javascript OOP inherit methods to multiple DOM elements

Question

I need a way to add new methods to the existing one on some DOM elements (in my case, canvas). I want to do something like this :

var c= document.getElementById('myCanvas');
c.foo = function(){
  console.log('foo' + this.id);
}
c.bar = function(){
  console.log('bar' + this.id);
}

Because it will be used for several canvas elements I don't want to write this code each time.

I could use a method that add this methods to all of my elements, but I was wondering if there is a way to do this with OOP (inheritance ?).

Could I do something like this ?

var extendedCanvas = function(){
  this.foo = function(){
    console.log('foo' + this.id);
  };
  this.bar = function(){
    console.log('bar' + this.id);
  }
}
var c= document.getElementById('myCanvas');
c.prototype = new extendedCanvas();
c.foo();

score 3 · Accepted Answer · edited May 23 '17 at 10:26

3

var c= document.getElementById('myCanvas');
c.prototype = new extendedCanvas();
c.foo();

Nope, assigning anything to the .prototype property of an instance does not have any effect. You could add your methods to HTMLCanvasElement.prototype, but extending the native DOM is considered a bad practise.

Instead, try the mixin pattern:

var c= document.getElementById('myCanvas');
extendedCanvas.call(c); // give properties .foo and .bar to 'c' instance
c.foo();

edited May 23 '17 at 10:26

Community

1
1

answered Feb 13 '14 at 16:08

Bergi

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148
957
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i love `Learning JavaScript Design Patterns by Addy Osmani`, thanks for introduction – Amir Sherafatian Feb 14 '14 at 07:02
Exactly what I wanted. Thx – ylerjen Feb 14 '14 at 10:12

score 0 · Answer 2 · answered Feb 13 '14 at 15:33

0

Try adding your functions directly onto the Element object like this :

Element.prototype.foo = function() {
    console.log('foo' + this.id);
}
Element.prototype.bar = function(){
    console.log('bar' + this.id);
}

answered Feb 13 '14 at 15:33

Serge K.

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Amir Sherafatian · Answer 3 · 2014-02-14T06:02:05.697

no, if classA wants to be inherited from classB, prototype of ClassA must be set by an instance of classB....

function classB()
{
    this.Name = "";
}
classB.prototype.func = function(){};

function classA()
{
}
classA.prototype = new classB();

now Name and func are on every instance of classA

and if you want more things on classA you can then do this:

function classA()
{
    this.Family = "";
}
classA.prototype = new classB();
classA.prototype.func_new = function(){};

Family and func_new are only on every instance of classA not classB

score 0 · Answer 4 · answered Feb 13 '14 at 16:28

you can use this approach to extend canvas... something like decorator pattern:

    function DecoratedCanvas(id) {

        this.Canvas = document.getElementById(id);

        this.foo = function () { console.log('foo' + this.Canvas.id); };
        this.bar = function () { console.log('bar' + this.Canvas.id); }
    };

    var o = new DecoratedCanvas("myCanvas");
    o.foo();
    o.bar();
    var c = o.Canvas;

javascript OOP inherit methods to multiple DOM elements

4 Answers4