set_expanded()

This property takes a Boolean value, indicating whether the element will be expanded or not. It has effect when the HierarchyLoadMode setting is set to Client.

<telerik:RadGrid RenderMode="Lightweight" ID="RadGrid1" runat="server" AllowPaging="True" CellSpacing="0"
    GridLines="None" Width="800px" OnNeedDataSource="RadGrid1_NeedDataSource"
    OnDetailTableDataBind="RadGrid1_DetailTableDataBind">
    <ClientSettings EnableRowHoverStyle="true">
        <ClientEvents OnRowClick="rowClick" />
    </ClientSettings>
    <MasterTableView DataKeyNames="CustomerID" Name="Customers"
        HierarchyLoadMode="Client">
        <DetailTables>
            <telerik:GridTableView runat="server" DataKeyNames="OrderID" Name="Orders">
            </telerik:GridTableView>
        </DetailTables>
    </MasterTableView>
</telerik:RadGrid>

function rowClick(sender, args) {
    if (args.get_tableView().get_name() == "Customers") {
        args.get_item().set_expanded(!args.get_item().get_expanded());

        // alternative approaches including cases with different load mode
        //args.get_tableView().expandItem(args.get_item().get_element());
        //args.get_tableView().fireCommand("ExpandCollapse", args.get_item().get_itemIndex());
    }
}