div ,get css attributes from java script

hey there i want to change some css attributes from javascript

but i cant access css values for that div

here is what i have in my css file

#sidebar {
float: left;
width: 160px;
padding: 25px 10px 0 20px;
}

#sidebar ul {
margin: 0;
padding: 0;
list-style: none;
}

here is my html code for that div

 <div id="sidebar">
     <%@ include file="some_page.jsp" %>
 </div>

here is my javascript code on some click event

var element = document.getElementById('sidebar');
alert(element.style.length); //Will alert 0
alert(element.style.width);//Empty alert box

i want to change the width attribute , can u help me please ?

thank you

Try this code, based on the code from here: http://www.quirksmode.org/dom/getstyles.html ​

var sidebarWidth = getStyle('sidebar','width');

function getStyle(el,styleProp) {
    el = document.getElementById(el);
    return (el.currentStyle)
        ? el.currentStyle[styleProp] 
        : (window.getComputedStyle)
             ? document.defaultView.getComputedStyle(el,null)
                                    .getPropertyValue(styleProp)
             : 'unknown';
}​

EDIT:

To give perhaps a more readable version of the code, this is closer to the quirksmode version. I had changed it to get rid of the unnecessary variables.

var sidebarWidth = getStyle('sidebar','width');

function getStyle(el,styleProp) {
    el = document.getElementById(el);
    var result;
    if(el.currentStyle) {
        result = el.currentStyle[styleProp];
    } else if (window.getComputedStyle) {
        result = document.defaultView.getComputedStyle(el,null)
                                    .getPropertyValue(styleProp);
    } else {
        result = 'unknown';
    }
    return result;
}​