The nearest node that precedes the current node in the document source and is not filtered out, or null if there is none.
This method sets currentNode to the previous node (in document source order) that is not filtered out and returns that node. If there is no such node, or if the search for the previous node takes the TreeWalker outside of the root subtree, currentNode remains unchanged and the method returns null.
Note that this method "flattens" the document tree structure and returns nodes in the order in which they appear in the document source. Calling previousNode( ) may cause the current node to move down, sideways, or up the document tree. This type of flattening traversal can also be performed with NodeIterator.previousNode( ).