ASP.NET MVC2 in Action 讀書筆記 [12-1] Custom Ajax

<script type="text/javascript">        function getXmlHttpRequest() {            var xhr;            //check for IE implementation(s)            if (typeof ActiveXObject != 'undefined') {                try {                    xhr = new ActiveXObject("Msxml2.XMLHTTP");                } catch (e) {                    xhr = new ActiveXObject("Microsoft.XMLHTTP");                }            } else if (XMLHttpRequest) {                //this works for Firefox, Safari, Opera                    xhr = new XMLHttpRequest();            } else {                alert("Sorry, your browser doesn't support ajax");            }            return xhr;        }            function getMessage() {            //get our xml http request object            var xhr = getXmlHttpRequest();            //prepare the request            xhr.open("GET", "get_message.html", true)                        //setup the callback function            xhr.onreadystatechange = function() {                //readyState 4 means we're done                if(xhr.readyState != 4) return;                                    //populate the page with the result                document.getElementById('result').innerHTML = xhr.responseText;            };                    //fire our request            xhr.send(null);        }    </script><button type="button" onclick="getMessage()">Get the Message</button><div id="result"></div>