Question

I have a situation, where I need to create a new JavaScript object that is inherited from Array. I am using the following code:

// Create constructor function.
var SpecialArray = function () {};

// Create intermediate function to create closure upon Array's prototype.
// This prevents littering of native Array's prototype.
var ISpecialArray = function () {};
ISpecialArray.prototype = Array.prototype;
SpecialArray.prototype = new ISpecialArray();
SpecialArray.prototype.constructor = SpecialArray;


// Use Array's push() method to add two elements to the prototype itself.
SpecialArray.prototype.push('pushed proto 0', 'pushed proto 1');

// Use [] operator to add item to 4th position
SpecialArray.prototype[4] = 'direct [] proto to 4';

// Create new instance of Special Array
var x = new SpecialArray();

// Directly add items to this new instance.
x.push('pushed directly on X');
x[9] = 'direct [] to 9'

console.log(x, 'length: ' + x.length);

Quite interestingly, the [] operation seem to be useless and the console output is:

["pushed proto 0", "pushed proto 1", "pushed directly on X"] length: 3

What am I missing here?

Answer 1

It is not possible to subclass the Array class and use t this way.
The best solution for you is to extend just the array class and use it as it is.
There are two other options that I do not like but they exist

• http://ajaxian.com/archives/another-trick-to-allow-array-subclasses

• http://dean.edwards.name/weblog/2006/11/hooray/