NodeList Objects

I've often write javascripts that use this rather common code to get all tags in an XHTML document:

var alltags = document.getElementsByTagName('*')
... and then use a for loop to access the elements as an array, for example:
for (i=0;i<alltags.length;i++) {
elementClass=alltags.className
[i]Do stuff
}

This has always seemed to work in the past, but I recently learned that the getElementsByTagName method returns a DOM NodeList, not an array. I'm currenly working on a project that needs to access the children of an element conditionally upon its class.

I have three questions. First: How do I declare a global variable to be a DOM NodeList object? It wouldn't be assigned until called from a function, so something akin to var elementList = document.getElementsByTagName() in my global declarations is out of the question. Is there something like var elementList = new NodeList() in javascript? (I know that I can assign it in a function without the var and it will be global, but other people may have to work with this code, and I'd like to have it clearly declared at the head of the program.)

Second question: Assuming alltags is a NodeList returned from a getElementsByTagName call, which of these is proper? This:

childElements = alltags[i].getChildNodes()
... or this:
childElements = alltags.item(i).getChildNodes()
And finally, which of the above techniques has better browser support?