Combo Box onblur event

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ASP.NET

Javascript

I have combobox(editable mode) & text box. I want find the text when combo box text is changed means when select one item then change the itemnae(text) or enter new value, those text i want to find and copy to text box and do some validations in my application. For this i wrote OnBlur event in javascript of combobox but its not fired, in this any wrong . This way of copying is right or any alternative for this. Pls help me..
Thanks in advance.

<script type="text/javascript">
    function BatchnoChanged()
    {
    var cmb=document.getElementById("cmbBatchno");
    var txt=document.getElementById("txtBatchNo");
    txt.value=cmb.value;

    }
    </script>

<asp:ComboBox ID="cmbBatchno" OnSelectedIndexChanged="cmbBatchno_SelectedIndexChanged" OnBlur="BatchnoChanged()"
                                                                                   ItemInsertLocation="Prepend"  runat="server" AutoPostBack="true"  DropDownStyle="Simple"
                                                                                   AutoCompleteMode="Append">
                                                                               </asp:ComboBox>
                                                                               <asp:TextBox ID="txtBatchNo" runat="server"></asp:TextBox>