Ajax实现进度条

/*
　　*@url  --string下载路径
　　*@filename　　--string文件名称
*/

downloadprocess(url,filename){
			filename = filename || 'xxx.csv';
			// 创建xhr对象
			var req = new xmlhttprequest(); 
			//向服务器发送请求 open() send()
			req.open("post", url, true);    //(method-get/post ,url, async) 
			req.setrequestheader("content-type","application/x-www-form-urlencoded");//post时需要传递			
			//监听进度事件            
			req.addeventlistener("progress", function (evt) {                
				if (evt.lengthcomputable) {                    
					var percentcomplete = evt.loaded / evt.total;
					console.log(percentcomplete);
					$("#progressing").html((percentcomplete * 100) + "%");
				}
			}, false);
			/*
				xmlhttprequest 的 responsetype 属性
				一个枚举类型的属性，返回响应数据的类型,只能设置异步的请求
				1、''                 -- domstring (默认);  utf-16
				2、arraybuffer        -- arraybuffer对象，即类型化数组
				3、blob               -- blob对象， 二进制大数据对象
				4、document           -- document对象
				5、json
				6、text				
			*/
			//设置返回数据的类型
			req.responsetype = "blob";
			
			req.onreadystatechange = function () {  //同步的请求无需onreadystatechange      
				if (req.readystate === 4 && req.status === 200) {                    
					var filename = $(that).data('filename');                    
					if (typeof window.chrome !== 'undefined') {                        
						// chrome version
						var link = document.createelement('a');
						link.href = window.url.createobjecturl(req.response);
						link.download = filename;
						link.click();
					} else if (typeof window.navigator.mssaveblob !== 'undefined') {                        
						// ie version
						var blob = new blob([req.response], { type: 'application/force-download' });
						window.navigator.mssaveblob(blob, filename);
					} else {
						// firefox version
						var file = new file([req.response], filename, { type: 'application/force-download' });
						window.open(url.createobjecturl(file));
					}
				}
			};
			
			req.send("fname=henry&lname=ford");
		}