Used to record the parent node's expand status.
1. When zTree initialize the node data, if you set treeNode.open = true, zTree will default expand this parent node.
2. Leaf node's 'open' attribute is false.
3. In order to solve the problem of someone make json data, supporting "false", "true" format of the data string.
4. When setting.async.enable = false, the parent node will be expanded which have no child nodes and its attribute 'open' is true. (v3.5.15+)
Default: false
true means: the parent node is expanded.
false means: the parent node is collapsed.
var treeObj = $.fn.zTree.getZTreeObj("tree");
var sNodes = treeObj.getSelectedNodes();
if (sNodes.length > 0) {
var isOpen = sNodes[0].open;
}