RadTreeView for ASP.NET
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Expanding only one Node per Level Send comments on this topic.
Example scenarios (How to) > Client -side > Expanding only one Node per Level

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

Collapse previously expanded Node on the same Level Copy Code
       function BeforeClientToggle(node)
       {
           var parent = node.Parent;
           
           if (parent == null)
           {
               parent = node.TreeView;
           }
           
           var siblings = parent.Nodes;
           var siblingsCount = siblings.length;
           
           for (var nodeIndex = 0; nodeIndex < siblingsCount; nodeIndex++)
           {
               var siblingNode = siblings[nodeIndex];
               
               if ((siblingNode != node) &&
                   (siblingNode.Expanded))
               {
                   siblingNode.Collapse();
                   
                   return;
               }
           }
       }
In summary, before a Node, that has child Nodes, is toggled we collapse the previously expanded sibling. This will leave only one Node expanded at a time per Level.