I'm trying to check if asp.net textbox is empty or not using java script. my code working ok but if I enter only numbers will conder it empty so I have either enter letters and numbers or only letters.

Please advice.

here is my code

 <script type="text/javascript">
function check(){
  var txt = document.getElementById('<%=txt.ClientID%>').value;

              //(isNaN(cmbStateHome) == false

              if (isNaN(txt) == false) {
                  alert("Please enter some thing.");

              }
              else {alert("ok");}
          }
}
</script> 


 <asp:TextBox ID="txt" runat="server"></asp:TextBox>
                <asp:TextBox ID="txtZipCodeHome" runat="server" Style="top: 361px; left: 88px; position: absolute;

<asp:LinkButton ID="LinkButton1" runat="server" OnClientClick="check()">LinkButton</asp:LinkButton>