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I'm subclassing Array using prototype, in this way:

MyArray = function()
{
   Array.apply(this, arguments);
};

MyArray.prototype = new Array();

All works fine as expected and I can initialize a "MyArray" instance in this way:

var arr = new MyArray();
arr.push(1, 2, 3);

the problem is that fast enumeration is broken for MyArray instances, in fact the following loop:

for (var i in arr)
{
   console.log(i);
}

prints: 0, 1, 2, length

In practice every method/property of the class is included in the enumeration. In a normal array the code would print only 0, 1, 2 (those numbers are the indexes of the array).

So... my question is: how can I extend Array by preserving fast enumeration?

ps: In Objective C in order to implement fast enumeration you have to override a method... is there a special method to override in JavaScript?

Notes: obviously my real implementation of "MyArray" is slightly different, but the code I posted is enough to generate an abnormal fast enumeration loop. Don't ask me why I'm extending Array, I have my valid reasons ;)

daveoncode
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    [You can't *really* subclass Array](http://perfectionkills.com/how-ecmascript-5-still-does-not-allow-to-subclass-an-array/). Also that's not "fast enumeration" in JavaScript, and in fact with respect to Array instances it's better referred to as "incorrect enumeration" :-) – Pointy Sep 03 '12 at 16:42
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    (It's "incorrect" because among other things you're not even guaranteed by the spec to get the numbered properties back in order.) – Pointy Sep 03 '12 at 16:44
  • @Pointy can you post your comment as an answer in order to let me accept it? ;) – daveoncode Sep 03 '12 at 19:49
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    Well jfriend00 basically said the same thing and he typed in all that useful stuff too :-) – Pointy Sep 03 '12 at 19:50

1 Answers1

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Array elements should NEVER be enumerated with for (var x in obj) because that iterates enumerable properties of the object not just array elements which, as you can see, can include things besides array elements. Furthermore, this method of iterating is NOT guaranteed to return properties in any given order so your array elements might not be in the right order.

In the latest browsers, you can create properties with the Object.defineProperties() method and specify whether a property is enumerable or not, but that still doesn't mean it's ever correct to enumerate the elements of an array with for (var x in obj). One should always use this type of form to iterate array elements: for (var i = 0, len = arr.length; i < len; i++) or a built-in iterator like array.forEach(callback [, thisArg]).

jfriend00
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  • @JaredFarrish - my intention is any form that iterates by numeric index directly. Once can use any form of loop that iterates between two index values. `while()` is OK too if it's iterating numeric index values and determining bounds via `.length`. – jfriend00 Sep 03 '12 at 17:10
  • I usually use for/in to iterate "normal" Array and I have no problems, the elements returned are in the right order and no "extra properties" are returned... can you prove me the opposite? – daveoncode Sep 03 '12 at 19:47
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    @daveoncode - order is NOT guaranteed by the ECMA specification and that type of iteration is NOT meant for ordered arrays. If you want to keep doing it, I don't quite understand why, but go right ahead. When I first discovered this, I changed my methods because safe, reliable code is way more important to me than saving a few characters of typing. Further, if you or someone else who works on your project ever includes a library that adds some methods to the array object, all your code could suddenly break. Why program in a dangerous fashion? It's just not smart. – jfriend00 Sep 03 '12 at 19:54