Expand Only One Node per Level

This article shows hot to expand only one Node per Level with JavaScript. Hook to the OnClientNodeClicking event of the TreeView and handle it in the following way:

JavaScript

function OnClientNodeClicking(sender, eventArgs) {
    var node = eventArgs.get_node();

    node.toggle();

    if (node.get_nodes().get_count()) {
        CollapseSiblings(node);
    }
}

function CollapseSiblings(node) {
    var parent = node.get_parent();
    var siblings = parent.get_nodes();
    var siblingsCount = siblings.get_count();

    for (var nodeIndex = 0; nodeIndex < siblingsCount; nodeIndex++)
     {
        var siblingNode = siblings.getNode(nodeIndex);

        if ((siblingNode != node) && (siblingNode.get_expanded())) 
        {
            siblingNode.collapse();
            return;
        }
    }
}