Javascript collection of DOM objects - why can't I reverse with Array.reverse()?

Question

What could be the problem with reversing the array of DOM objects as in the following code:

var imagesArr = new Array();
imagesArr = document.getElementById("myDivHolderId").getElementsByTagName("img");
imagesArr.reverse();

In Firefox 3, when I call the reverse() method the script stops executing and shows the following error in the console of the Web Developer Toolbar:

imagesArr.reverse is not a function

The imagesArr variable can be iterated through with a for loop and elements like imagesArr[i] can be accessed, so why is it not seen as an array when calling the reverse() method?

Answer 1

Because getElementsByTag name actually returns a NodeList structure. It has similar array like indexing properties for syntactic convenience, but it is not an array. For example, the set of entries is actually constantly being dynamically updated - if you add a new img tag under myDivHolderId, it will automatically appear in imagesArr.

See http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-536297177 for more.