html button does not become invisible

I want to set a button visibility based on what I retrieve from a session attribute in window.onload.. The alert appears so the button should become invisible.. Please let me know why it is still visible.

<button type="button" id="ex" onclick="">Ex</button>    
   <script>
    window.onload = function(){
          <%
          Object ex = session.getAttribute("ex");
          if (ex == null)
          {
             %>
             alert("NULL");
             document.getElementById('ex').style.visibility = 'hidden';
          <%     
          }
          else
          {
              %>
              alert("NOT NULL");
         <%   
          }
          %>
        
        };
    </script>