<apex:page id="tstpage">
<script>
 alert ('test1');
 var x = document.getElementById("{!$Component.tstpage.frm}");
 alert('x value ' +x);
</script>

<apex:form id="frm">
   testing1
</apex:form>

  <h1>Congratulations</h1>
    This is your new Page
     <!-- End Default Content REMOVE THIS -->
   </apex:page>