How can i get all checked items and values in RadTreeView on client side?

Question 1

You can use TreeView.get_nodes() and TreeNode.get_nodes() but these only give you the immediate nodes, not their full descendants.

So for RadTreeView with varying levels of nodes, we need to have a recursive function to drill down into the whole nodes hierarchy.

This answer is adapted from wizpert's answer and this Telerik blog: http://www.telerik.com/blogs/how-to-get-the-checked-items-from-a-treeview-with-checkboxes

function OnClientDropDownClosed(sender, eventArgs) 
{
    // first let's get hold of the treeview (sender is the combobox)
    var tree = sender.get_items().getItem(0).findControl("RadTreeView1");

    // pass the treeview nodes to another function to find the checked nodes
    var items = getCheckedNodes(tree.get_nodes());

    var text = 'None selected'; // default text

    if (items.length > 0)
    {
        var texts = [];
        for (var i = 0; i < items.length; i++) 
        {
            // extract the text only of the checked nodes
            texts.push(items[i].get_text());
        }
        text = texts.join(', '); // comma separated text
    }

    sender.set_text(text); // set the combobox text
}

function getCheckedNodes(nodes) {
    var node, childCheckedNodes;
    var checkedNodes = []; // array of the checked nodes

    for (var i = 0; i < nodes.get_count(); i++) {
        node = nodes.getNode(i);
        if (node.get_checked()) {
            checkedNodes.push(node); // add to array if checked
        }

        // to understand recursion, first you must understand recursion
        if (node.get_nodes().get_count() > 0)
        {
            // recursive function call
            childCheckedNodes = getCheckedNodes(node.get_nodes());

            if (childCheckedNodes.length > 0)
            {
                // append array with results from recursive call
                checkedNodes = checkedNodes.concat(childCheckedNodes);
            }
        }
    }

    return checkedNodes;
}

Question 2

if I understood your question right you want to add the checked Nodes text and Value to an array in javacript , here is the Solution:

function OnClientDropDownClosed(sender, eventArgs)
{ 
var arr_of_nodes = new Array();

 var tree=sender.get_items().getItem(0).findControl("RadTreeView1");
 var treenodes=tree.get_nodes(); 
  for (i=0; i<treenodes.get_count(); i++) 
   { 

       if (treenodes.getNode(i).get_checked()) 
       { 
        var _nodeText=treenodes.getNode(i).get_text();
        var _nodeValue=treenodes.getNode(i).get_value();
       arr_of_nodes.push(_nodeText+":"+_nodeValue);

       } 
   } 
 //do something with the array arr
}