window.onbeforeunload executed on page refresh instead of on page close

I'm using window.onbeforeunload to pop up a confirm dialog when a close event occurs, but the confirm dialog appears on page refresh and doesn't execute on page close.

Here's the JavaScript code:

<script language="JavaScript">
    window.onbeforeunload = confirmWinClose();
    function confirmWinClose() {
        var confirmClose = confirm('Close?');
        return confirmClose;
    }    
</script>

I tried it on Chrome, Firefox and Internet Explorer.