Ecco il codice:

var vopacity;
var interval = 0;

function fade_out(id_div_to_handle)
{
for(i=0;i<arguments.length;i++)
{
ref = document.getElementById(arguments[i]);
if (ref.style.visibility == 'hidden' || ref.style.visibility=='')
{
ref.style.visibility = 'visible';
handle_div1('copri');
}
else
{
vopacity = 100; // parte da 100 per scendere a 0
interval = setInterval('opacity(\''+arguments[i]+'\')',20);
}
}
}

function opacity(div){
if(vopacity>0){
vopacity -= 8;
object = document.getElementById(div).style;
object.MozOpacity = (vopacity / 100)
object.opacity = (vopacity / 100);
object.KhtmlOpacity = (vopacity / 100);
object.filter = 'alpha(opacity=' + vopacity + ')';
}
else {
clearInterval(interval);
document.getElementById(div).style.visibility = 'hidden';
vopacity = 100;
object.MozOpacity = (vopacity / 100);
object.opacity = (vopacity / 100);
object.KhtmlOpacity = (vopacity / 100);
object.filter = 'alpha(opacity=' + vopacity + ')';
handle_div1('copri');
// resetta i campi html
destroyElement('map');
document.getElementById('window_title').innerHTML = '';
}
}