototype of a function like you're doing; just add the functions you want. And constructor functions by convention start with a capital letter.

function Shepherd(mydog) { 
    this.dog = mydog;
}
Shepherd.prototype.guard = function () {
    alert( this.dog.getName() + " is barking." );
}

function Dog(age, name) {
  this.age = age;
  this.name = name;
}

Dog.prototype.getName = function () {
    return this.name;
}

Dog.prototype.bark = function () {
    alert(this.getName() + " is barking." );
}

window.onload = function () {
  var someDog = new Dog(4, "Ubu");
  var someShepherd = new Shepherd(someDog);      
  someShepherd.guard();
}

jsfiddle

Krzysztof · Accepted Answer · 2011-11-26 16:54:42Z

0

I don't know why you need such code, but you can try this:

dog = function () {
  this.age;
  this.name;
  this.dog1 = new dog.shepherd(this);

  var d = this.dog1;
  for (var i in dog.shepherd.prototype) {
    this[i] = function(){
       return this.dog1[i].apply(d, arguments);
    }
  }
}

dog.prototype = {
  bark : function () {
    alert( (this.getName()) + " is barking." );
  },

  getName : function () {
    return this.name;
  }
}

dog.shepherd = function (parent) {
  this.parent = parent;
  this.something;
}

dog.shepherd.prototype = {
  guard : function () {
        alert( (this.parent.getName()) + " is barking." );
  }
}

window.onload = function () {
  var n = new dog();
  n.guard();
}

answered Nov 26, 2011 at 16:54

Krzysztof

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