js 文件下载进度监控

     var xhr = new XMLHttpRequest();
     xhr.open('GET', '文件地址.mp4');
     xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
     xhr.responseType = "blob";
     xhr.onprogress = function (event) {
         if (event.lengthComputable) {
             console.log(event.loaded);
             console.log(event.total);         }
     };
     xhr.onload = function (oEvent) {
         console.log(oEvent);
         console.log(xhr.status);
         console.log(xhr.response);
         if (xhr.readyState === 4 && xhr.status === 200) {
             // var name = xhr.getResponseHeader("Content-disposition");
             // var filename = name.substring(20,name.length);
             var blob = new Blob([xhr.response], {type: 'video/mp4'});
             var csvUrl = URL.createObjectURL(blob);
             var link = document.createElement('a');
             link.href = csvUrl;
             link.download = 'abababababba.mp4';
             link.click();          
         }
     }
     xhr.send();