It's very easy to pass any Control Id from Code Behind to Javascript Function.
With the help of
Control'ClientId property we can pass any control id from code-behind to Javascript function.
we can understand it by an example:-
Suppose,we have a textbox control and button control as shown:-
<asp:TextBox ID="txt_name" runat="server" Text="Vishal"></asp:TextBox>
<asp:Button ID="btn_get_value" runat="server" Text="Get Value" />
We make a Javascript function:-
<script type="text/javascript" language="javascript">
function get_value(id)
{
var value = document.getElementById(id).value;
alert(value);
}
</script>
Now,from code-behind in button click event write:-
protected void btn_get_value_Click(Object sender,EventArgs e)
{
ScriptManager.RegisterStartupScript(Page, typeof(Page), "message", "javascript:get_value('"+ txt_name.ClientID +"');", true);
}