javascript - 不能改变全局变量？

Question

我检查了所有内容，但找不到更改全局变量持续时间的问题，这是我的代码：

<script type="text/javascript">

$('#clock').countdown({ layout: '{mnn} : {snn}', timeSeparator: ':', until: 5, onExpiry: restartCountdown });

function restartCountdown() {

        var duration = 10;
        var xmlhttp = new XMLHttpRequest();
        xmlhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            duration = this.responseText;
            alert (duration);
            }
        };
        xmlhttp.open("GET", "waitingtime.php?date_entry=$date_entry,roomname=$rname2,uid=$uid", true);
        xmlhttp.send();

        $('#clock').countdown('option', {until: duration, onExpiry: reloadpage} );
    }

    function reloadpage() {
        location.reload();
    }

</script>

标签： javascriptjquery