JavaScript - How Do I Set A Page Element As A Variable And Change It?

<style>
#table {display: hidden; z-index: 100;}
</style>

<table id='table' ><td> ......data</td></table>

<script>
function change(){
document.getElementById("table").style.display='block';
}
</script>

<a href='javascript:void(0)' onclick='change()'>Change</a>

<script>
window.addEvent('domready', function() {
var tabs = $$('div[id^="tab"]');
tabs.fade('hide');

$$('#buttons li a').each(function(element,index){
    element.addEvent('click',function(){
        tabs.fade(0);
        tabs[index].fade(1);
    });
});
});
</script> 

<ul id="buttons">
     <li><a href="#">button 1</a></li>
     <li><a href="#">button 2</a></li>
     <li><a href="#">button 3</a></li>
     <li><a href="#">button 4</a></li>
</ul>

<div id="tab1">content for button 1</div>
<div id="tab2">content for button 2</div>
<div id="tab3">content for button 3</div>
<div id="tab4">content for button 4</div>

function something()
{
    if(section1.delta < 0)
	do something here;
}

var xmlHttp;

function ajax_compatible() 
{
   // Post: Returns an object if browser is AJAX compatible else false.
   
   if (window.XMLHttpRequest)
   {
      // users web browser can support AJAX
      return ( new XMLHttpRequest() );
   }
   else if (window.ActiveXObject)
   {
      // users web browser is IE6 and IE5
      return ( new ActiveXObject("Microsoft.XMLHTTP") );
   }
   else {
       alert("Unfortunately your browser does not support AJAX.");
      return null;
   }
}


function get_cookie(cookie_name) 
{ 
	if (document.cookie.length > 0) 
	{ 
	  c_start = document.cookie.indexOf(cookie_name + "="); 
	  
	  if (c_start != -1) 
	  {  
		c_start = c_start + cookie_name.length+1; 
		c_end   = document.cookie.indexOf(";",c_start) 
		
		if (c_end == -1) 
		{
		   c_end = document.cookie.length;
		}
		
		return unescape(document.cookie.substring(c_start,c_end)) 
	  }  
	} 
	return "" 
} 


function add_comment()
{
   // Post: Add user comment & rating to database and update new table row.
   // DO WE NEED TO SEND COOKIE ASWELL OR IS IT ALREADY DONE???
   
   xmlHttp = ajax_compatible();
   
   if (xmlHttp == null) {
       return;
   }
   
   var url = 'http://localhost:8000/cgi-bin/main.py';
   
   alert("add_comment() running");
   
   // Extract CGI variables to pass onto python script
   var id = document.getElementById('videoID').value;
   var video_id = 'videoID=' + id;
   var rating   = 'rating=' + document.getElementsByName('rating').value;
   var comment  = 'comment=' + document.getElementById('comment').value;
   var add_rating = 'addRating=' + document.getElementById('addRating').value;
   //var cookie = document.cookie;
   
   cgi_var = video_id + '&' + rating + '&' + comment + '&' + add_rating; // + '&' + cookie;
   
   // open python script 
   xmlHttp.open('POST',url,true);
   xmlHttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded'); 
	   
   // Error is in below line
   xmlHttp.onreadystatechange = state_changed(id); 
   // if I use the following line then it works
   // xmlHttp.onreadystatechange = test; 
   xmlHttp.send(cgi_var);
}


function state_changed(element_id)
{
   // Post: Use one function that will be used for each page update.
   //       This function sends the text returned by the python server script
   //       onto the browser(HTML element with the id 'element_id') to be displayed.
   
   if (xmlHttp.readyState == 4) 
	{
	   document.getElementById(element_id).innerHTML = xmlHttp.responseText;
	}

}

function test()
{
   // Post: used to test where the problem occurs
   // This function works
   
   if (xmlHttp.readyState == 4) 
	{
	   document.getElementById('result').innerHTML = xmlHttp.responseText;
	}

}