您可以手动获取XHR响应头数据：
http://www.w3.org/TR/XMLHttpRequest/#the-
getresponseheader()-方法
此函数将获取所请求URL的文件大小：
function get_filesize(url, callback) { var xhr = new XMLHttpRequest(); xhr.open("HEAD", url, true); // Notice "HEAD" instead of "GET",// to get only the header xhr.onreadystatechange = function() { if (this.readyState == this.DONE) { callback(parseInt(xhr.getResponseHeader("Content-Length"))); } }; xhr.send();}get_filesize("http://example.com/foo.exe", function(size) { alert("The size of foo.exe is: " + size + " bytes.");});
