Judge whether the node has been hidden.
1. When initialize zTree, the nodes which be set 'isHidden = true' will be hidden.
2. Please don't change this attribute of the nodes which have been created. If you want to hide or show nodes, please use 'hideNode() / hideNodes() / showNode() / showNodes()' methods.
true means: this node is hidden.
false means: this node is shown.
var treeObj = $.fn.zTree.getZTreeObj("tree");
var sNodes = treeObj.getNodes();
if (sNodes.length > 0) {
var isHidden = sNodes[0].isHidden;
}