io farei cosi'

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "DTD/xhtml1-transitional.dtd">
<html>
<head>
    <script type="text/javascript">
        function Oggetto(width, height) {
            this.div = document.createElement('div');
            this.div.style.width = width + "px";
            this.div.style.height = height + "px";
            this.div.style.backgroundColor = "#cccccc";
            
        }

    window.onload=function()
    {
            oggetto = new Oggetto(200, 100)
            document.getElementById("qui").appendChild(oggetto.div);
    }
    </script>
</head>
<body>
    <div id="qui">
        
    </div>

            
</body>
</html>