session Destroy question

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Javascript

With hi
I wrote a script and when I shut herself user page automatically unset all session I do this I wrote this code

<script type="text/javascript">
var logout = function() {
    var xhr = new XMLHttpRequest;
    xhr.open("GET", "logout.php", false);
    xhr.send(null);
};

var winNumber=1;
window.onload = function() {
   winNumber=winNumber+1;
};

window.onbeforeunload = function() {
    winNumber=winNumber-1;

    if (winNumber === 1) {
        logout();
    }
};
</script> 

Here is my issue with the screen refresh time of session gone if I want to happen just after the closing of this page