apex body onload

<script>

   sforce.connection.sessionId = '{!$Api.Session_ID}';

   

   var previousOnload = window.onload;        

   window.onload = function() { 

       if (previousOnload) { 

           previousOnload();

       }

       init();

   }

   

   function init() {

       alert('Hello!! = ' + sforce.connection.sessionId);

   }

</script>