alert对ajax阻塞调查（IE, Chrome, FF）

<div id="result"></div>

<script>
    function loadXMLDoc(url) {
        var xmlhttp;
        xmlhttp = null;
        xmlhttp = new XMLHttpRequest();
        xmlhttp.onreadystatechange = function () {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                if (xmlhttp.responseText == ‘1‘) {
                    alert(1);
                    document.getElementById(‘result‘).innerHTML=‘‘;
                }else if(xmlhttp.responseText == ‘2‘) {
                    document.getElementById(‘result‘).innerHTML=‘123456‘;
                }
            }
        };
        xmlhttp.open("GET", url, true);
        xmlhttp.send(null);
    }

    loadXMLDoc(‘http://localhost/echo.php?s=1‘);//返回1，为了让alert弹出
    loadXMLDoc(‘http://localhost/echo.php?s=2‘);//返回2，为了让div#result部分修改值

</script>