Using RadTreeNode Methods Against а Group of Nodes
To run RadTreeNode methods against every node in a group, iterate the RadTreeNodeCollection and call the method for every RadTreeNode instance. For example, to check all nodes in a tree the RadTreeView get_allNodes() method returns the collection of all nodes, the nodes are iterated and set_checked(true) is called for every node instance. This same general pattern can be followed for all kinds of operations against collections of nodes.
function CheckAll() {
var tree = $find("RadTreeView1");
var nodes = tree.get_allNodes();
for (var i = 0; i < nodes.length; i++) {
nodes[i].set_checked(true);
}
}