javascript ajax请求

<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <title></title>
    <meta charset="utf-8" />

</head>
<body>
    <input id="btn1" type="button" value="提交" />
    <script type="text/javascript">
        var oBtn = document.getElementById("btn1");
        oBtn.onclick = function () {
            var oAjax = null;
            if (window.XMLHttpRequest) {
                oAjax = new XMLHttpRequest();
            } else {
                oAjax = new ActiveXObject("");
            }
            oAjax.open("get", "aaa.txt", true);
            oAjax.send();
            oAjax.onreadystatechange = function () {
                if (oAjax.readyState == 4) {
                    if (oAjax.status == 200) {
                        alert(oAjax.responseText);
                    }
                }
            };
        };
    </script>
</body>
</html>