TreeWalker.currentNode

node = treeWalker.currentNode;
treeWalker.currentNode = node;

var treeWalker = document.createTreeWalker(
    document.body,
    NodeFilter.SHOW_ELEMENT,
    { acceptNode: function(node) { return NodeFilter.FILTER_ACCEPT; } },
    false
);
root = treeWalker.currentNode; // the root element as it is the first element!