How to check if an object is an instance of a NodeList in IE?

Question

Why is NodeList undefined in IE6/7?

Answer 1

"Duck Typing" should always work:

...

if (typeof el.length == 'number' 
    && typeof el.item == 'function'
    && typeof el.nextNode == 'function'
    && typeof el.reset == 'function')
{
    alert("I'm a NodeList");
}