vs中模拟计算器最后答案显示不出

点击等号没有反应

<script>
    function BindDdl(){
        var sltClg = document.getElementById("ddlOp");

        sltClg.options.add(new Option("+","01"));
        sltClg.options.add(new Option("-","02"));
        sltClg.options.add(new Option("x","03"));
        sltClg.options.add(new Option("/","04"));
    }
    function GetSelectedItem(){
        var ddlOp = document.getElementById("ddlOp");
        var intSelectedIndex = ddlOp.SelectedIndex;
        var itemValue = ddlOp.options[intSelectedIndex].value;
        return itemValue;
    }
    function Calc(){
        var txtNum1 = document.getElementById("txtNum1");
        var txtNum2 = document.getElementById("txtNum2");
        var strNum1 = txtNum1.value;
        var strNum2 = txtNum2.value;
        var dblNum1 = parseFloat(strNum1);
        var dblNum2 = parseFloat(strNum2);
        var dblResult = 0;
        var strOpValue = GetSelectedItem();
        switch (strOpValue){
            case "01":
            dblResult = dblNum1 + dblNum2;
            break;
            case "02":
            dblResult = dblNum1 - dblNum2;
            break;
            case "03":
            dblResult = dblNum1 * dblNum2;
            break;
            case "04":
            dblResult = dblNum1 / dblNum2;
            break;
        }
        var lblResult = document.getElementById("lblResult");
        lblResult.innerText = dblResult.toString();
    }
</script>
<div>
    <table style="width: 50%;">
        <tr>
            <td>数1</td>
            <td>
                <input id="txtNum1" type="text" />
            </td>
            <td>
                <select id="ddlOp" name="D1"></select>
            </td>
            <td>数2</td>
            <td>
                <input id="txtNum2" type="text" />
            </td>
            <td>
                <input id="btnCalc" type="button" value="=" onclick="Calc();" />
            </td>
            <td>
                <asp:Label ID="lblResult" runat="server" Text="Label"></asp:Label>
            </td>
        </tr>
    </table>
    <script>
        BindDdl();
    </script>
</div>

你不绑定事件怎么点击获取值？