Whats the '!' for in javascript?

Hey, i found the following piece of code on the internet:

var newwindow = '';

function popitup(url)
{
if (!newwindow.closed && newwindow.location)
{
newwindow.location.href = url;
}
else
{
newwindow=window.open(url,'name','height=200,width=150');
if (!newwindow.opener) newwindow.opener = self;
}

I was just wondering what the '!' is for in '!newwindow.closed'. I've come across it many times before but never actually understood what it does.

Thanks, BIOSTALL

It's the not operator. Not Operator
:afrog: