How to check Empty field using JavaScript

function CheckIfEmpty(ctl)
{
/*
Here ctl is the name of
the control, whose value you want to check

*/
if (
      document.getElementById
      ("<%=ctl.ClientID%>").value==""
    )
    {
      alert("Field can not be blank");
      document.getElementById("<%=ctl.ClientID%>").focus();
      return false;
    }
    return true;
} 

<asp:TextBox ID="myTextBox" runat="server"      onblur="javascript:CheckIfEmpty(form1.TextBox1)">
</asp:TextBox>