js 獲取圖片大小

<input type="button" value="GET Size" onclick="getFileSize(document.getElementById('xx'))" />   <img src="a.jpg" id="xx" alt="" />   <script>     function getFileSize(o) {       x = window.XMLHttpRequest ? new window.XMLHttpRequest : new ActiveXObject("MSxml2.XMLHTTP");       x.open("HEAD", o.src, false);       x.send();       alert("大小：" + x.getResponseHeader("Content-Length"));     }   </script>