actually this code is working well in firefox mozilla but it's not working in IE8

<asp:Button ID=btnSubmit runat=server Text=Submit CssClass=btnPrimary OnClientClick=return doSubmit('this');

        OnClick=btnSubmit_Click />

      <script type=text/javascript>

           function doSubmit() 

           {

            var ansLength = oDOM.body.innerText.trim().length;

            if (ansLength == 0 && smielyPresent == -1) 

              {

              alert(you cannot submit blank answer);

              return false;

              }

           }

    </script>



     protected void btnSubmit_Click(object sender, EventArgs e)

    {

      // i am doing some stuff

    }

here i want to prevent postback when answerlength == 0 ;
but when answer length ==0 then it alert alert(u can't submit blank answer) message and postback to server io want to prevent it how i do ?