深入浅出Ajax（二）

<script type="text/javascript">
        window.onload = initPage;

        function initPage()
        {
            var btn = document.getElementById("username").onblur = checkUsername;
        }

        function checkUsername()
        {
            request = createRequest();
            if (request == null)
            {
                alert("没有请求对象！");
            }
            var url = "DetailForm.aspx?username=" + escape(document.getElementById("username").value);//向何处发请求
            request.onreadystatechange = showUsernameStatus;//函数引用，设置回调函数
            request.open("GET", url, true); //请求方式,true表示将异步的发送请求
            request.send(null);//没有随请求发送额外东西
        }


        function showUsernameStatus()
        {
            if (request.readyState == 4)
            {
                if (request.status == 200)
                {
                    document.getElementById("username").value = request.responseText;
                }

            }
        }
        function createRequest()
        {
            try
            {
                request = new XMLHttpRequest();
            } catch (tryMS)
            {
                try
                {
                    request = new ActiveXObject("Msxml2.XMLHTTP");
                } catch (otherMS)
                {
                    try
                    {
                        request = new ActiveXObject("Microsoft.XMLHTTP");
                    } catch (failed)
                    {
                        request = null;
                    }
                }
            }
            return request;
        } 
    </script>

第二章和第一章内容几乎一样，可以把有共同点的代码提取出来，以后用。