TreeWalker.filter

The TreeWalker.filter read-only property returns a NodeFilter that is the filtering object associated with the TreeWalker.

When creating the TreeWalker, the filter object is passed in as the third parameter, and its method NodeFilter.acceptNode() is called on every single node to determine whether or not to accept it.

var treeWalker = document.createTreeWalker(
    document.body,
    NodeFilter.SHOW_ELEMENT,
    { acceptNode: function(node) { return NodeFilter.FILTER_ACCEPT; } },
    false
);
nodeFilter = treeWalker.filter; // document.body in this case