Node previousNode( ) throws DOMException;
The previous node in the sequence of nodes represented by this NodeIterator, or null if there is no previous node.
If this method is called after a call to detach( ), it throws a DOMException with a code of INVALID_STATE_ERR.
This method iterates backward through the sequence of nodes represented by this NodeIterator. It returns the node before the one that was most recently returned by previousNode( ) or nextNode( ). If there is no such node in the sequence, it returns null.