Question

How can I override a method of the Array class?

Asked by: remmuh

I am creating a helper class that for all purposes could simply extend the Array class. However, I need to override the push method with my own code while still calling the super's method.

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Asked On
2008-12-30 at 12:35:51ID24016548
Tags

javascript

,

arrays

,

web development

Topic

JavaScript

Participating Experts
1
Points
350
Comments
4

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Answers

 

by: SnowFlakePosted on 2008-12-30 at 13:14:41ID: 23266120

<html>
<head>
<script>

Array.prototype.basePush=Array.prototype.push;
Array.prototype.push=function(el){
      alert(el);
      this.basePush(el);
}

var arr=new Array();
arr.push(2);
arr.push(3);
alert(arr.toString());
</script>
</head>
</html>

 

by: remmuhPosted on 2008-12-30 at 13:21:54ID: 23266181

Great! That looks like it will work perfect. What if I want to subclass the Array class?

// Constructor
 
var GetURLHelper = function()
{
};
 
GetURLHelper.prototype = new Array;
 
GetURLHelper.prototype.push = function(url)
{
	if(typeof url == 'string' && !this.exists(url))
	{
		this.push(url);
	}
};
 
GetURLHelper.prototype.exists = function(url)
{
	//TODO: Implement iterator
	for(var i=0; i < this.items.length; i++)
	{
		if(this.items[i] == url)
			return true
	}
	return false
};

                                              
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by: SnowFlakePosted on 2008-12-30 at 23:24:52ID: 23268479

using this.push(url)  (line 13)  resolves to the pus of the prototype of GetURLHelper.

you might think that you can call this.prototype.push(url)  which will can the push version of the Array class but then it will push the value on to the prototype of this (which is the Array instance you create on line 7 - by the way - dont you need a () in new Array()  ???).

so - you can eithr use
this.prototype.prototype.push.call(this,url);
which will invoke the push method of the prototype object on the current instance.
or you can directly assign it to a member of the subClass with a different name as in my own example


GetURLHelper.prototype = new Array;
GetURLHelper.prototype.ArrayPush=GetURLHelper.prototype.push;
GetURLHelper.prototype.push = function(url)
{
	if(typeof url == 'string' && !this.exists(url))
	{
		this.ArrayPush(url);
	}
};
                                              
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by: remmuhPosted on 2008-12-31 at 08:26:02ID: 31529825

I'm finally getting a hang of the flexibility of incorporating complex design patterns into JavaScript, and this answer really helped a lot. Thanks for a great easy to understand solution. It provided exactly what I was looking for!

20120131-EE-VQP-002

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