how to show/hide panel inside datalist using javascript

Make a button field. On item command event of the datalist find this button.

protected void DataList1_ItemCommand(object source, DataListCommandEventArgs e)
{
   Button ButtonHide = (Button)e.Item.FindControl("ButtonID");
}

if the button is inside a panel use;

protected void DataList1_ItemCommand(object source, DataListCommandEventArgs e)
{
   Control panelControl = e.Item.FindControl("PanelID");
   Button ButtonHide = panelControl.FindControl("ButtonID") as Button;
}

so on the click event of the button that you are using hide/unhide the panel;

maybe something like;

ClientScript.RegisterStartupScript(GetType(), "Javascript", "HidePanel()", true);

and your HidePanel() function;

function HidePanel()
{
    var panel = document.getElementById('PanelID');
    if(panel.style.display == 'none')
    {
        panel.style.display = 'block';
    }
    else
    {
        panel.style.display = 'none';
    }
}