Extending the XML Node prototype

I have added custom methods and property getters to the XML DOM before
using javascript code such as:

document.prototype.myMethod = function(){ ...

Now I would like to add a custom getter to all instances of XML node
objects. The trouble is, I don't know what the name of the constructor
is. I'd assume something like:

xmlNode.prototype.myMethod = ...

But it doesnt' work. I have tried several variations of the "xmlNode"
but always get a javascript error that it doesn't exist. Does anybody
where I could figure out what the correct name for this is, or even if
it's possible?

Thanks!