How to call javascript method on browser close event

i have to call the javescript function on browser close event for that i have write the fallow code

    <script type="text/javascript">
        window.onbeforeunload = function (event) {           
                      alert('U have to leave this page');
        };
    </script>

but the bproblem is that it get fired when

1) reload the page

2)redirect to another page

3)close browser

i want only thired once plz help to catch the browser close event in javascript