在JavaScript中,如何使用[]操作符访问从数组继承的对象?

我遇到了一种情况,需要创建一个从数组继承的新JavaScript对象。我正在使用以下代码:

// 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);

非常有趣的是,[]操作似乎毫无用处,控制台输出是:

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

我错过了什么?