CALLING A JAVASCRIPT FUNCTION FROM THE ASPX CLICK

<input id="btn1" type="button" class="btn" value="" onclick="return btn_click()" title="Search Patients" />

 <script type="text/javascript" language="javascript">

function btn_click(){
        if($get("search_q_Header").value==''){
            alert('Please enter value to Search');
            $get("search_q_Header").focus();
            return false;
        }

   </script>