The NodeIterator.filter read-only property returns a NodeFilter object, that is an object which implements an acceptNode(node) method, used to screen nodes.
When creating the NodeIterator, the filter object is passed in as the third parameter, and the object method acceptNode(node) is called on every single node to determine whether or not to accept it. This function should return the constant NodeFilter.FILTER_ACCEPT for cases when the node should be accepted and NodeFilter.FILTER_REJECT for cases when the node should be rejected.