How Can I Populate My Dropdownlist

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C#

Javascript

DROPDOWNLIST1: i have a table say "cat" with columns-id,category and categoryid... i successfully populated its value from the database.
DROPDOWNLIST2: Also have another table called "products" with columns-id,name,categoryid.i need to populate my DROPDOWNLIST2 based on categoryid i retrieve from DROPDOWNLIST1

i did it using C#,but i have one problem ie each time i choose an option in DROPDOWNLIST1 after the page load, only then the value of DROPDOWNLIST2 is updated.i need to avoid this hence i need code for the same in javascript.im new to javascrpt.please help me
here is my working code

protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            bindcatdropdown();

        }
    }
    protected void bindcatdropdown()
    {

        DataTable table = new DataTable();
        using (SqlConnection con = new SqlConnection(constr))
        {
            string sql = "select category,categoryid from cat ORDER BY categoryid";
            using (SqlCommand cmd = new SqlCommand(sql, con))
            {
                using (SqlDataAdapter da = new SqlDataAdapter(cmd))
                {
                    da.Fill(table);
                }
            }

        }
        DropDownList1.DataSource = table;
        DropDownList1.DataBind();
        DropDownList1.Items.Insert(0, new ListItem("--Select--", "0"));
       }

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
   {
       DataTable table = new DataTable();
       //int id = Convert.ToInt32(DropDownList1.SelectedValue);
       int id = int.Parse(DropDownList1.SelectedValue.ToString());
       using (SqlConnection con = new SqlConnection(constr))
       {
           string sql = "select name,categoryid from products WHERE categoryid=@cid ORDER BY categoryid";
           using (SqlCommand cmd = new SqlCommand(sql, con))
           {
               using (SqlDataAdapter da = new SqlDataAdapter(cmd))
               {
                   cmd.Parameters.AddWithValue("@cid", id);
                   da.Fill(table);
               }
           }

       }
       DropDownList2.DataSource = table;
       DropDownList2.DataBind();
   }